Unraveling the Ecological Importance of Elasmobranchs

Chapter · March 2010with 234 Reads
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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  • ... *Significant cluster (P < 0.05). and indirect (e.g., competition) effects (Heithaus et al., 2010;Navia et al., 2012). Sharks also may have indirect, nonconsumptive effects by altering the behaviour of potential prey and competitors (Lima and Dill, 1990), but it is their role as consumers that directly impacts their fitness and characterizes their function in marine food webs. ...
    ... 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). ...
    ... Additionally, the ecological impacts of sharks extend beyond direct consumption of prey. Interspecific competition and indirect predatory effects influence behaviours, interactions, and community dynamics (Heithaus et al., 2010). Robust dietary analyses and improved tracking methods have enabled an assessment of marine food web dynamics and the inference of critical functional responses to changes in environmental conditions and anthropogenic influences across ecologically relevant spatial and temporal scales. ...
    Chapter
    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.
  • ... Some shark species have experienced population declines, mainly due to overfishing, bycatch, pollution and habitat deg- radation ( 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 implementa- tion of effective strategies for the conservation and manage- ment of sharks is often hampered by the lack of information regarding their diet, life history and behaviour (Shiffman et al., 2012). ...
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    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 penetra- tion into sediments, ex tending the zone of oxygena- tion ( 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 activi- ties ( VanBlaricom 1982, Heithaus et al. 2010). Kiszka et al. (2015) de tected the association of southern stingrays Hypanus americanus and bar jacks Ca - ranx ruber, where stingray bioturbation allowed C. ruber to access re sources otherwise unavailable. ...
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    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). ...
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    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). Because they feed on a wide variety of prey along the Ecuadorian Pacific coast, C. leucas and G. cuvier are generalist predators, and because they occupy a high position in the marine food web, these sharks are important regulators of lower trophic levels. ...
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    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
  • ... 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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    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.
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    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.
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    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.
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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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    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.
  • 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).
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    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.
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    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.
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    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.
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    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.
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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.