Unraveling the Ecological Importance of Elasmobranchs

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DOI: 10.1201/9781420080483-c16
In book: Sharks and Their Relatives II: Biodiversity, adaptive physiology, and conservation, Publisher: CRC Press, Editors: Jeffery C. Carrier, John A. Musick, Michael R Heithaus, pp.611-637
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  • ... Predation is an essential piece of information for determining natural mortality and can significantly influence estimates of maximum sustainable yield (MSY) for fish stocks (Tyrrell et al. 2011). Sharks are a particularly important predator group, functioning as apex predators in many marine environments (Heithaus et al. 2010). Most predatory sharks are tertiary consumers (trophic level > 4), occupying the same trophic role as marine mammals and surpassing that of seabirds and most teleost fishes (Cortés 1999). ...
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    Though the feeding habits of Spiny Dogfish Squalus acanthias in the northwestern Atlantic Ocean have received much attention due to their potential interactions with fisheries, currently no estimate of daily ration specific to the northwestern Atlantic population has been determined. To assess the diet of these sharks in the southern extent of their range, stomach contents were collected from 255 Spiny Dogfish captured by bottom trawl in nearshore North Carolina waters; 244 were mature females. Prey items were identified and percent index of relative importance was calculated for each prey taxon and category. To determine daily ration, 15 mature female Spiny Dogfish were captured by hook and line in North Carolina waters and kept in captivity for 2 weeks of feeding trials. Dogfish were fed preweighed frozen Atlantic Menhaden Brevoortia tryannus and allowed to digest their prey for predetermined periods, after which remaining food was removed using stomach-tube gastric lavage. Gastric evacuation rates determined by feeding trials were combined with data collected from sampling stomach contents to determine the daily ration and the amount of important prey taxa consumed during the Spiny Dogfish overwintering period. Teleost fishes were the dominant prey category, and Atlantic Menhaden and Bay Anchovy Anchoa mitchilli were the most important prey taxa. Daily ration estimates ranged from 0.26% to 0.56% of the shark's body weight per day. Spiny Dogfish potentially consumed an equivalent of 1.55–3.33% of the Atlantic Menhaden stock while overwintering in North Carolina waters.
  • ... Despite the ecological significance of large sharks (Heithaus et al. 2010) and their importance to both commercial and recreational fisheries (Walker 1998), quantitative data on their spatial and seasonal abundance are scarce. Studies that have examined spatial and temporal partitioning among elasmobranchs have typically focussed on juvenile sharks or smaller sharks in shallow-water habitats (Simpfendorfer and Milward 1993;Speed et al. 2010). ...
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    Our understanding of the ecological role of larger elasmobranchs is limited by a lack of information on their spatial and seasonal abundance. Analysis of 14 years of gill-net catch data in south-eastern Queensland, Australia, revealed that the species composition of large sharks and other elasmobranchs significantly differed among beaches and seasons. Spinner sharks (Carcharhinus brevipinna) and hammerhead sharks (Sphyrna spp.) comprised nearly half the catch of all elasmobranchs. Although the distribution of these sharks overlapped, spatial variation existed in their abundance. Spinner sharks characterised the catch at Sunshine Coast beaches, whereas the catch at Gold Coast beaches was dominated by hammerhead sharks. Seasonal differences in elasmobranch community structure were also apparent, driven largely by a lower abundance of many species during the winter and the predominance of species such as spinner sharks and hammerheads in spring and summer. The present study provides the first quantitative data for numerous species of Carcharhiniformes in south-eastern Queensland and demonstrates that analysis of catch-rate data can improve our understanding of how larger sharks partition resources.
  • ... While the extent of these declines is debated (Burgess et al., 2005;Heupel et al., 2009;Braccini, 2015), there is general consensus that they are primarily caused by elevated fishing mortality through targeted fisheries that supply shark and manta ray products, and bycatch in other fisheries (Friedlander and Demartini, 2002;Dulvy et al., 2008;Davidson et al., 2015). Over the last decade, scientists and conservation practitioners have highlighted the urgent need for improved fisheries management to stem the large-scale exploitation of shark and ray species, many of which are critically important apex predators and valuable marine tourism assets (Heithaus et al., 2010;Gallagher and Hammerschlag, 2011;Vianna et al., 2012;Dulvy et al., 2014). In countries where a large part of the population has a high dependency on marine resources for livelihoods and protein, assessments and management of these fisheries are often hindered by the presence of extensive fleets of unregistered vessels and widespread unregulated small-scale fisheries, as well as a lack of enforcement of existing regulations on registered vessels (Blaber et al., 2009). ...
  • ... While the extent of these declines is debated (Burgess et al., 2005;Heupel et al., 2009;Braccini, 2015), there is general consensus that they are primarily caused by elevated fishing mortality through targeted fisheries that supply shark and manta ray products, and bycatch in other fisheries (Friedlander and Demartini, 2002;Dulvy et al., 2008;Davidson et al., 2015). Over the last decade, scientists and conservation practitioners have highlighted the urgent need for improved fisheries management to stem the large-scale exploitation of shark and ray species, many of which are critically important apex predators and valuable marine tourism assets (Heithaus et al., 2010;Gallagher and Hammerschlag, 2011;Vianna et al., 2012;Dulvy et al., 2014). In countries where a large part of the population has a high dependency on marine resources for livelihoods and protein, assessments and management of these fisheries are often hindered by the presence of extensive fleets of unregistered vessels and widespread unregulated small-scale fisheries, as well as a lack of enforcement of existing regulations on registered vessels (Blaber et al., 2009). ...
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    Fisheries are complex social-ecological systems, where managers struggle to balance the socio-economic interests of fishing communities with the biology and ecology of fisheries species. Spatial closures are a popular measure to address conservation and fisheries management goals, including the protection of shark populations. However, very little research has been published on the effectiveness of shark-specific closures to protect sharks, or their impacts on fisher behavior. Situated within the global center of tropical marine biodiversity, Indonesia’s shark fishery contributes more to the international shark fin trade than any other nation. Here we evaluate the effect of shark-specific closures on sharks and other species of interest, as well as shark fishers’ responses to losing access to their former fishing grounds. We assessed shark diversity and abundance in an open access zone (OAZ) and two No-Take Zones (NTZs) of a Marine Protected Area within the recently established shark sanctuary in Raja Ampat, Indonesia, where sharks have high monetary value as a tourism attraction. Shark abundance was significantly higher in the privately managed NTZs than in the OAZ. Across all management zones, neither zone size, depth nor reef complexity explained variations in shark abundance, suggesting that governance is the main driver of successful shark conservation areas. These trends were also reflected in species targeted by small-scale reef fisheries, including snappers, emperor, groupers, tunas, mackerels, and large-bodied wrasse and parrotfish. Interviews with shark fishers who lost access to their primary fishing grounds when the shark sanctuary was established showed that while most fishers (88%) knew that sharks were protected in Raja Ampat, many were unsure about the purpose of the sanctuary. Few fishers felt that the agencies implementing fishing bans understood their livelihood needs. We found that shark fishers adapted to the loss of former fishing grounds by shifting fishing effort to other locations or diversifying their livelihoods, including illegal petrol transport. While conserving sharks for tourism can be effective, it may inadvertently result in displacing fishing effort to unprotected regions. We propose that effective shark conservation in Indonesia will need to combine strategic spatial protection with efforts to support livelihood security and diversification.
  • ... Elasmobranchs are important trophic components of marine ecosystems, playing a role in the control of populations of prey species (Camhi et al., 1998;Heithaus et al., 2010). The removal of such organisms can lead to changes in the environment as well in species abundance (Heithaus et al., 2008;Baum & Worm, 2009;Bornatowski et al., 2014). ...
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    The reproductive biology of Urotrygon microphthalmum was studied based on specimens caught as by-catch in shrimp (Litopenaeus schmitti, Xiphopenaeus kroyeri, Farfantepenaeus subtilis and Farfantepenaeus brasiliensis) fishing operations between March 2010 and March 2012 on the coast of Pernambuco, Brazil. Females reached a larger total length (LT ) and total body mass (MT ) (298 mm and 148 g) than males (250 mm and 90 g). Length at maturity was estimated to be 188 and 199 mm LT for males and females. Uterine fecundity ranged from one to four embryos (mean ± s.d.: 1·85 ± 0·45). Size at birth was estimated to be 105 mm LT . Gestation lasted between 4 and 5 months. The reproductive cycle of U. microphthalmum is hypothesized to be asynchronous and biannual.
  • ... Given the importance of sharks to ecosystem functioning, information on the trophic ecology of individual species is critical for understanding how changes in mortality (e.g., through directed fisheries, incidental catch, or pollution) may impact species of concern and overall food web structure. Conversely, it is also crucial to understand how impacts on ecosystem components (e.g., habitat alteration, fishing on prey species) may influence the productivity of shark populations (Stevens 2000;Heithaus et al. 2010;Simpfendorfer et al. 2011). Despite its relevance to ecosystem-based management, the trophic ecology of many shark species remains poorly understood, in part due to the laborintensive and expensive nature of sampling (Wetherbee and Cortes 2004;Wetherbee et al. 2012). ...
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    Identifying feeding patterns of large-bodied predators is necessary for predicting their potential effects on food web dynamics. However, diet information from stomach contents can be impractical to obtain because required sample sizes can be prohibitively large. In contrast, diet estimates obtained using Bayesian stable isotope mixing models require less sampling effort and can also reveal both population- and individual-level variation in diet. Here, we used an extensive stable isotope data set to evaluate the trophic role of bluntnose sixgill shark (Hexanchus griseus), a globally distributed species and among the largest sharks in the North Pacific. In total, 43 subadult sixgill sharks were sampled from Puget Sound, Washington, USA. Mixing model results indicated that the population feeds primarily on benthic fish and invertebrates (estimated median diet percentages: 33 and 35%, respectively). Further, the model indicated low individual variation in diets and that the feeding behavior of both individuals and the population as a whole tended towards generalism. Specifically, sixgill sharks appear to feed on prey groups approximately in proportion to their average biomass densities in the Puget Sound food web. As generalists, sixgill sharks are less likely to be affected by changes in the abundance of any single prey resource, and our results suggest they are unlikely to be important predators to at least some species of management concern. In addition, stable isotope data obtained opportunistically from an adult sixgill shark supports previously suggested ontogenetic movement patterns, whereby some adults make brief migrations into Puget Sound from outer coastal habitats, likely to birth, and pups feed, grow, and remain resident in Puget Sound for several years. Our findings provide insights into the trophic role of this important but understudied species and demonstrate how stable isotope analyses can further understanding of shark ecology.
  • ... Because of their relatively high abundance, skates may play an important role in demersal food webs and benthic communities (Ebert & Bizzarro, 2007). The first step to determine how they can influence the dynamics of the community is to know the diet composition and trophic trends (Heithaus et al., 2010). The diet of Z. chilensis, studied in Argentina from 1996 to 1998 from 41 ∘ to 46 ∘ S from commercial fishing catches, was composed mainly of fishes (Lucifora et al., 2000; Koen Alonso et al., 2001). ...
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    The diet and trophic level (TL ) of the yellownose skate Zearaja chilensis in the south-western Atlantic Ocean (35°-54° S), and how these varied in relation to body size, sex, maturity stage, depth and region were determined by analysis of stomach contents. From 776 specimens analysed, 671 (86·5%) ranging from 180 to 1190 mm total length (LT ) had prey in their stomachs. The diet was dominated by fishes, mainly the notothenioid Patagonotothen ramsayi and the Argentine hake Merluccius hubbsi. The consumption of fishes and crabs increased with increasing predator size, and these preys were more important in the north than in the south. Isopods and other crustaceans were consumed more in the south and their consumption decreased as the size of Z. chilensis increased. The TL of Z. chilensis increased with LT from 4·29 to 4·59 (mean 4·53), confirming their ecological role as a top predator. The small and large size classes exhibited a low diet overlap and the highest spatial segregation, whereas medium and large specimens had higher co-occurrence and dietary overlap indices. A clear distinction in tooth shape was noted between sexes in adult specimens, with males having longer cusps. This sexual heterodonty may be related to reproductive behaviour, increasing the grasping ability of males during courtship, because there were no differences in diet between the sexes.
  • ... Apex predators play an important role in marine community structure through both consumptive and non-consumptive effects on prey [1,2,3,4]. Within marine systems, several species of large sharks are thought to be apex predators that may act as keystone species [2,5]. Owing to their life history characteristics, sharks are thought to be particularly sensitive to overfishing and although there is much debate as to the extent, there is evidence of substantial declines in some locations (e.g. ...
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    White sharks are highly migratory and segregate by sex, age and size. Unlike marine mammals, they neither surface to breathe nor frequent haul-out sites, hindering generation of abundance data required to estimate population size. A recent tag-recapture study used photographic identifications of white sharks at two aggregation sites to estimate abundance in "central California" at 219 mature and sub-adult individuals. They concluded this represented approximately one-half of the total abundance of mature and sub-adult sharks in the entire eastern North Pacific Ocean (ENP). This low estimate generated great concern within the conservation community, prompting petitions for governmental endangered species designations. We critically examine that study and find violations of model assumptions that, when considered in total, lead to population underestimates. We also use a Bayesian mixture model to demonstrate that the inclusion of transient sharks, characteristic of white shark aggregation sites, would substantially increase abundance estimates for the adults and sub-adults in the surveyed sub-population. Using a dataset obtained from the same sampling locations and widely accepted demographic methodology, our analysis indicates a minimum all-life stages population size of >2000 individuals in the California subpopulation is required to account for the number and size range of individual sharks observed at the two sampled sites. Even accounting for methodological and conceptual biases, an extrapolation of these data to estimate the white shark population size throughout the ENP is inappropriate. The true ENP white shark population size is likely several-fold greater as both our study and the original published estimate exclude non-aggregating sharks and those that independently aggregate at other important ENP sites. Accurately estimating the central California and ENP white shark population size requires methodologies that account for biases introduced by sampling a limited number of sites and that account for all life history stages across the species' range of habitats.
  • ... Predation is an essential piece of information for determining natural mortality and can significantly influence estimates of maximum sustainable yield (MSY) for fish stocks (Tyrrell et al. 2011). Sharks are a particularly important predator group, functioning as apex predators in many marine environments (Heithaus et al. 2010). Most predatory sharks are tertiary consumers (trophic level > 4), occupying the same trophic role as marine mammals and surpassing that of seabirds and most teleost fishes (Cortés 1999). ...
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    Full-text available
    Though the feeding habits of Spiny Dogfish Squalus acanthias in the northwestern Atlantic Ocean have received much attention due to their potential interactions with fisheries, currently no estimate of daily ration specific to the northwestern Atlantic population has been determined. To assess the diet of these sharks in the southern extent of their range, stomach contents were collected from 255 Spiny Dogfish captured by bottom trawl in nearshore North Carolina waters; 244 were mature females. Prey items were identified and percent index of relative importance was calculated for each prey taxon and category. To determine daily ration, 15 mature female Spiny Dogfish were captured by hook and line in North Carolina waters and kept in captivity for 2 weeks of feeding trials. Dogfish were fed preweighed frozen Atlantic Menhaden Brevoortia tryannus and allowed to digest their prey for predetermined periods, after which remaining food was removed using stomach-tube gastric lavage. Gastric evacuation rates determined by feeding trials were combined with data collected from sampling stomach contents to determine the daily ration and the amount of important prey taxa consumed during the Spiny Dogfish overwintering period. Teleost fishes were the dominant prey category, and Atlantic Menhaden and Bay Anchovy Anchoa mitchilli were the most important prey taxa. Daily ration estimates ranged from 0.26% to 0.56% of the shark's body weight per day. Spiny Dogfish potentially consumed an equivalent of 1.55–3.33% of the Atlantic Menhaden stock while overwintering in North Carolina waters.Received July 23, 2013; accepted March 5, 2014
  • ... Marine predators can significantly influence the distributions of other species and local community dynamics . Elasmobranchs typically occupy high trophic levels within marine communities, and some species function as apex predators in these ecosystems (Cortés 1999, Heithaus et al. 2010, Hussey et al. 2015, Shaw et al. 2016). Even at relatively small spatial and temporal scales, elasmobranchs can have significant, population-level, top-down effects on prey species (Beamish et al. 1992, Lacroix andFleming 2014). ...
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    In marine communities, resource partitioning can be as important as abiotic environmental preferences in determining habitat use patterns. Elasmobranchs are generally assumed to be crepuscular or nocturnal, but diel temporal habitat partitioning is poorly studied in this group. We attempted to identify habitat preferences and find evidence of resource partitioning among the elasmobranch community in Back and Core Sounds, North Carolina, using a multi-gear, fishery-independent survey with a temporal focus on the diurnal-nocturnal transition. Gillnet, longline, drumline, and rod-and-reel sampling captured a total of 160 elasmobranchs, representing 12 species within the estuary, and differences between the seven most abundant species were assessed in terms of temporal, environmental, and spatial habitat factors. The elasmobranch community was broadly divided into cool and warm temperature assemblages. Most species showed evidence of generalist habitat preferences, but spatial overlap between species was generally low. Blacknose sharks [Carcharhinus acronotus (Poey, 1860)] appeared to be nocturnal, and aggregations of smooth dogfish [Mustelus canis (Mitchill, 1815)] and spiny dogfish (Squalus acanthias Linnaeus, 1758) were found during mid-afternoon hours. Blacknose sharks and blacktip sharks [Carcharhinus limbatus (Müller and Henle, 1839)] showed evidence of spatial resource partitioning based on distance from the nearest inlet. Temperature appears to be a strong influence on the presence of elasmobranch species within Back and Core Sounds, but behavioral interspecific avoidance may be a greater influence on fine-scale habitat use by elasmobranchs in this estuarine system.
  • ... The possibility that oceanic whitetip shark may be insufficiently resilient to withstand current or future levels of fishing pressure raises additional questions about the potential for ecological effects associated with removal of apex predators. In general , sharks are believed capable of top-down influence on food webs by direct predation, by generating risk of predation that evokes changes in behavior of prey species with consequent negative effects on their fitness, and by initiating trophic cascades (Heithaus et al. 2010). Kitchell et al. (2002) used an Ecopath with Ecosim model to investigate fishing effects on and the ecological importance of " brown sharks " (i.e., oceanic whitetip and silky (Carcharhinus falciformis) sharks) in North Pacific food webs. ...
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    One key issue for standardizing catch per unit effort (CPUE) of bycatch species is how to model observations of zero catch per fishing operation. Typically, the fraction of zero catches is high, and catch counts may be overdispersed. In this study, we develop a model selection and multimodel inference approach to standardize CPUE in a case study of oceanic whitetip shark (Carcharhinus longimanus) bycatch in the Hawaii-based pelagic longline fishery. Alternative hypotheses for shark catch per longline set were characterized by the variance to mean ratio of the count distribution. Zero-inflated and non-inflated Poisson, negative binomial, and delta-gamma models were fit to fishery observer data using stepwise variable selection. Alternative hypotheses were compared using multimodel inference. Results from the best-fitting zero-inflated negative binomial model showed that standardized CPUE of oceanic whitetip sharks decreased by about 90% during 1995–2010 because of increased zero catch sets and decreased CPUE on sets with positive catch. Our model selection approach provides an objective way to address the question of how to treat zero catches when analyzing bycatch CPUE.
  • ... Los batoideos, con un nivel trófico bajo (<4), están considerados como consumidores secundarios cuando son comparados con la mayoría de las especies de tiburones. Sin embargo, todavía se desconoce el nivel trófico estándar de muchos de ellos (Heithaus et al. 2010). Se predice que los batoideos tienen un papel crítico en la dinámica de los ecosistemas marinos como especies depredadoras a niveles intermedios (Ritchie y Johnson 2009), donde son una liga importante en las redes tróficas de las Feeding habits of the speckled guitarfish Rhinobatos glaucostigma (Elasmobranchii, Batoidea) in the southeastern Gulf of California Hábitos alimentarios de la guitarra punteada Rhinobatos glaucostigma (Elasmobranchii, Batoidea) en el sureste del golfo de California ABSTRACT. ...
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    To assess the feeding habits of Rhinobatos glaucostigma (speckled guitarfish), the stomach contents of 176 individuals were examined. Specimens were obtained from the artisanal gillnet fisheries operating off Nayarit, Mexico, in the southeastern region of the Gulf of California, during January–August 2008 and January–May 2009. Relative measures of prey quantities and nonparametric multivariate methods were used to analyze diet differences between sex and maturity stages. The trophic relationship and feeding strategy were determined using Levin’s niche amplitude index, the Costello method modified by Amundsen, and Pianka’s diet overlap index. The overall diet was dominated by crustaceans, primarily decapods, brachyurans, and cumaceans. Diets were similar between sexes and maturity stages, except between mature females and males where only slight differences were found. Mature females and immature specimens of both sexes showed specialized diets, while mature males had a generalist diet. A significant overlap between sexes and maturity stage diets was found. Diet compositions allowed us to estimate a standardized trophic level of 3.57 for R. glaucostigma. The diet compositions between sexes and maturity stages suggest that all individuals consume similar prey items and have similar trophic roles in the benthic community of the southeastern region of the Gulf of California, where R. glaucostigma should be considered a secondary consumer. Para caracterizar los habitos alimentarios de Rhinobatos glaucostigma (guitarra punteada), se muestrearon los contenidos estomacales de 176 individuos. Los especimenes fueron capturados con redes agalleras por la pesca artesanal que opera frente a Nayarit, Mexico, en la region sureste del golfo de California, durante los periodos de enero a agosto de 2008 y de enero a mayo de 2009. Para analizar las diferencias de dieta entre sexos y estadios de madurez, se utilizaron las medidas relativas de cuantificacion de dieta y metodos no parametricos multivariados. Las relaciones troficas y la estrategia de alimentacion fueron determinadas con el indice de amplitud de nicho de Levin, el metodo grafico de Costello modificado por Amundsen y el indice de traslapo de dietas de Pianka. La dieta global estuvo dominada por crustaceos, principalmente por decapodos, braquiuros y cumaceos. Las dietas fueron similares entre sexos y estadios de madurez, excepto entre hembras y machos maduros. Las hembras maduras y los individuos inmaduros de ambos sexos presentaron una dieta especialista, mientras que los machos maduros tuvieron una dieta generalista. Se encontro un traslapo significativo entre las dietas de ambos sexos y los estadios de madurez. La composicion especifica de la dieta permitio estimar un nivel trofico estandar de 3.57 para R. glaucostigma. La composicion de la dieta entre sexos y estadios de madurez sugiere que todos los individuos consumen presas similares y tienen un papel trofico similar en la comunidad bentonica de la region sureste del golfo de California, en donde R. glaucostigma debe ser considerada como un consumidor secundario.
  • ... To date, elasmobranch habitat connectivity has been described in the context of ontogenetic changes in habitat use of populations (e.g., Chin et al. 2013), without an appreciation of how individuals may connect habitats on finer temporal and spatial scales. Adult Spotted Eagle Rays may thus serve as a model predator to test future hypotheses related to benthic habitat connectivity and spatial subsidies (sensu Heithaus et al. 2010). Future work should concentrate on the frequency and extent of these habitat exchanges. ...
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    Batoids are important mesopredators whose high mobility and extensive migrations can link seemingly distant food webs in coastal ecosystems. Despite this recognition, our knowledge of the movement patterns of many species is limited due to the logistical challenge of tracking these animals on multiple scales. Smart Positioning or Temperature (SPOT) satellite-linked transmitters allow for precise, multi-scale tracking of species that regularly use surface waters. To date, SPOTs have been predominantly used on sharks, with only a single application to a batoid. Given the epipelagic nature of myliobatid stingrays, we examined the potential for towed-float SPOT transmitters to monitor large-scale movements of two representative species: the Cownose Ray (Rhinoptera bonasus; n = 15) and Spotted Eagle Ray (Aetobatus narinari; n = 9). Tracking data identified several consistent outmigration patterns of Cownose Rays along the Mississippi-Alabama shelf and seasonal variation in movement rates along barrier island habitats. We also documented sex-related differences in movement rates and habitat use of Spotted Eagle Rays along the Bermuda platform, where males exhibited significantly higher movement rates than females and more transient behavior between inshore lagoons and outer coral reefs. Both Cownose and Spotted Eagle Rays were shown to exhibit connectivity among several habitat types along continental shelves in their respective locales, demonstrating future challenges to the management of these species over large spatial scales. While reductions in tag size and improved tethering techniques would undoubtedly broaden the applicability of towed-float satellite telemetry to other species and sizes, our work highlights the strong potential for this technology to provide insights into the spatial ecology and habitat use of myliobatid rays.
  • ... Globally, the removal of sharks due to fishing has accelerated rapidly in recent decades, with the result that many species are now threatened with or vulnerable to extinction in many regions [7][8][9][10][11][12]. There is a growing recognition of the importance of sharks as keystone species in the structuring of marine ecosystems through their influence on species composition, biomass and the trophic roles of prey assemblages [10,[13][14][15]. However, we remain largely unaware of some of the most basic aspects of the ecology of many species, including movement patterns and habitat requirements. ...
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    Tiger sharks (Galeocerdo cuvier) are apex predators occurring in most tropical and warm temperate marine ecosystems, but we know relatively little of their patterns of residency and movement over large spatial and temporal scales. We deployed satellite tags on eleven tiger sharks off the north-western coast of Western Australia and used the Brownian Bridge kernel method to calculate home ranges and analyse movement behaviour. One individual recorded one of the largest geographical ranges of movement ever reported for the species, travelling over 4000 km during 517 days of monitoring. Tags on the remainder of the sharks reported for shorter periods (7-191 days). Most of these sharks had restricted movements and long-term (30-188 days) residency in coastal waters in the vicinity of the area where they were tagged. Core home range areas of sharks varied greatly from 1166.9 to 634,944 km2. Tiger sharks spent most of their time in water temperatures between 23°-26°C but experienced temperatures ranging from 6°C to 33°C. One shark displayed seasonal movements among three distinct home range cores spread along most of the coast of Western Australia and generalized linear models showed that this individual had different patterns of temperature and depth occupancy in each region of the coast, with the highest probability of residency occurring in the shallowest areas of the coast with water temperatures above 23°C. These results suggest that tiger sharks can migrate over very large distances and across latitudes ranging from tropical to the cool temperate waters. Such extensive long-term movements may be a key element influencing the connectivity of populations within and among ocean basins.
  • ... Shifts in prey community structure are possibly a result of both removal of prey as well as risk effects, although there is some controversy surrounding this hypothesis due to nonconvergence of dietary links and differences in the distribution of meso-and apex predators (Heithaus et al., 2010). The role sharks play in shaping marine ecosystems is therefore likely related to their abundance, distribution and foraging strategies, as well as prey removal. ...
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    In this chapter we examine the biodiversity and the status of conservation and management of shark species in Australasia and Indonesia. Almost 17% of shark species in the region are listed by the International Union for the Conservation of Nature (IUCN) as threatened, and approximately 40% are of conservation concern, their future being dependent on the implementation of appropriate management strategies. Overfishing is a major threat to sharks, as their life-history strategies make them susceptible to even modest levels of fishing mortality. In Australia and New Zealand many shark stocks experienced dramatic declines as a consequence of overfishing; however, in the past few decades substantial improvements in the management of shark fisheries have taken place. On the other hand, shark fishing in Indonesia is largely unreported and unregulated and fishing by Indonesian vessels is likely to have consequences that go beyond the depletion of local populations, affecting shark populations in neighbouring countries such as Australia. We illustrate examples of over fishing in the region, discuss the potential effects of habitat degradation and climate change in the future and examine current management frameworks for the conservation of shark species in the region with an emphasis on the implementation of Nation Plans of Action for the Conservation and Management of Sharks (NPoAs).
  • ... The causes of annual variation in population size remain unclear for many species, and we are unaware of any previous studies that have assessed these causes in detail for lemon sharks. Furthermore, little is known regarding mortality rates of both the larger juveniles that have left the nursery site (ages [3][4][5][6][7][8][9][10][11] and mature adults (ages 12+) [36,37]. ...
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    Long-lived marine megavertebrates (e.g. sharks, turtles, mammals, and seabirds) are inherently vulnerable to anthropogenic mortality. Although some mathematical models have been applied successfully to manage these animals, more detailed treatments are often needed to assess potential drivers of population dynamics. In particular, factors such as age-structure, density-dependent feedbacks on reproduction, and demographic stochasticity are important for understanding population trends, but are often difficult to assess. Lemon sharks (Negaprion brevirostris) have apelagic adult phase that makes them logistically difficult to study. However, juveniles use coastal nursery areas where their densities can be high. We use a stage-structured, Markov-chain stochastic model to describe lemon shark population dynamics from a 17-year longitudinal dataset at a coastal nursery area at Bimini, Bahamas. We found that the interaction between delayed breeding, density-dependence, and demographic stochasticity accounts for 33 to 49% of the variance in population size. Demographic stochasticity contributed all random effects in this model, suggesting that the existence of unmodeled environmental factors may be driving the majority of interannual population fluctuations. In addition, we are able to use our model to estimate the natural mortality rate of older age classes of lemon sharks that are difficult to study. Further, we use our model to examine what effect the length of a time series plays on deciphering ecological patterns. We find that-even with a relatively long time series-our sampling still misses important rare events. Our approach can be used more broadly to infer population dynamics of other large vertebrates in which age structure anddemographic stochasticity are important.ReviewersThis article was reviewed by Yang Kuang, Christine Jacob, and Ollivier Hyrien.
  • ... The eco- logical implications of these declines have been diYcult to detect, but a range of theoretical and empirical studies sug- gest that in some cases their removal may have cascading eVects (reviewed in Ferretti et al. 2010). Compared to tele- osts, our knowledge of elasmobranch feeding habits and movement patterns lags considerably and impairs our abil- ity to understand their ecological roles and predict the con- sequences of their declines ( Heithaus et al. 2010). ...
    Article
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    We used variance decomposition to explore the importance of body size, sex, location, and sampling period as predictors of intrapopulation variation in 􏶉15N and 􏶉13C values in spiny dogWsh Squalus suckleyi from the Puget Sound–Strait of Georgia basin. Isotopes in two tissues with long (dorsal white muscle) and short (liver) isotopic turn- over rates (»1 year and »3–4 months, respectively) were sampled to evaluate whether the relative importance of each variable diVered depending on the time span over which diet information was integrated. SigniWcant spatial variation was observed in both muscle and liver isotopic composition, whereby location uniquely explained 25 and 17 % of the total variance, respectively. The remaining variables explained considerably less variation in both tis- sue types. Furthermore, evidence of seasonal isotopic shifts in 􏶉15N and 􏶉13C values was apparent, but diVered widely in direction and magnitude among groups. These Wndings suggest that members of spiny dogWsh schools may share a common feeding history, possibly by spending extended time periods (weeks to months) foraging in a spatially Wxed region. Another explanation is that individuals may move and feed in aggregations that exist for extended periods. These complex group-level patterns suggest that even for large-bodied, motile predators such as sharks, population- level diet estimates derived from averaging isotope ratios of individuals collected from only a few locations may poorly reXect the true population mean.
  • ... Changes in elasmobranch abundance may be indicative of or may be responses to ecosystem-level restructuring after the effects of fishing (Kitchell et al. 2002;Myers et al. 2007;Baum and Worm 2009); sharks may function as keystone predators and therefore would be essential to the maintenance and stability of food webs (Myers et al. 2007). In the high-latitude northeast Pacific Ocean, changes in the relative abundance of Pacific Sleeper Sharks could have both direct trophic effects on the ecosystem (Yang and Page 1999;Hulbert et al. 2006;Sigler et al. 2006;Yano et al. 2007;Courtney and Foy 2012;Mellish 2012, 2014) and indirect effects on the ecosystem, mediated through the behavioral responses of potential prey (Frid et al. 2006(Frid et al. , 2007a(Frid et al. , 2007b(Frid et al. , 2009Heithaus et al. 2008Heithaus et al. , 2010Wirsing et al. 2008;Kuker and Barrett-Lennard 2010). ...
    Article
    Monte Carlo simulation was used to investigate the sustainability of incidental exploitation rates (U) for Pacific Sleeper Sharks Somniosus pacificus in the Gulf of Alaska (GOA) under status quo management. Monte Carlo simulations were implemented with a standard, length-based, age-structured model that was evaluated with forward projection. Given the paucity of relevant data, we investigated the sensitivity of simulation results to a range of assumptions about key model parameters by using 24 alternative model configurations, each simulated 1,000 times. The risk analysis results were most sensitive to Pacific Sleeper Shark U-values. The aggregate fraction of simulations ending in an overfished condition increased from 0% under the low-U scenario to 59% under the high-U scenario. Risk analysis results were also sensitive to the assumed shape of the length-based selectivity curve (asymptotic or dome shaped) but were less sensitive to the range of assumptions about other key model parameters, including maximum age and stock productivity. These results indicate that a priority for Pacific Sleeper Shark management is to reduce the uncertainty in U. This major uncertainty will be decreased by an observer program that is now in place to monitor the historically unobserved GOA Pacific Halibut Hippoglossus stenolepis fishery, which incidentally catches Pacific Sleeper Sharks.Received March 19, 2015; accepted December 7, 2015
  • ... Elasmobranchii are highly skilled predators (Holmgren and Nilsson 1999;Lima et al. 2000;Xavier et al. 2012) being at the top of the marine food chain (Gadig 1998;Heithaus et al. 2008;Schwingel and Assunc ßão 2009;Xavier et al. 2012). They have an important role in population control (Camhi et al. 1998;Stevens et al. 2000;Xavier et al. 2012), connecting low and high trophic levels (Ferretti et al. 2010;Heithaus et al. 2010;Bornatowski et al. 2014). Therefore, fluctuations in Elasmobranch populations may lead to changes in communities at all trophic levels (Myers et al. 2007;Heithaus et al. 2010). ...
  • ... Currently, three-quarters of pelagic elasmobranchs are classified as Threatened or near Threatened (IUCN Red List Status), and 11 species are globally threatened with a high risk of extinction (Dulvy et al., 2008). Despite clear evidence for shark population declines, including in oceanic ecosystems (Baum et al., 2003;Myers and Worm, 2003;Ferretti et al., 2010), relatively little is known on the feeding ecology of many species and the ecological importance of this guild is poorly understood (Ferretti et al., 2010;Heithaus et al., 2010;Kitchell et al., 2002). Sharks can play important roles in marine ecosystems through diverse mechanisms, but their relative importance may vary significantly among ecosystems, species and contexts (Heithaus et al., 2008. ...
    Article
    Full-text available
    Sharks are a major component of the top predator guild in oceanic ecosystems, but the trophic relationships of many populations remain poorly understood. We examined chemical tracers of diet and habitat (δ15N and δ13C, respectively) and total mercury (Hg) concentrations in muscle tissue of seven pelagic sharks: blue shark (Prionace glauca), short-fin mako shark (Isurus oxyrinchus), oceanic whitetip shark (Carcharhinus longimanus), scalloped hammerhead shark (Sphyrna lewini), pelagic thresher shark (Alopias pelagicus), crocodile shark (Pseudocarcharias kamoharai) and silky shark (Carcharhinus falciformis), from the data poor south-western tropical Indian Ocean. Minimal interspecific variation in mean δ15N values and a large degree of isotopic niche overlap - driven by high intraspecific variation in δ15N values-was observed among pelagic sharks. Similarly, δ13C values of sharks overlapped considerably for all species with the exception of P. glauca, which had more 13C-depleted values indicating possibly longer residence times in purely pelagic waters. Geographic variation in δ13C, δ15N and Hg were observed for P. glauca and I. oxyrinchus. Mean Hg levels were similar among species with the exception of P. kamoharai which had significantly higher Hg concentrations likely related to mesopelagic feeding. Hg concentrations increased with body size in I. oxyrinchus, P. glauca and C. longimanus. Values of δ15N and δ13C varied with size only in P. glauca, suggesting ontogenetic shifts in diets or habitats. Together, isotopic data indicate that–with few exceptions-variance within species in trophic interactions or foraging habitats is greater than differentiation among pelagic sharks in the south-western Indian Ocean. Therefore, it is possible that this group exhibits some level of trophic redundancy, but further studies of diets and fine-scale habitat use are needed to fully test this hypothesis.
  • ... Life history traits that are characteristic of most batoids and other chondrichthyans lead to increased vulnerability of populations to depletion from overexploitation (Hoenig and Gruber 1990), particularly species with large maximum sizes (Dulvy et al. 2000(Dulvy et al. , 2014. As both mesopredators and prey that link upper and lower trophic levels, skates and rays may also play important ecological roles in the structure and dynamics of coastal ecosystems (e.g., Murawski 1991;Heithaus et al. 2010;Bornatowski et al. 2014). Thus, perturbations to coastal batoid populations may also impact the stability and productivity of co-occurring species of ecological and economical value. ...
    Article
    Full-text available
    Life history strategies of batoid fishes have evolved within dynamic marine ecosystems. Adaptations in reproductive and developmental biology are paramount to the survival of species, and therefore knowledge of growth rates to maturity is fundamental for identifying constraints on the conservation of populations. The butterfly rays (Myliobatiformes: Gymnuridae) are highly derived batoids with generally low reproductive potentials for which age and growth information remains unknown. In this study we applied high-resolution X-ray computed tomography (HRXCT) to vertebral centra from a stingray for the first time to estimate age, and used a multimodel approach to investigate growth of spiny butterfly ray, Gymnura altavela. Estimated ages of the oldest male and female were 11 and 18 yrs. at disk widths (WD) 1355 mm and 2150 mm, respectively. Disk width-at-age data were analyzed using three growth models (von Bertalanffy, logistic, Gompertz), and the most parsimonious and empirically supported model was the logistic function with sex treated as a fixed effect on asymptotic disk width (WD∞) and k parameters. Model parameter estimates were (males) WD∞ = 1285.46 ± 67.27 mm, k = 0.60 ± 0.10, and (females) WD∞ = 2173.51 ± 129.78 mm, k = 0.27 ± 0.04. Results indicated sexually dimorphic growth patterns, with males growing faster and reaching asymptotic size at earlier ages than females. These age and growth results are the first reported for the genus, and suggest that G. altavela grows at a similar rate as some teleosts and batoids, and relatively fast among chondrichthyans.
  • ... The study of elasmobranch feeding ecology and trophic interactions provides basic and warranted knowledge to understanding energy flow and applying such knowledge in an ecosystem-based fisheries management (Brodeur et al., 2017). Chondrichthyes fish play an important role in their communities (Heithaus et al., 2010). Some species are top predators with high trophic level (Cortés, 1999;Ebert & Bizzarro, 2007), whereas others are mesopredators with intermediate trophic levels and, in general, modifier of the benthic communities of marine ecosystems (Stevens et al., 2000;Myers et al., 2007). ...
    Article
    Full-text available
    A global analysis of the diet of chondrichthyes (23 species) was conducted during one bottom-trawl research cruise in the region where the highest catches of skates occurred off southwestern Atlantic. The diet was quantified by calculating the percentage frequency of occurrence of each prey. The influence of variables total length, sex, maturity stage, predator species, morphtype (sharks, skates, batoids), and depth on the diet was evaluated by fitting generalized linear models. The diet composition of chondrichthyes consisted mainly of crustaceans, polychaetes, and fish. The study region constituted a feeding ground for the majority of chondrichthyans species analyzed. The diet of chondrichthyes exhibited inter-specific differences sorted into three major trophic guilds and global ontogenetic shift. The consumption of crustaceans and polychaetes decreases with the total length of the chondrichthyes, whereas the consumption of fish increases as the predator grew in size. In addition, prey size of fish and crabs increased with increasing body size of predator, but large chondrichthyes also continued to feed on relatively small crabs. Trophic partitioning and ontogenetic diet shifts may act synergistically, favoring the coexistence of at least 23 species of chondrichthyes at all stages of their life histories, limiting the direct competition for food.
  • ... Batoids (rays and skates) are common coastal mesopredators, which can structure benthic communities and ecosystems through the removal of invertebrate prey and bioturbation (Peterson et al. 2001;Heithaus et al. 2010;Ajemian et al. 2012). Since stingrays are generally long-lived with low fecundity (Musick 1999;McEachran 2002;Dulvy et al. 2014), they are predicted to invest heavily in anti-predator behavior (see Werner 1998;Frid et al. 2012). ...
    Article
    Emerging conservation efforts for the world’s large predators may, if successful, restore natural predator–prey interactions. Marine reserves, where large predators tend to be relatively common, offer an experimental manipulation to investigate interactions between large-bodied marine predators and their prey. We hypothesized that southern stingrays—large, long-lived and highly interactive mesopredators—would invest in anti-predator behavior in marine reserves where predatory large sharks, the primary predator of stingrays, are more abundant. Specifically, we predicted southern stingrays in marine reserves would reduce the use of deep forereef habitats in the favor of shallow flats where the risk of shark encounters is lower. Baited remote underwater video was used to survey stingrays and reef sharks in flats and forereef habitats of two reserves and two fished sites in Belize. The interaction between “protection status” and “habitat” was the most important factor determining stingray presence. As predicted, southern stingrays spent more time interacting with baited remote underwater videos in the safer flats habitats, were more likely to have predator-inflicted damage inside reserves, and were less abundant in marine reserves but only in the forereef habitat. These results are consistent with a predation-sensitive habitat shift rather than southern stingray populations being reduced by direct predation from reef sharks. Our study provides evidence that roving predators can induce pronounced habitat shifts in prey that rely on crypsis and refuging, rather than active escape, in high-visibility, heterogeneous marine habitats. Given documented impacts of stingrays on benthic communities it is possible restoration of reef shark populations with reserves could induce reef ecosystem changes through behavior-mediated trophic cascades.
  • ... Top predators can influence the abundance and population structure of organisms at lower trophic levels through direct effects, such as predation mortality, and indirect interactions, such as eliciting risk averse behaviours of prey that have fitness consequences (Estes 1998;Frank 2005;Ainley 2007). In marine ecosystems, including the North Pacific, large sharks such as the salmon shark (Lamna ditropis) are generally considered to be positioned at a high trophic level (Wetherbee et al. 2004;Heithaus et al. 2010). Several studies have investigated the effects of top predators on the abundance of their prey despite the logistical challenges of studying large, highly mobile species (Estes 1998;Frank 2005;Ainley 2007;Navia et al. 2010;Bornatowski et al. 2014). ...
    Article
    Top predators, such as salmon sharks (Lamna ditropis), can influence the abundance and population structure of organisms at lower trophic levels through direct effects, such as predation mortality, and indirect interactions. As a first step towards better understanding the average annual prey consumption for individual adult salmon sharks, we bracketed consumption estimates using three methods: (1) daily ration requirement; (2) bioenergetic mass balance; and (3) a Bayesian model of shark growth. In the first method, we applied ration estimates for related lamnid shark species that yielded salmon shark estimates of 1461 and 2202 kg year⁻¹. The second method used a mass-balance technique to incorporate life history information from salmon sharks and physiological parameters from other species and produced estimates of 1870, 2070, 1610 and 1762 kg year⁻¹, depending on assumed diet. Growth modelling used salmon shark growth histories and yielded estimates of 16 900 or 20 800 kg year⁻¹, depending on assumed assimilation efficiency. Of the consumption estimates, those from the mass-balance technique may be the most realistic because they incorporated salmon shark life history data and do not produce extreme values. Taken as a whole, these estimates suggest that salmon sharks have similar energetic requirements to piscivorous marine mammals.
  • ... Around 39% of the coastal and continental chondrichthyans species listed in the IUCN Red List are categorized as Data Deficient, while 43.6% are considered Threatened . Thus, from a conservation point of view, it is crucial to understand the role these species play in coastal ecosystems (Heithaus et al. 2010). ...
    Article
    Full-text available
    Although chondrichthyans are conspicuously present in shallow waters, many ecological aspects of neritic species in the Humboldt Current System remain unknown. This study provides a first assessment of the diet of seven commercially exploited and understudied sympatric chondrichthyans inhabiting nearshore habitats off the central coast of Peru: four stingrays (Hypanus dipterurus, Myliobatis peruvianus, M. chilensis, and Urotrygon chilensis), a guitarfish (Pseudobatos planiceps), a smooth-hound shark (Mustelus mento), and a chimaera (Callorhinchus callorynchus). A total of 166 stomachs were examined between 2012 and 2015 and prey items were pooled for the total of years for analysis. Although our analysis did not account for inter seasons variability, our results suggest diet partitioning among species, except for the stingrays’ group. A diet based on soft-bottom polychaetes and fish was shared by H. dipterurus, M. peruvianus, and M. chilensis, while soft-bottom polychaetes and crabs were more important in U. chilensis. The smooth-hound shark and guitarfish exhibited a diet dominated by crabs, and the chimaera consumed mainly hard-bottom mollusks. Foraging habitat estimations distinguished two main habitats of association: Benthic, including the stingray U. chilensis, the chimaera, and the smooth-hound shark; and benthic-demersal, including the guitarfish, and the rest of stingrays. A pattern of feeding specialization was observed for H. dipterurus, P. planiceps, and C. callorynchus. Preliminary trophic level estimations based on diet composition placed these species as secondary consumers. Intraspecific dietary variation was assessed for P. planiceps and H. dipterurus as their sampled sizes allowed meaningful comparisons. The diet of P. planiceps varied from small to large sizes but not for H. dipterurus. No differences were detected on diet composition between males and females in either species. Despite the limited temporal resolution, this study provides the first insights of chondrichthyans predatory activity, suggesting diet partitioning among the species of this assemblage in a nearshore habitat of the central coast of Peru. Enhancing the temporal resolution of this type of studies would improve our knowledge on trophic functioning in the Humboldt Current ecosystem.
  • ... Some shark species have experienced population declines, mainly due to overfishing, bycatch, pollution and habitat degradation (Dulvy et al., 2008). Recent studies suggest that populations of large sharks have declined by 90% or more in some regions (Myers et al., 2007), making them one of the most threatened group of marine animals worldwide (Heithaus et al., 2010;Lucifora et al., 2011). The implementation of effective strategies for the conservation and management of sharks is often hampered by the lack of information regarding their diet, life history and behaviour (Shiffman et al., 2012). ...
    Article
    As apex predators, sharks are known to play an important role in marine food webs. Detailed information on their diet and trophic level is however needed to make clear inferences about their role in the ecosystem. A total of 335 stomachs of smooth hammerhead sharks, Sphyrna zygaena , were obtained from commercial fishing vessels operating in the Ecuadorian Pacific between January and December 2004. A total of 53 prey items were found in the stomachs. According to the Index of Relative Importance (%IRI), cephalopods were the main prey ( Dosidicus gigas, Sthenoteuthis oualaniensis, Ancistrocheirus lesueurii and Lolliguncula [Loliolopsis] diomedeae ). Sphyrna zygaena was thus confirmed to be a teutophagous species. The estimated trophic level of S. zygaena was between 4.6 and 5.1 (mean ± SD: 4.7 ± 0.16; males: 4.7; females: 4.8). Levin's index (B A ) was low (overall: 0.07; males: 0.08; females: 0.09), indicating a narrow trophic niche. We found that sharks <150 cm in total length consumed prey of coastal origin, whereas sharks ≥150 cm foraged in oceanic waters and near the continental shelf. The analyses indicate that S. zygaena is a specialized predator consuming mainly squids.
  • ... At a fine scale, the formation of feeding pits facilitates oxygen penetration into sediments, ex tending the zone of oxygenation (Gilbert et al. 1995) and affecting the nitrogen cycle (Kogure & Wada 2005). Bioturbation may also enable other species to benefit from prey items that are disturbed or excavated during foraging activities (VanBlaricom 1982, Heithaus et al. 2010. Kiszka et al. (2015) de tected the association of southern stingrays Hypanus americanus and bar jacks Caranx ruber, where stingray bioturbation allowed C. ruber to access re sources otherwise unavailable. ...
    Article
    Nursery areas are crucial for many elasmobranch species, providing benefits that increase fitness and survival. Shark nurseries are well studied and our knowledge of their function and importance has expanded over the past few decades. However, little attention has been given to batoid nurseries, with studies covering less than 6% of the 663 currently described species. Threats of extinction faced by batoids reinforce the importance of defining these critical habitats. This review synthesises current knowledge of batoid nursery areas to provide a better understanding of their ecological roles and importance. Historically, different criteria have been used to define viviparous and oviparous batoid nurseries, causing confusion that could lead to failure of conservation and management strategies by under- or overestimating the importance of areas and delaying effective action. We suggest the criteria used to identify shark nurseries be applied to juvenile batoids, standardizing this nursery definition for all elasmobranchs, but we also advocate for a second set of criteria that identifies egg case nurseries. Batoids are thought to play 3 main ecological roles in nursery areas: energetic links, bioturbators and mesopredators. Biotic and abiotic features affect abundance and distribution of batoids within nurseries and likely play a key role in their habitat use. However, analysis of batoid ecological roles in nursery areas is limited by the lack of research on their early life history stages. Thus, identification of areas that support sensitive life stages and an improved understanding of early life history are crucial for the efficient management and conservation of batoid species and their nurseries.
  • ... Declining populations of sharks in the tropics are of concern because of increasing evidence of their important trophic role ( Heithaus et al., 2010;Roff et al., 2016;Ruppert et al., 2013). The presence of sharks has been shown to affect the diet, condition and morphology of their prey ( Barley et al., 2017aBarley et al., , 2017bHammerschlag et al., 2018) and food chain structure ( Barley et al., 2017a). ...
    Article
    There is limited evidence on the rate at which the shark populations of coral reefs can rebound from over-exploitation, the baselines that might signify when recovery has occurred and the role of no-take Marine Protected Areas (MPA) in aiding this process. We surveyed shark assemblages at Ashmore Reef in Western Australia using baited remote underwater video stations in 2004 prior to enforcement of MPA status and then again in 2016 after eight years of strict enforcement. We found an increase in the relative mean abundance of Carcharhinus amblyrhynchos from 0.16 ± 0.06 individuals h−1 in 2004 to 0.74 ± 0.11 individuals h−1 in 2016, a change that was also accompanied by a shift in the assemblage of sharks to greater proportions of apex species (from 7.1% to 11.9%) and reef sharks (from 28.6% to 57.6%), and a decrease in the proportional abundance of lower trophic level species (from 64.3% to 30.5%). Abundances and trophic assemblage of sharks at Ashmore Reef in 2004 resembled those of the Scott Reefs, where targeted fishing for sharks still occurs, whereas in 2016, abundances and trophic structures had recovered to resemble those of the Rowley Shoals, a reef system that has been a strictly enforced MPA for over 25 years. The shift in abundance and community structure coincident with strict enforcement of the MPA at Ashmore Reef has occurred at a rate greater than predicted by demographic models, implying the action of compensatory processes in recovery. Our study shows that shark communities can recover rapidly after exploitation in a well-managed no-take MPA.
  • ... Being top predators makes them important in controlling lower trophic levels (Stevens et al., 2000;Myers et al., 2007;Heithaus et al., 2008;Navia et al., 2010). Thus, the reduction of their populations may result, through trophic cascade effects, in changes in marine populations (Ferretti et al., 2010;Heithaus et al., 2010). ...
    Article
    Full-text available
    This study presents information on the diet of two shark species, Carcharhinus leucas and Galeocerdo cuvier that inhabit the southeastern Pacific Ocean. The stomachs were collected from October 2003 to July 2005 in Ecuador. Stomachs of 41 C. leucas and six G. cuvier were analyzed. According to the index of relative importance (IRI), the most important prey for C. leucas were fishes: Ophichthidae family (13.41), Tylosurus pacificus (9.79), Katsuwonus pelamis (4.54) and fish remains (44.81). G. cuvier, for its part, consumed squids: Ancistrocheirus lesueuri (45.14), Pholidoteuthis boschmaii (7.81) and Octopoteuthis spp. (5.17), as well as turtles: Caretta caretta (9.7), Lepidochelys cf. kempii (5) and turtle remains (16.5). Results show that C. leucas (trophic level, ITR; 4.32 ± 0.13) and G. cuvier (ITR; 4.26 ± 0.09) are tertiary consumers, occupying high positions in the food chain, but also generalist predators that feed on a variety of prey. The high frequency of sea turtles in the stomachs of G. cuvier (> 300 cm) suggests that this shark species is an important predator of turtles, which are commonly found along the southeastern Pacific coasts
  • ... In particular, changes in shark abundance are most likely to cause ecosystem effects when: the shark species is preying upon or inducing antipredatory behavior in longer-lived species, the shark species of concern is the primary predator for a limited number of prey species, the shark species preys upon a keystone or high trophic-level species, the shark species alters community structure, the shark species preys on a species during a life history stage where density dependence occurs, or there are no other predators of the same trophic level present in the ecosystem ). It has also been proposed that sharks (and other apex predatory species) have a stabilizing effect on their ecosystem Heithaus et al. 2010). ...
    Thesis
    [PREAMBLE: For complete and updated results, please refer to the following publications: (1) Peterson et al. (2017) Fish and Fisheries. 18(5): 845-859.; (2) Peterson et al. (2017) Marine Ecology Progress Series. 579: 81-96.] Broad scale analyses of shark population and community dynamics are particularly challenging given the complex life history strategies employed and their vast migratory patterns. Consequently, studies are generally limited to analyzing small-scale, localized dynamics that can be examined from easily accessible, nearshore environments. In particular, fishery-independent shark surveys are frequently limited by spatial political boundaries, such that they only sample a discrete portion of a migratory coastal shark’s distribution. Given the age- and sex-structured movements of these species, a localized survey is likely unable to represent stock-wide changes in abundance, such that several small ranging surveys are treated as independent measures of abundance. Survey-based trends in abundance frequently display data conflict, likely due to high levels of uncertainty and variable timing in migrations. Similarly, sharks within communities interact, with the capacity of one species to alter the population size and growth rate of another species. However, these interactions have never been assessed at a wide geographic scale. In the current thesis, I used generalized linear models (GLMs) to estimate annual indices of abundance from eight species of Atlantic coastal sharks from six fishery-independent surveys along the U.S. east coast and within the Gulf of Mexico. These conflicting indices of abundance were input into a dynamic factor analysis (DFA) model with large-scale climatic indices and anthropogenic forces as covariates to produce simplified species-specific trends of abundance for each species throughout the sampled distribution. These common trends were then input into a multivariate, first-order autoregressive, state-space (MARSS-1) model to estimate interspecies interactions and density dependence. These broad-scale interactions were compared to localized interactions generated from conducting MARSS-1 analyses on GLM-based indices of abundance calculated from individual surveys. Resulting DFA common trends suggested that large coastal species followed similar patterns of abundance since 1975, where abundance was high at the beginning of the time series, declined into the early 1990s, was depressed for a length of time corresponding to age at maturity, and then showed initial signs of rebounding. The small coastal species showed more regional variability in abundance, likely due to separate Atlantic and Gulf of Mexico stocks for several of these species. Broad-scale community analysis results showed that seven out of ten coastal shark populations exhibited density dependence, and an additional seven interspecies interactions were identified that significantly influence the population growth rate of affected species. The localized, survey-specific MARSS-1 modeling results produced different results, suggesting that small scale results cannot be extrapolated across the entire stock. Nevertheless, results from these survey-specific models greatly assisted interpretation of the large scale results. Overall, by analyzing coastal shark population and community dynamics from a broader perspective, we can interpret broad trends in abundance and account for interactions that were previously unknown. These results may assist in assessment efforts by reducing conflicting information input into stock assessment models, and accounting for community relationships that may affect population growth rate of various species.
  • ... Diet and trophic ecology provide insight into predator-prey interactions: community linkages, food web dynamics, and energy transfer in marine environments (Heithaus et al., 2010;Vaudo and Heithaus, 2011;Wetherbee et al., 2012; see chapter "Diet composition and trophic ecology of Northeast Pacific sharks" by Bizzarro et al., this volume). Diet is typically assessed through analysing stomach contents, but this method only provides short-term resolution (Cort es, 1999;Hyslop, 1980;Peterson and Fry, 1987). ...
    Chapter
    Full-text available
    The sharks, batoids, and chimaeras, collectively the class Chondrichthyes, are one of the most successful groups of fishes, with over 1250 species globally. Recent taxonomic revisions have increased their diversity by about 20% over the past 17 years (2000–2016). The Northeast Pacific Ocean is one of the top 20 most diverse regions/ countries on the globe with 77 chondrichthyan species, a number less than a quarter that of the most species-rich area (Australia) but that has increased by 10% since 2000 to include three new species (two skates and a chimaera). In this chapter we discuss the species richness of chondrichthyans occurring in the Northeast Pacific Ocean, characterize their life histories, briefly review several fisheries, and summarize the conservation status of those chondrichthyans occurring in the region. Detailed descriptions and evaluations of fisheries can be found in Chapter 7 of AMB Volume 78.
  • ... Though the influence of sharks can be critical in shaping population and community dynamics (Ferretti et al., 2010;Heithaus et al., 2008Heithaus et al., , 2010Heupel et al., 2014), their ecological roles are complex and often plastic. Diets will shift across life spans, with sharks targeting different types of prey or trophic positions through ontogeny (see Section 3). ...
    Chapter
    Full-text available
    Although there is a general perception of sharks as large pelagic, apex predators, most sharks are smaller, meso- and upper-trophic level predators that are associated with the seafloor. Among 73 shark species documented in the eastern North Pacific (ENP), less than half reach maximum lengths >. 200. cm, and 78% occur in demersal or benthic regions of the continental shelf or slope. Most small (≤. 200. cm) species (e.g., houndsharks) and demersal, nearshore juveniles of larger species (e.g., requiem sharks) consume small teleosts and decapod crustaceans, whereas large species in pelagic coastal and oceanic environments feed on large teleosts and squids. Several large, pelagic apex predator species occur in the ENP, but the largest species (i.e., Basking Shark, Whale Shark) consume zooplankton or small nekton. Size-based dietary variability is substantial for many species, and segregation of juvenile and adult foraging habitats also is common (e.g., Horn Shark, Shortfin Mako). Temporal dietary differences are most pronounced for temperate, nearshore species with wide size ranges, and least pronounced for smaller species in extreme latitudes and deep-water regions. Sympatric sharks often occupy various trophic positions, with resource overlap differing by space and time and some sharks serving as prey to other species. Most coastal species remain in the same general region over time and feed opportunistically on variable prey inputs (e.g., season migrations, spawning, or recruitment events), whereas pelagic, oceanic species actively seek hot spots of prey abundance that are spatiotemporally variable. The influence of sharks on ecosystem structure and regulation has been downplayed compared to that of large teleosts species with higher per capita consumption rates (e.g., tunas, billfishes). However, sharks also exert indirect influences on prey populations by causing behavioural changes that may result in restricted ranges and reduced fitness. Except for food web modelling efforts in Alaskan waters, the trophic impacts of sharks are poorly incorporated into current ecosystem approaches to fisheries management in the NEP.
  • ... LPP occupy high positions in marine food webs and therefore act as important regulators of lower trophic level species, playing a fundamental role in marine ecosystems through direct (predation) and indirect (competition) ecological interactions (Heithaus et al. 2008;Ferretti et al. 2010;Bornatowski et al. 2014a;Navia et al. 2016). These organisms could exert topdown effects on marine food webs and their reductions could alter the ecosystem structure and functioning (Myers et al. 2007;Baum and Worm 2009;Ritchie and Johnson 2009;Heithaus et al. 2010;Ferretti et al. 2010). ...
    Article
    Large pelagic predators occupy high positions in food webs and could control lower trophic level species by direct and indirect ecological interactions. In this study we aimed to test the hypotheses: (1) pelagic predators are keystone species, and their removals could trigger impacts on the food chain; (2) higher landings of pelagic predators could trigger fishing impacts with time leading to a drop in the mean trophic level of catches; and (3) recovery in the pelagic predators populations, especially for sharks, could be achieved with fishing effort reduction. We performed a food web approach using an Ecopath with Ecosim model to represent the Southeastern and Southern Brazil, a subtropical marine ecosystem, in 2001. We then calibrated the baseline model using catch and fishing effort time series from 2001 to 2012. Afterwards, we simulated the impact of fishing effort changes on species and assessed the ecological impacts on the pelagic community from 2012 to 2025. Results showed that the model was well fitted to landing data for the majority of groups. The pelagic predators species were classified as keystone species impacting mainly on pelagic community. The ecosystem was resilient and fisheries seem sustainable at that time. However, the temporal simulation, from 2001 to 2012, revealed declines in the biomass of three sharks, tuna and billfish groups. It was possible observe declines in the mean trophic level of the catch and in the mean total length of landings. Longline fisheries particularly affected the sharks, billfish and swordfish, while hammerhead sharks were mostly impacted by gillnet fishery. Model simulations showed that large sharks’ biomasses could be recovered or maintained only after strong fishing effort reduction.
  • ... Large sharks as top predators can influence lower trophic levels in the food chain through direct and indirect effects (Stevens et al., 2000;Myers et al., 2007;Heithaus et al., 2008;Navia et al., 2010). In this sense, reduction of top predator populations can result in community changes through cascading effects (Ferretti et al., 2010;Heithaus et al., 2010). So, determining the real influence of large sharks in subtropical food webs requires not only an understanding of diet and trophic relationships of sharks, but also the status of elasmobranch populations. ...
    Article
    The diets of six shark species, Sphyrna lewini, Sphyrna zygaena, Carcharhinus obscurus, Carcharhinus limbatus, Rhizoprionodon lalandii and Galeocerdo cuvier, were investigated in a subtropical coastal ecosystem of southern Brazil. Stomach content data were obtained to assess foraging niche segregation and ontogenetic shifts in the diets of these sharks. Five of the shark species off the Paraná coast were ichthyophagous, with the exception of S. zygaena, which was teutophagous. With the exception of G. cuvier, which had a generalist diet, the other five species displayed specialization in their feeding. Ontogenetic shifts were observed in C. obscurus and S. lewini with large individuals consuming elasmobranchs. Owing to the diet overlap between C. obscurus and S. lewini, C. obscurus and C. limbatus and R. lalandii and C. limbatus, future studies on the spatial and temporal distributions of these species are needed to understand the extent of competitive interactions.
  • ... 'elasmobranch, ' Lowe et al. 1996, Simpfendorfer et al. 2001'unidentified teleosts,' Bethea et al. 2004), such that inferring specific predator−prey interactions is challenging (Grubbs et al. 2016). Furthermore, diet composition does not directly reflect the population-level importance of interactions between predator and prey species (Heithaus et al. 2010). ...
    Article
    Studies aiming to assess intra- and interspecies community relationships in marine habitats are typically limited to accessible, nearshore areas of restricted temporal and spatial scale, within which only segments of the populations of interest are available. Using multivariate first-order auto regressive state-space (MARSS-1) models, we estimated measures of interspecies interactions and density dependence of 7 Atlantic coastal shark species (4 large and 3 small coastal sharks) at 2 spatial scales. Localized analyses were based on data from 4 relatively spatially limited, fishery-independent surveys conducted along the southeast US Atlantic coast and within the Gulf of Mexico. We then compared these localized results to those generated using broad-scale indices of relative abundance estimated as common trends across the collection of 6 spatially restricted surveys. The MARSS-1 framework was also used to estimate relative community stability. Localized MARSS-1 analyses identified density-dependent compensation in all populations in addition to 9 interspecies interactions, while results of broad-scale MARSS-1 analyses revealed density dependence in 5 species and 9 interspecies interactions. More specifically, our results support the manifestation of density-dependent compensation of neonate and juvenile shark life stages within nursery areas. Overall, interactions within smaller spatial areas differed from those identified using the broad-scale relative abundance trends, indicating that small-scale interactions cannot be extrapolated to shark population growth rates of an entire stock.
  • ... Predators like E. morio and M. bonaci could trigger indirect effects in the community (e.g. Heithaus et al., 2008Heithaus et al., , 2010Ferretti et al., 2010) and influence a large range of ecological processes (Babcock et al., 1999;Pinnegar et al., 2000;Willis, Anderson, 2003;Silveira et al., 2015), such as linkages between top and intermediate predators, and intermediate predators and their resources (Pace et al., 1999). For example, the high consumption of scarines, by both M. bonaci and E. morio, illustrate how these groupers could have a controlling role in the abundance of parrotfishes, which in turn play key roles as grazers and sand producers on coral reefs. ...
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    Red and black groupers are large-bodied opportunistic ambush predators commonly found in Southwestern Atlantic tropical reefs. We investigated the diet of both species in order to detail ontogenetic, spatial and temporal trends, and to assess the extent of overlap in resource use between these two sympatric predators on the Abrolhos Bank, Brazil. Decapods and fishes were the main food items of Epinephelus morio while fishes were the main prey of Mycteroperca bonaci. Both diets were significantly influenced by body size and habitat, but only smaller individuals of E. morio feed almost exclusively on crustaceans. While the two groupers rely on many of the same prey types, coexistence may be facilitated by E. morio feeding more heavily on crustaceans, particularly the blackpoint sculling crab Cronius ruber, while black grouper take comparatively few crustaceans but lots of fish prey. Predators like red and black groupers could trigger indirect effects in the community and influence a large range of ecological processes, such as linkages between top and intermediate predators, and intermediate predators and their resources.
  • ... The ecological consequences resultant of niche partitioning among sharks within lagoon food webs, however, suggest cooccurrence may play a large part in shaping the ecological roles blacktip reef sharks and lemon sharks play in Moorea. Sharks serve as predators within their respective ecosystems (Heithaus et al., 2010). However, variability in food web structure, including the presence of competitors, can lead to variability in species' trophic interactions (Paine, 1966;Polis and Strong, 1996). ...
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    Food web structure is shaped by interactions within and across trophic levels. As such, understanding how the presence and absence of predators, prey, and competitors affect species foraging patterns is important for predicting the consequences of changes in species abundances, distributions, and behaviors. Here, we used plasma δ¹³C and δ¹⁵N values from juvenile blacktip reef sharks (Carcharhinus melanopterus) and juvenile sicklefin lemon sharks (Negaprion acutidens) to investigate how species co-occurrence affects their trophic interactions in littoral waters of Moorea, French Polynesia. Co-occurrence led to isotopic niche partitioning among sharks within nurseries, with significant increases in δ¹⁵N values among sicklefin lemon sharks, and significant decreases in δ¹⁵N among blacktip reef sharks. Niche segregation likely promotes coexistence of these two predators during early years of growth and development, but data do not suggest coexistence affects life history traits, such as body size, body condition, and ontogenetic niche shifts. Plasticity in trophic niches among juvenile blacktip reef sharks and sicklefin lemon sharks also suggests these predators are able to account for changes in community structure, resource availability, and intra-guild competition, and may fill similar functional roles in the absence of the other species, which is important as environmental change and human impacts persist in coral reef ecosystems.
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    Knowledge of the trophic ecology and interactions of marine top predators is fundamental for understanding community structure and dynamics as well as ecosystem function. We examined the feeding relationships of 4 heavily exploited elasmobranchs caught in coastal artisanal shark fisheries in south-western Madagascar in 2009 and 2010-Sphyrna lewini, Loxodon macrorhinus, Carcharhinus falciformis and Rhynchobatus djiddensis-using stable isotope (delta N-15 and delta C-13) analysis. Relative trophic position (indicated by delta N-15) and foraging location (indicated by delta C-13) differed among species. Isotopic niche width was highly variable: more pelagic species, such as S. lewini and C. falciformis, had the broadest isotopic niches while the benthic R. djiddensis had the narrowest. A high percentage of niche overlap occurred between R. djiddensis and 2 of the species, C. falciformis (93.2%) and L. macrorhinus (73.2%), and to a lesser extent S. lewini (13.3%). Relative trophic position of S. lewini significantly increased with size, suggesting a dietary shift with age. Sex differences in delta N-15 values were observed in L. macrorhinus, suggesting intraspecific niche partitioning. Variation in stable isotope values among these 4 highly exploited elasmobranch species indicates trophic structuring, likely driven by differences in diet and habitat use as well as by size and sex. This study provides the first baseline information on the trophic ecology of elasmobranchs caught in artisanal fisheries from south-western Madagascar.
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    Large‐bodied predators are well represented among the world's threatened and endangered species. A significant body of literature shows that in terrestrial and marine ecosystems large predators can play important roles in ecosystem structure and functioning. By contrast, the ecological roles and importance of large predators within freshwater ecosystems are poorly understood, constraining the design and implementation of optimal conservation strategies for freshwater ecosystems. Conservationists and environmentalists frequently promulgate ecological roles that crocodylians are assumed to fulfil, but often with limited evidence supporting those claims. Here, we review the available information on the ecological importance of crocodylians, a widely distributed group of predominantly freshwater‐dwelling, large‐bodied predators. We synthesise information regarding the role of crocodylians under five criteria within the context of modern ecological concepts: as indicators of ecological health, as ecosystem engineers, apex predators, keystone species, and as contributors to nutrient and energy translocation across ecosystems. Some crocodylians play a role as indicators of ecosystem health, but this is largely untested across the order Crocodylia. By contrast, the role of crocodylian activities in ecosystem engineering is largely anecdotal, and information supporting their assumed role as apex predators is currently limited to only a few species. Whether crocodylians contribute significantly to nutrient and energy translocation through cross‐ecosystem movements is unknown. We conclude that most claims regarding the importance of crocodylians as apex predators, keystone species, ecosystem engineers, and as contributors to nutrient and energy translocation across ecosystems are mostly unsubstantiated speculation, drawn from anecdotal observations made during research carried out primarily for other purposes. There is a paucity of biological research targeted directly at: understanding population dynamics; trophic interactions within their ecological communities; and quantifying the short‐ and long‐term ecological impacts of crocodylian population declines, extirpations, and recoveries. Conservation practices ideally need evidence‐based planning, decision making and justification. Addressing the knowledge gaps identified here will be important for achieving effective conservation of crocodylians.
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    Marine mammals feed at a variety of trophic levels, occur from freshwater to open-ocean ecosystems, and are found across virtually all latitudes. Due to their high historical – and sometimes present-day - abundances, capability for large-scale movements and high metabolic rates, they have the potential to affect the structure and function of ecosystems through a variety of mechanisms over both ecological and evolutionary time. Usually, the effects of marine mammals on ecosystems are explicitly or implicitly considered to occur through their ability to remove prey through direct predation. Recent empirical studies and a rich theoretical framework, however, demonstrate marine mammals can affect ecosystems through more diverse pathways, including those that are driven by marine mammal behaviour. Thus, non-consumptive effects of and on marine mammals may be critical in shaping their ecological importance. Non-consumptive effects may include risk effects, whereby predators induce costly changes to prey behaviour, that impact prey population sizes or the magnitude and spatiotemporal patterns of prey impacts on communities (e.g. behaviour-mediated trophic cascades; BMTC). Changes in the abundance of large apex predators and the introduction of perceived and real risks (human disturbance) may also affect behaviours of marine mammals and their prey that cascade to the wider ecosystem; the conditions under which such cascading effects might be most important, however, remain poorly understood. Other behaviour-driven ecological roles of marine mammals may include foraging tactics that facilitate the foraging of other species (especially seabirds), the translocation of nutrients and linking the dynamics of spatially distinct food webs.
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    Knowledge of movements and habitat use is necessary to assess a species' ecological role and is especially important for mesopredators because they provide the link between upper and lower trophic levels. Using acoustic telemetry, we examined coarse-scale diel and seasonal movements of elasmobranch mesopredators on a shallow sandflat in Shark Bay, Western Australia. Giant shovelnose rays (Glaucostegus typus) and reticulate whiprays (Himantura uarnak) were most often detected in nearshore microhabitats and were regularly detected throughout the day and year, although reticulate whiprays tended to frequent the monitored array over longer periods. Pink whiprays (H. fai) and cowtail stingrays (Pastinachus atrus) were also detected throughout the day, but were far less frequently detected. Overall, there was no apparent spatial or temporal partitioning of the sandflats, but residency to the area varied between species. In addition, ray presence throughout the year suggests that previously observed differences in seasonal abundance are likely because of seasonal changes in habitat use rather than large-scale migrations. Continuous use of the sandflats and limited movements within this ray community suggests that rays have the potential to be a structuring force on this system and that focusing on nearshore habitats is important for managing subtropical ray populations.
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    Ontogenetic niche shifts are common among animals, yet most studies only investigate niche shifts at the population level, which may overlook considerable differences among individuals in the timing and dynamics of these shifts. Such divergent behaviors within size-/age-classes have important implications for the roles a population-and specific age-classes-play in their respective ecosystem(s). Using acoustic telemetry, we tracked the movements of juvenile bull sharks in the Shark River Estuary of Everglades National Park, Florida, and found that sharks increased their use of marine microhabitats with age to take advantage of more abundant resources, but continued to use freshwater and estuarine microhabitats as refuges from marine predators. Within this population-level ontogenetic niche shift, however, movement patterns varied among individual sharks, with 47 % of sharks exhibiting condition-dependent habitat use and 53 % appearing risk-averse regardless of body condition. Among sharks older than age 0, fifty percent made regular movements between adjacent regions of the estuary, while the other half made less predictable movements that often featured long-term residence in specific regions. Individual differences were apparently shaped by both intrinsic and extrinsic factors, including individual responses to food-risk trade-offs and body condition. These differences appear to develop early in the lives of bull sharks, and persist throughout their residencies in nursery habitats. The widespread occurrence of intraspecific variation in behavior among mobile taxa suggests it is important in shaping population dynamics of at least some species, and elucidating the contexts and timing in which it develops and persists is important for understanding its role within communities.
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    There is no conclusive evidence of any nonhuman animal using the sun as part of its predation strategy. Here, we show that the world’s largest predatory fish—the white shark (Carcharodon carcharias)—exploits the sun when approaching baits by positioning the sun directly behind them. On sunny days, sharks reversed their direction of approach along an east-west axis from morning to afternoon but had uniformly distributed approach directions during overcast conditions. These results show that white sharks have sufficient behavioral flexibility to exploit fluctuating environmental features when predating. This sun-tracking predation strategy has a number of potential functional roles, including improvement of prey detection, avoidance of retinal overstimulation, and predator concealment.
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    Vertical movements of shortfin mako sharks Isurus oxyrinchus in the western North Atlantic Ocean are strongly influenced by temperature Jeremy J. Vaudo*, Bradley M. Wetherbee, Anthony D. Wood, Kevin Weng, Lucy A. Howey-Jordan, Guy M. Harvey, Mahmood S. Shivji *Email: jvaudo@nova.edu ABSTRACT: Although shortfin mako sharks Isurus oxyrinchus are regularly encountered in pelagic fisheries, limited information is available on their vertical distribution and is primarily restricted to cooler areas of their distribution. We investigated the vertical movements of mako sharks across differing temperature regimes within the western North Atlantic by tagging 8 individuals with pop-up satellite archival tags off the northeastern United States and the Yucatan Peninsula, Mexico. Depth and temperature records across 587 d showed vertical movements strongly associated with ocean temperature. Temperatures <15° C created a lower depth limit to most diving behaviors, and shifts in depths used coincided with changes in the thermal properties of the vertical habitat. In the warmest water columns, sharks spent 36% of the daytime at depths >150 m compared to only 1% in the coldest water columns. The sharks showed diel diving behavior, with deeper dives occurring primarily during the daytime (maximum depth: 866 m). Overall, sharks experienced temperatures between 5.2 and 31.1°C. When the opportunity was available, sharks spent considerable time in waters ranging from 22 to 27° C, indicating underestimation of the previously reported upper limit of the mako sharks’ preferred temperature. The preference for higher temperatures does not support endothermy as an adaption for niche expansion in mako sharks. The strong influence of thermal habitat on movement behavior suggests potentially strong impacts of rising ocean temperatures on the ecology of this highly migratory top predator.
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    Tiger sharks were sampled off the western (Ningaloo Reef, Shark Bay) and eastern (the Great Barrier Reef; GBR, Queensland and New South Wales; NSW) coastlines of Australia. Multiple tissues were collected from each shark to investigate the effects of location, size and sex of sharks on δ¹³C and δ¹⁵N stable isotopes among these locations. Isotopic composition of sharks sampled in reef and seagrass habitats (Shark Bay, GBR) reflected seagrass-based food-webs, whereas at Ningaloo Reef analysis revealed a dietary transition between pelagic and seagrass food-webs. In temperate habitats off southern Queensland and NSW coasts, shark diets relied on pelagic food-webs. Tiger sharks occupied roles at the top of food-webs at Shark Bay and on the GBR, but not at Ningaloo Reef or off the coast of NSW. Composition of δ¹³C in tissues was influenced by body size and sex of sharks, in addition to residency and diet stability. This variability in stable isotopic composition of tissues is likely to be a result of adaptive foraging strategies that allow these sharks to exploit multiple shelf and offshore habitats. The trophic role of tiger sharks is therefore both context- and habitat-dependent, consistent with a generalist, opportunistic diet at the population level.
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    Apex predators often have strong top-down effects on ecosystem components and are therefore a priority for conservation and management. Due to their large size and conspicuous predatory behaviour, reef sharks are typically assumed to be apex predators, but their functional role is yet to be confirmed. In this study, we used stomach contents and stable isotopes to estimate diet, trophic position and carbon sources for three common species of reef shark (Triaenodon obesus, Carcharhinus melanopterus and C. amblyrhynchos) from the Great Barrier Reef (Australia) and evaluated their assumed functional role as apex predators by qualitative and quantitative comparisons with other sharks and large predatory fishes. We found that reef sharks do not occupy the apex of coral reef food chains, but instead have functional roles similar to those of large predatory fishes such as snappers, emperors and groupers, which are typically regarded as high-level mesopredators. We hypothesise that a degree of functional redundancy exists within this guild of predators, potentially explaining why shark-induced trophic cascades are rare or subtle in coral reef ecosystems. We also found that reef sharks participate in multiple food webs (pelagic and benthic) and are sustained by multiple sources of primary production. We conclude that large conspicuous predators, be they elasmobranchs or any other taxon, should not axiomatically be regarded as apex predators without thorough analysis of their diet. In the case of reef sharks, our dietary analyses suggest they should be reassigned to an alternative trophic group such as high-level mesopredators. This change will facilitate improved understanding of how reef communities function and how removal of predators (e.g., via fishing) might affect ecosystem properties.
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    In northeastern Alberta, Canada, continued expansion of the oil and gas industry along with timber harvesting has raised concerns that the resulting environmental changes may negatively affect the woodland caribou (Rangifer tarandus caribou) population in this region. Caribou are a threatened species in Alberta, and populations in northeastern Alberta appear to be stable or slightly decreasing. The spatial distribution of caribou in relation to alternative prey (commonly moose [Alces alces]) has been hypothesized to affect the level of wolf (Canis lupus) predation on caribou populations. We monitored radiomarked caribou, moose, and wolves between 1993 and 1997, and we found that selection of fen/bog complexes by caribou and selection of well-drained habitats by moose and wolves resulted in spatial separation. This spatial separation in turn reduced wolf predation pressure on caribou but did not provide a total refuge from wolves. Any management activities that increase the density of moose and wolves or increase access of wolves into fen/bog complexes will likely reduce the refuge effect provided by large fen/bog complexes.
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    Extrapolating ecological processes from small‐scale experimental systems to scales of natural populations usually entails a considerable increase in spatial heterogeneity, which may affect process rates and, ultimately, population dynamics. We demonstrate how information on the heterogeneity of natural populations can be taken into account when scaling up laboratory‐derived process functions, using the technique of moment approximation. We apply moment approximation to a benthic crustacean predator‐prey system, where a laboratory‐derived functional response is made spatial by including correction terms for the variance in prey density and the covariance between prey and predator densities observed in the field. We also show how moment approximation may be used to incorporate spatial information into a dynamic model of the system. While the nonspatial model predicts stable dynamics, its spatial equivalent also produces bounded fluctuations, in agreement with observed dynamics. A detailed analysis shows that predator‐prey covariance, but not prey variance, destabilizes the dynamics. We conclude that second‐order moment approximation may provide a useful technique for including spatial information in population models. The main advantage of the method is its conceptual value: by providing explicit estimates of variance and covariance effects, it offers the possibility of understanding how heterogeneity affects ecological processes.
  • Chapter
    Productivity is a major factor affecting food web and ecosystem dynamics in natural systems (Slobodkin, 1960, 1962; Odum, 1969; Fretwell, 1977, 1987; Oksanen et al., 1981, this volume). Productivity can influence aspects of food weds like food chain length, stability, interaction strength, and species diversity (Rosenzweig, 1971; Oksanen et al., 1981; DeAngelis et al., 1989a, 1989b; DeAngelis, 1992; Moore et al., 1993; and Abrams and Roth, 1994a, 1994b). Among these, food chain length has been the most discussed. Food chain length has been suggested to lengthen with productivity because trophic transfers from resources to consumers entail losses to heat and waste. Therefore more trophic levels (longer food chains) should be supported if the web receives more energy or limiting materials or if trophic transfers are more efficient. These classical trophic transfer arguments posed by Elton (1927) and developed by Hutchinson (1959) and Slobodkin (1960) are basic to productivity-based food chain models (Oksanen et al., 1981; Fretwell, 1987). The actual support for the argument that energetic constraints limit food chain length in natural systems is, however, open to debate (Pimm and Kitching, 1987; Oksanen, 1988; Lawton, 1989; Pimm, 1991; Persson et al. , 1992; Hairston and Hairston, 1993; Wootton and Power, 1993).
  • Article
    Experimental determination of sediment reworking rates in a subtropical intertidal flat environment yielded information about the amount, nature and implications of sediment reworking in nearshore deposits. Callianassid shrimp in Bahía La Choya, Sonora, Mexico, overturn the sediment in the inner flats at an average rate of 0.56 m3/m2/year. Elasmobranch rays overturn the sediment in the midflats at an average rate of 1.01 m3/m2/year. Resin castings indicate that the shrimp are capable of burrowing to a depth of at least 1.15 m and, where present, can completely rework this interval in Bahía La Choya in two years. The rays reach a maximum observed burrowing depth of 20 cm and, where present, can completely rework this interval in Bahía La Choya in 72 days. Reworking rates are high enough to preclude the preservation of most physical sedimentary structures under normal conditions. Only large-scale sedimentary structures or those buried deeply and rapidly are likely to escape reworking. Rates of biogenic sedimentation by callianassid shrimp are high enough to generate subsurface shell beds. Short-term biogenic sedimentation rates are higher than long-term rates, indicating that such intertidal sediments are not only thoroughly reworked, but are incomplete at time scales of weeks to months.
  • Article
    Ages were estimated for 115 of 899 cownose rays, Rhinoptera bonasus, collected primarily from commercial fishing gear, in lower Chesapeake Bay and vicinity from May through October, 1976-78. Age determinations were made using sectioned vertebral centra and estimates of von Bertalanffy parameters were for males DW∞=119.2, K=0.126, and t0=-3.699, and for females DW∞=125.0, K=0.119, and t0=-3.764. Females attained a larger adult size and the oldest specimen aged was a female 13 years old and 107 cm disc width. Both sexes mature after reaching about 70% of their maximum size and ages at maturity were estimated at 5 to 6 years for males and 7 to 8 years for females. In spring migrating rays schooled by size; they arrived along the North Carolina coast by April and entered Chesapeake Bay by early May. Rays were abundant in the major Virginia tributaries of Chesapeake Bay throughout summer and occurred in salinities as low as 8‰ and at water temperatures between 15-29 °C. Size segregation continued during summer and adults schooled by sex. Most rays left Chesapeake Bay by early October.
  • Article
    Study of the flux and fate of reactive organic material (OM) within Debidue Flat, an intertidal sandflat in the North Inlet estuary, South Carolina, demonstrated that this coarse-grained deposit is a dynamic, open system that experiences rapid OM decomposition and exchange of solutes in the top 30 cm of the sediment column. The fluxes of reactive OM through Debidue Flat were high during all seasons (27-170 mmol C m2 d-1) and were comparable to fluxes in muddy portions of the North Inlet estuary. Porewater decomposition products were N- and P-rich, the modeled reactivity of organic carbon undergoing decomposition was high (first-order rate constant, k = 0.02 d-1), and abundant extractable chlorophyll a was measured year-round; all properties were consistent with marine algal-derived substrates. Porewater solute profiles were controlled by advective flow that rapidly exchanged porewater with overlying waters to ∼25 cm depth on timescales of hours. Thus, these sandflats act like an unsteady "trickling bed filter," capturing or generating reactive organic particles, rapidly remineralizing OM, and recycling nutrients. Macrobiological structures within the flat altered the amounts and reaction rates of OM on various spatial and temporal scales. Relatively elevated OM decay rates were associated with the burrows of Callichirus major, a deep-burrowing thalassinid shrimp. Large stingray feeding pits accumulated fine grained OM, locally clogging the "trickling bed filter," and inhibiting porewater advection. As illustrated by Debidue Flat, intertidal sands can be sites of high OM flux and turnover and play an important role in biogeochemical cycling in estuarine systems.
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    Mammalian predator-prey systems are behaviorally sophisticated games of stealth and fear. But, traditional mass-action models of predator prey dynamics treat individuals as behaviorally unresponsive "molecules" in Brownian motion. Foraging theory should provide the conceptual framework to envision the interaction. But, current models of predator feeding behavior generally envision a clever predator consuming large numbers of sessile and behaviorally inert prey (e.g., kangaroo rats, Dipodomys, collecting seeds from food patches). Here, we extend foraging theory to consider a predator-prey game of stealth and fear and then embed this game into the modeling of predator-prey population dynamics. The melding of the prey and predator's optimal behaviors with their population and community-level consequences constitutes the ecology of fear. The ecology of fear identifies the endpoints of a continuum of N-driven (population size) versus mu-driven (fear) systems. In N-driven systems, the major direct dynamical feedback involves predators killing prey, whereas mu-driven systems involve the indirect effects from changes in fear levels and prey catchability. In mu-driven systems, prey respond to predators by becoming more vigilant or by moving away from suspected predators. In this way, a predator (e.g., mountain lion, Puma concolor) depletes a food patch (e.g., local herd of mule deer, Odocoileus hemionus) by frightening prey rather than by actually killing prey. Behavior buffers the system: a reduction in predator numbers should rapidly engender less vigilant and more catchable prey. The ecology of fear explains why big fierce carnivores should be and can be rare. In carnivore systems, ignore the behavioral game at one's peril.
  • Article
    The @'predation-sensitive food@' (PSF) hypothesis proposes that both food and predation necessarily limit populations, because as food becomes limiting animals take greater risks to obtain more food, and some of these are killed. Alternative hypotheses are @'predator regulation@' where predators hold the prey population well below starvation levels; and @'surplus@' predation where predators kill only those prey that are excluded from optimal habitat and are dying from starvation. The predictions from these hypotheses were tested by examining body condition of Serengeti Wildebeest (Connochaetes taurinus) over 24 yr (1968-1991). Two phases of population growth were examined: 1968-1973 when the population was increased with superabundant food; and 1977-1991 when the population was stationary and regulated by intraspecific competition for food. Three categories of data were compared: live animals, predation kills, and nonpredation deaths. Body condition was measured from bone marrow, the last reserves of fat in ungulates. The predator regulation hypothesis predicts that the marrow condition should be similar to the predation and live samples. The surplus hypothesis predicts the predation and nonpredation samples should be similar. The PSF hypothesis predicts that marrow condition of the predation sample should be (1) poorer than that of the live sample, (2) better than that of the nonpredation sample, and (3) better when food is limiting than when it is abundant. Analyses of the frequency distribution of marrow categories showed that both the predation and nonpredation samples were significantly poorer than that of the live population. In both increase and stationary phases of population growth, the predation sample was in better condition than the nonpredation sample. The predation sample was not quite significantly better (P = 0.052) when food was limiting. These results are consistent with the PSF hypothesis and inconsistent with both of the alternative hypotheses. Female and male condition was similar in the predation sample, but females were killed at a younger age. Lions and hyenas killed animals in similar condition, but lions took animals at a younger age. The results suggest that (1) body condition affects the vulnerability of individual wildebeest to predation, and (2) predation jointly limits the population with intraspecific competition by removing animals from the population that are in better condition than those that are starving.
  • Article
    Multiscale patterns of spatial and temporal variation in density and population structure were used to evaluate the generality of a three-trophic-level cascade among sea otters (Enhydra lutris), invertebrate herbivores, and macroalgae in Alaska. The paradigm holds that where sea otters occur herbivores are rare and plants are abundant, whereas when sea otters are absent herbivores are relatively common and plants are rare. Spatial patterns were based on 20 randomly placed quadrats at 153 randomly selected sites distributed among five locations with and four locations without sea otters. Both sea urchin and kelp abundance differed significantly among locations with vs. without sea otters in the Aleutian Islands and southeast Alaska. There was little (Aleutian Islands) or no (southeast Alaska) overlap between sites with and without sea otters, in plots of kelp density against urchin biomass. Despite intersite variation in the abundance of kelps and herbivores, these analyses demonstrate that sea otter predation has a predictable and broadly generalizable influence on the structure of Alaskan kelp forests. The percent cover of algal turf and suspension feeder assemblages also differed significantly (although less dramatically) between locations with and without sea otters. Temporal variation in community structure was assessed over periods of from 3 to 15 yr at sites in the Aleutian Islands and southeast Alaska where sea otters were 1) continuously present, 2) continuously absent, or 3) becoming reestablished because of natural range expansion. Kelp and sea urchin abundance remained largely unchanged at most sites where sea otters were continuously present or absent, the one exception being at Torch Bay (southeast Alaska), where kelp abundance varied significantly through time and urchin abundance varied significantly among sites because of episodic and patchy disturbances. In contrast, kelp and sea urchin abundances changed significantly, and in the expected directions, at sites that were being recolonized by sea otters. Sea urchin biomass declined by 50% in the Aleutian Islands and by nearly 100% in southeast Alaska following the spread of sea otters into previously unoccupied habitats. In response to these different rates and magnitudes of urchin reduction by sea otter predation, increases in kelp abundance were abrupt and highly significant in southeast Alaska but much smaller and slower over similar time periods in the Aleutian Islands. The different kelp colonization rates between southeast Alaska and the Aleutian Islands appear to be caused by large-scale differences in echinoid recruitment coupled with size-selective predation by sea otters for larger urchins. The length of urchin jaws (correlated with test diameter, r^2 = 0.968) in sea otter scats indicates that sea urchins <15-20 mm test diameter are rarely eaten by foraging sea otters. Sea urchin populations in the Aleutian Islands included high densities of small individuals (<20 mm test diameter) at all sites and during all years sampled, whereas in southeast Alaska similarly sized urchins were absent from most populations during most years. Small (<30-35 mm test diameter) tetracycline-marked urchins in the Aleutian Islands grew at a maximum rate of @?10 mm/yr; thus the population must have significant recruitment annually, or at least every several years. In contrast, echinoid recruitment in southeast Alaska was more episodic, with many years to perhaps decades separating significant events. Our findings help explain regional differences in recovery rates of kelp forests following recolonization by sea otters.
  • Article
    Previous research suggests that nursery areas provide an abundant food source as well as protection from predation for young sharks, and that these benefits are the reasons they use these areas. This study examined the abundance of prey species within a known nursery area, Terra Ceia Bay, Florida, and compared those data with the amount of time blacktip sharks spent within various geographic zones within the nursery. The most abundant prey species within the study site were pinfish, Lagodon rhomboides, pigfish, Orthopristis chrysoptera, spotfin mojarra, Eucinostomus argenteus, and silver perch, Bairdiella chrysoura. Prey species were found to be most abundant in the mid to southern portion of the nursery area, whereas sharks spent the majority of their time within the northern portion of the study site. There was no correlation between the amount of time sharks (as a whole and by individual) spent within a geographic zone and the abundance of prey species within that area. These results suggest that prey abundance is not the main factor directing the movement patterns and habitat choice of juvenile Carcharhinus limbatus within Terra Ceia Bay. Predator avoidance may be more important in the use of the nursery grounds by these young animals than prey abundance.
  • Article
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    Many theoretical and experimental studies suggest that synergistic interactions between resources and predators influence foraging decisions and their fitness consequences. This framework, however, has been ignored almost completely by hypotheses on causes of the population decline of Steller sea lions (SSLs) (Eumetopias jubatus) in western Alaska. By comparing predictions from a dynamic state variable model to empirical data on the behaviour of individuals instrumented with satellite-linked time-at-depth recorders, we develop and find preliminary support for the hypothesis that, during winter in Prince William Sound, juvenile SSLs (a) underutilise walleye pollock, a predictable resource in deep strata, due to predation risk from Pacific sleeper sharks, and (b) underutilise the potential energy bonanza of inshore aggregations of Pacific herring due to risk from either killer whales, larger conspecifics, or both. Further, under conditions of resource scarcity—induced by overfishing, long-term oceanographic cycles, or their combination—trade-offs between mortality risk and energy gain may influence demographic parameters. Accordingly, computer simulations illustrated the theoretical plausibility that a decline of Pacific herring in shallow strata would greatly increase the number of deep foraging dives, thereby increasing exposure to sleeper sharks and mortality rates. These results suggest that hypotheses on the decline of SSLs should consider synergistic effects of predators and resources on behaviour and mortality rates. Empirical support for our model, however, is limited and we outline tasks for empirical research that emerge from these limitations. More generally, in the context of today's conservation crises, our work illustrates that the greater the dearth of system-specific data, the greater the need to apply principles of behavioural ecology toward the understanding and management of large-scale marine systems.
  • Article
    In this paper we review the empirical studies documenting trait-mediated indirect interactions (TMIIs) in food webs. Basic models and empirical approaches that form the foundation of our conceptualization of species interactions generally assume that interactions, are an intrinsic property of the two interacting species and therefore are governed by their respective densities. However, if a species reacts to the presence of a second species by altering its phenotype, then the trait changes in the reacting species can alter the per capita effect of the reacting species on other species and, consequently, population density or fitness of the other species. Such trait-mediated indirect interactions can reinforce or oppose density-mediated effects and have been largely overlooked by community ecologists. We first briefly develop the case for the broad mechanistic basis for TMIIs and then review the direct evidence for TMIIs in various permutations of simple three- to four-species food webs. We find strong evidence for quantitatively significant effects of TMIIs in a variety of aquatic and terrestrial systems. We further highlight those few studies that address the question of the relative magnitudes of density- and trait-mediated effects and the role of species densities in their transmission. These studies indicate that trait effects are often as strong or stronger than density effects. We conclude that ecological communities are replete with TMIIs arising from trait plasticity and that these effects are quantitatively important to community dynamics. Finally, we synthesize our results and indicate profitable directions for future research.
  • Article
    A North Carolina reef fish community was resurveyed with scuba gear to determine if changes occurred in community structure after 15 years of intense fishing. Generally, fishes important in the recreational and commercial fisheries were smaller, and large changes occurred in relative abundance and species composition. Indicative of a warming trend, total species composition of fishes had become more tropical, and a tropical sponge previously unrecorded at this latitude off the North Carolina coast became common. Two new (to the area) families and 29 new species of tropical fishes were recorded. Observations of 28 species of tropical reef fishes increased significantly. No new temperate species were observed, and the most abundant temperate species decreased by a factor of 22. Mean monthly bottom water temperatures in winter were 1–6°C warmer during the recent study. An increase in fish-cleaning symbiosis was especially noticeable.
  • Article
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    The impact of fishing on chondrichthyan stocks around the world is currently the focus of considerable international concern. Most chondrichthyan populations are of low productivity relative to teleost fishes, a consequence of their different life-history strategies. This is reflected in the poor record of sustainability of target shark fisheries. Most sharks and some batoids are predators at, or near, the top of marine food webs. The effects of fishing are examined at the single-species level and through trophic interactions. We summarize the status of chondrichthyan fisheries from around the world. Some 50% of the estimated global catch of chondrichthyans is taken as by-catch, does not appear in official fishery statistics, and is almost totally unmanaged. When taken as by-catch, they are often subjected to high fishing mortality directed at teleost target species. Consequently, some skates, sawfish, and deep-water dogfish have been virtually extirpated From large regions. Some chondrichthyans are more resilient to fishing and we examine predictions on the vulnerability of different species based on their life-history and population parameters. At the species level, fishing may alter size structure and population parameters in response to changes in species abundance. We review the evidence for such density-dependent change. Fishing can affect trophic interactions and we examine cases of apparent species replacement and shifts in community composition. Sharks and rays learn to associate trawlers with food and feeding on discards may increase their populations. Using ECOSIM, we make some predictions about the long-term response of ecosystems to fishing on sharks. Three different environments are analysed: a tropical shelf ecosystem in Venezuela, a Hawaiian coral reef ecosystem, and a North Pacific oceanic ecosystem. (C) 2000 International Council for the Exploration of the Sea.
  • Article
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    Harbor seals Phoca vitulina and other pinnipeds in the Gulf of Alaska have declined since the 1980s. The search for causation has considered top-down and bottom-up influences as independent factors. Research on other systems, however, has revealed that resource availability and predator densities synergistically determine the predation rates experienced by intermediate consumers. From this premise we developed a dynamic state variable model of behavior for the declining harbor seal population of Prince William Sound, Alaska. We modeled separate scenarios in which seals were prey to (1) transient killer whales Orcinus orca at and near the surface and Pacific sleeper sharks Somniosus pacificus throughout the water column, or (2) killer whales only. In both scenarios, resource decrements reduced the time spent by seals at the haulout (a refuge lacking food), increased the time spent at foraging areas, and lengthened surface intervals and dive durations. Because of this behavioral compensation, per capita fish consumption remained relatively constant, but predation rates increased as resources declined, despite fixed predator densities. Foraging effort and predation rates increased further when energy stores were lower at the onset of simulation periods, but in all scenarios seals not killed by predators had achieved a high level of energy stores by the reproductive season. These behavioral mechanisms proposed by the model potentially explain - at least partially - why the population has been declining while seals have maintained good energy stores throughout temporal shifts in resource availability. More generally, simulations suggest that overfishing and other factors that reduce fish populations indirectly increase predation rates on seals, but data are needed to test this hypothesis. Our model also encompasses a broader ecosystem perspective by predicting how resource level determines the relative strength of trait- and density-mediated interactions, whereby predators of seals indirectly affect fish populations by influencing the foraging behavior and density of seals. The behavioral modeling approach presented here is an additional tool for resource managers attempting to optimize fisheries exploitation and pinniped conservation.
  • Article
    In Moreton Bay, Australia, dugongs (Dugong dugon) often graze in large herds at the same location for weeks to months. Such grazing reduced seagrass shoot density by 65 to 95%, aboveground biomass by 73 to 96% and belowground biomass by 31 to 71% at 3 sites ranging in size from 2 to 75 ha. Following even the most intense and sustained grazing, the space between surviving tufts of seagrass remains small (< 1 m(2)) and recovery is usually rapid (months). In this regard, intensive grazing differs from disturbances caused by storms, sedimentation or disease. However, recovery of seagrass meadows can be suppressed by low levels of sustained grazing pressure. The species composition of seagrass meadows can be altered by intensive grazing, which favours rapidly growing, early pioneer species, such as Halophila ovalis, at the expense of slower growing but dominant species such as Zostera capricorni. In Moreton Bay, H. ovalis is the most nutritious (high nitrogen, low fibre) and the most preferred seagrass grazed by dugongs. Z. capricorni is the least preferred species. By preventing the expansion of Z. capricorni and increasing the abundance of H. ovalis, this grazing system, termed cultivation grazing, can improve the quality of the dugong's diet.
  • Article
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    An experiment was designed to assess the role of 2 different predators in determining the macrobenthic community structure of an intertidal sandflat, The 2 predators were: shorebirds which feed throughout the year by removing individual prey items, and eagle rays Myliobatis tenuicaudatus which are only present during the summer and disturb large volumes of sediment where extracting prey. The experiment consisted of bird exclusion, ray + bird exclusion and reference plots. Samples were collected from each plot on 2 occasions: 6 mo after the initiation of the experiment, when rays were absent and common bivalve densities were high following recruitment, and 8 mo later when rays were present and bivalve population structure was not dominated by new recruits. At the end of the experiment analysis of surficial sediment features did not indicate the experiment was confounded by localised modifications of sediment or hydrodynamic conditions. Community level differences on both occasions were driven by effects on common taxa. The seasonality of effects in our experiment precluded direct comparison of the 2 predators. However, the 6 mo results indicated that bird predation resulted in indirect effects due to adult/juvenile interactions amongst the dominant bivalve Macomona liliana. At the end of the experiment, 14 mo after its initiation, analysis of common taxa generally revealed direct negative effects of predation, with significantly high densities in the ray + bird exclusion treatment. Infaunal density changes in response to the exclusion of shorebirds and rays did not indicate the presence of multiple trophic levels in this infaunal assemblage. Differences between the results obtained from the bird exclusion and the ray + bird exclusion treatments on the first sampling occasion were attributed to an edge effect around the bird exclusion plots which effectively increased their area. This edge effect emphasises the importance of infaunal mobility and its potential to swamp predator effects. The results of this experiment highlight the importance of considering the role of predators within an appropriate spatial and temporal context.
  • Article
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    Studies of the influence of small-scale disturbances on soft-bottom communities have usually been conducted on one type of community only. We utilized polychaete and bivalve dominated macrobenthic communities in 2 physically similar intertidal sandflat sites to study differences in recolonization of Pits created by feeding eagle rays Myliobatis tenuicaudatus. In both communities ray pits were sampled on 6 occasions until 12 d after their creation. Ray pits rapidly infilled with sediment of similar grain size to that of the surrounding sandflat. Organic carbon content only became elevated in the pits in the polychaete dominated community. The intensity of ray disturbance indicated that sediment within an area of 700 to 800 m2 at either site would be turned over about every 70 d. Rapid recolonization of pits by macrofauna occurred in both communities, although bivalves tended to recolonize more rapidly than polychaetes. An epibenthic crustacean was the only species to indicate possible preferential exploitation of pits. Rapid recolonization and sediment infilling emphasise the importance of passive transport of adults into pits. The tube-mat forming polychaete Boccardia syrtis, dominant at one site, was the only common species which did not colonize pits in the same proportion to that found in the sediment adjacent to pits. The rate of sediment turnover by rays and the rapid recolonization by macrofauna indicate that rays may tend to smooth out distribution patterns, particularly those of long-lived infauna, and play a role in maintaining dominance patterns in both polychaete and bivalve communities.
  • Article
    Biologists have long known that predators play a key role in structuring ecological communities, but recent research suggests that predator richness - the number of genotypes, species, and functional groups that comprise predator assemblages - can also have cascading effects on communities and ecosystem properties. Changes in predator richness, including the decreases resulting from extinctions and the increases resulting from exotic invasions, can alter the composition, diversity, and population dynamics of lower trophic levels. However, the magnitude and direction of these effects are highly variable and depend on environmental context and natural history, and so are difficult to predict. This is because species at higher trophic levels exhibit many indirect, non-additive, and behavioral interactions. The next steps in predator biodiversity research will be to increase experimental realism and to incorporate current knowledge about the functional role of predator richness into ecosystem management.
  • Article
    The high trophic connectivity of many communities can produce large numbers of indirect interactions. Although many trait-mediated indirect interactions (TMII) are caused by changes in prey behavior, less is known about the effects of changes in predator behavior such as prey switching or multiple predator effects (MPE) on indirect interactions, especially in marine systems. We performed a series of field caging experiments off the Isles of Shoals, Maine (USA) from 2000 to 2002 to test for the presence of behaviorally mediated indirect effects in a shallow subtidal food web. Specifically, crab (Cancer borealis) predation on sea urchins (Strongylocentrotus droebachiensis) was quantified in three habitats (Codium fragile algal beds, barrens, and mussel beds) representing differing combinations of food and shelter to examine the effects of prey switching by crabs. A second predator (the lobster, Homarus americanus) was added to the crab treatments to examine MPE effects. Urchin mortality was significantly lower in the mussel habitat than in the Codium and barren habitats. Mussels produced a positive indirect effect on urchins by changing the behavior of crabs; crabs fed on mussels instead of urchins (prey switching). In the barrens, crab predation on urchins indirectly increased the abundance of the introduced ascidian, Diplosoma sp., whereas Codium density did not change among treatments. A significant risk reduction for urchins occurred in Codium and barren habitats, but not in mussel habitats when crabs and lobsters were combined. Lobsters also produced a positive indirect effect on mussels by reducing crab predation. Thus, lobsters modify crab behavior and dampen changes in community structure. Our results illustrate the importance of predator behavior and habitat context in modifying consumer pressure and community structure, and argue for the consideration of these factors in other multi-predator systems where habitats represent food and/or shelter.
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    Sex differences in foraging behaviour have been attributed to size dimorphism, niche divergence, and sex-specific fitness-maximizing strategies. Although sex differences in diving behaviour of marine carnivores are thought to result in sex differences in diet, this is not known for any species over temporal scales relevant to life-history characteristics. We examined blubber fatty acid (FA) profiles of gray seals, Halichoerus grypus (Fabricius, 1791), a sexually size-dimorphic species in which sex differences in foraging behaviour have been observed. FA profiles reflect prey consumed over a period of weeks or months. FA profiles of adult males and females varied significantly by season but there was a season by sex interaction, indicating that seasonal changes in diet differed by sex. FA profiles of adults also varied interannually, with a significant sex by year interaction. Interannual variability may have been a response to changes in ocean-bottom temperatures affecting prey availability or changes in prey abundance. Adult FA profiles differed from those of 6-month-old juveniles; however, there was no evidence of sex differences in the diet of younger animals. Our results indicate that sex differences in the foraging behaviour of adults are reflected in differences in diet at multiple temporal scales.
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    Predation has long been implicated as a major selective force in the evolution of several morphological and behavioral characteristics of animals. The importance of predation during evolutionary time is clear, but growing evidence suggests that animals also have the ability to assess and behaviorally influence their risk of being preyed upon in ecological time (i.e., during their lifetime). We develop an abstraction of the predation process in which several components of predation risk are identified. A review of the literature indicates that an animal's ability to assess and behaviorally control one or more of these components strongly influences decision making in feeding animals, as well as in animals deciding when and how to escape predators, when and how to be social, or even, for fishes, when and how to breathe air. This review also reveals that such decision making reflects apparent trade-offs between the risk of predation and the benefits to be gained from engaging in a given activity. Despite this body of evidence, several areas in the study of animal behavior have received little or no attention from a predation perspective. We identify several such areas, the most important of which is that dealing with animal reproduction. Much work also remains regarding the precise nature of the risk of predation and how it is actually perceived by animals, and the extent to which they can behaviorally control their risk of predation. Mathematical models will likely play a major role in future work, and we suggest that modelers strive to consider the potential complexity in behavioral responses to predation risk. Overall, since virtually every animal is potential prey for others, research that seriously considers the influence of predation risk will provide significant insight into the nature of animal behavior.Predation has long been implicated as a major selective force in the evolution of several morphological and behavioral characteristics of animals. The importance of predation during evolutionary time is clear, but growing evidence suggests that animals also have the ability to assess and behaviorally influence their risk of being preyed upon in ecological time (i.e., during their lifetime). We develop an abstraction of the predation process in which several components of predation risk are identified. A review of the literature indicates that an animal's ability to assess and behaviorally control one or more of these components strongly influences decision making in feeding animals, as well as in animals deciding when and how to escape predators, when and how to be social, or even, for fishes, when and how to breathe air. This review also reveals that such decision making reflects apparent trade-offs between the risk of predation and the benefits to be gained from engaging in a given activity. Despite this body of evidence, several areas in the study of animal behavior have received little or no attention from a predation perspective. We identify several such areas, the most important of which is that dealing with animal reproduction. Much work also remains regarding the precise nature of the risk of predation and how it is actually perceived by animals, and the extent to which they can behaviorally control their risk of predation. Mathematical models will likely play a major role in future work, and we suggest that modelers strive to consider the potential complexity in behavioral responses to predation risk. Overall, since virtually every animal is potential prey for others, research that seriously considers the influence of predation risk will provide significant insight into the nature of animal behavior.
  • Article
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    Mathematical models examine the relationship between harvesting effort and stock size for a predator species when the prey adapts to the risk of predation. In one set of models, the prey can increase its own reproductive rate if it increases its vulnerability to the predator. In the second set of models, each of two prey species has fixed characteristics, but changes in the average characteristics within the prey trophic level occur via shifts in the relative abundance of the two species. In both models, the equilibrium predator population can increase as harvest of that species increases. In the case of two-prey models, the predator's equilibrium population always increases with an increased harvest rate if the two prey coexist and share a single resource. The predator's equilibrium population often decreases from its maximum size to zero over a very small range of harvest rates, once those rates become high enough. Because increased stock size is often used to justify increased harvest rates, this relationship poses a risk that harvest rate will increase to the point where the stock quickly collapses. The results are relevant to understanding changes in the population size of a species experiencing declining environmental conditions.
  • Article
    Physical variables, standing crop and dugong activity were monitored over 14 months in a subtidal community dominated by the tropical seagrass Halodule uninervis and the green alga Penicillus nodulosus in a small cove in subtropical Shark Bay, Western Australia. Water temperature ranged from 14.5 (June) to 30.5°C (February), salinity from 48 (August) to 62‰ (March). Attenuation coefficients were 0.18–0.32m−1 in February and 0.10m−1 in June. Mean daily PAR at the sea bed was 200μmolm−2s−1 in September and 400μmolm−2s−1 in January. Visually, Halodule appeared dominant, but Penicillus biomass exceeded Halodule biomass by ≈3% on ridges and averaged ≈ nine times higher in gullies. Total Halodule biomass on ridges was 46.5gm−2 in May and 69.8gm−2 in March, rhizome biomass (40–65gdry weightm−2) was four to six times leaf biomass (7–16gdry weightm−2). Productivity on the ridges, measured over 4–6 week intervals was 0.12gdry weightm−2 per day in August–October and 1.56gdry weightm−2 per day in March–April. Growth persisted throughout the year, and was not limited to temperatures of ≥21°C. Productivity was estimated as 295gdry weightm−2 per year. The plastochrone interval for rhizomes (PIR) was 12.3 days in May and June and 5.9 days in February. The temporal peak in productivity did not coincide with peak insolation, but did coincide with high temperatures and high and continuous dugong activity. Statistical analysis indicated that light and temperature influenced leaf productivity more than they did rhizome, root, or total productivity. Dugongs (Dugong dugon) rooted in the community from January through April, raising levels of suspended sediments and attenuation coefficients and reducing PAR. Halodule biomass in dugong exclosures at the end of the grazing season was 1.8 times that in adjacent unprotected areas. Dugongs departed when autumn temperatures fell below 19°C. During the grazing season loss of biomass resulting from dugong activity exceeded 50% of production.
  • Article
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    Field experiments involving local manipulations of predator densities are an important tool for studying the role of predation in natural communities. I use mathematical models to investigate how treatment effects should vary with the size of the area manipulated in open predation experiments, i.e., in studies that allow prey to migrate in and out from experimental units. The most general result is that the influence of prey movements on prey densities decreases with increasing spatial scale of an experiment, while the effect of predation rate is independent of scale. Thus, the results seen in small-scale experiments tend to reflect prey movements, while the effects seen in large-scale experiments are due mainly to predator-related mortality. If predation rates are spatially variable (e.g., the habitat consists of refuge patches and predator-rich patches) the problem becomes more complex, because movement rates influence predation rates. Movements that are independent of predator density can swamp the effects of predation mortality in small-scale experiments. However, in larger scale experiments, the effect of movements is to increase the predation rate. This is because a larger proportion of the prey is exposed to predators at high movement rates. Predators can also affect local densities through effects on prey behavior. If prey increase movement rates out from predator patches, this will lead to increased treatment effects in small-scale experiments. In contrast, the result of such behaviors is to decrease treatment effects in large-scale experiments, because most prey will be in refuge patches and predation rates will be low. Alternatively, sometimes predators cause prey to decrease activity when close to a predator. This kind of behavioral response leads to decreased treatment effects in small-scale experiments. If the response is strong, the result can even be a reversed effect, i.e., reduced movements out from predator cages lead to increased prey densities in such cages. I argue that the effect of this type of behavior in large-scale experiments should also be to decrease treatment effects. This is because predation rates are usually low on inactive prey. However, freezing behaviors cannot lead to reversed predator effects on a large scale.
  • Article
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    Birds and mammals are the leading marine predators at high latitudes, while sharks and other large fish occupy top positions in tropical waters. The present study proposes that temperature-dependent predation success (TPS) explains global patterns of marine vertebrate community structure. Burst speed increases with temperature in ectotherms but is independent of temperature in endotherms. If capture success depends on relative swimming speeds of predator and prey, ectothermic prey will be more vulnerable to attack by endothermic predators at low temperatures. Conversely, high temperatures should enhance the ability of ectothermic predators to prey on endotherms. Pursuit-diving seabirds (penguins, auks and some cormorants) and pinnipeds (seals and sea lions) are ubiquitous in ocean waters with summer surface temperatures cooler than the mid-teens to low 20s (degrees C) but are virtually absent in warmer regions. We suggest that the near-absence of these animals at low latitudes is due to TPS, as warm water increases the difficulty of capturing fish prey and increases vulnerability to predation by large ectothermic and partially endothermic sharks. Pursuit-diving birds and pinnipeds are virtually absent from warm temperate and tropical waters, even where primary productivity and fisheries data suggest that food supplies are ample. This indicates that the low productivity that prevails in much of the tropical zone cannot explain the worldwide distributional patterns of pursuit-diving birds and pinnipeds. Endothermy in marine communities increases with cooler temperatures and with animal size. Pursuit-diving birds and pinnipeds are sensitive to temperature limits and may suffer important range contractions as oceans warm.
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