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An ecological assessment of large coastal shark communities in South Florida
Abstract
Given population declines of many shark species, coupled with increasing conservation efforts and recoveries, there is a need for baseline assessments and continued monitoring of shark populations to support management. The waters of South Florida, USA, are used by a diverse array of sharks that occur among a mosaic of habitats and management zones. Here we conducted standardized drumline surveys from Miami through the middle Florida Keys to examine spatial, seasonal and environmental patterns in shark occurrence, catch per unit effort, composition, and demographic structure. Between 2009 and 2021, a total of 21,755 drumlines were deployed, capturing 3398 sharks, comprising fifteen species. Ginglymostoma cirratum, (n = 1335), Carcharhinus limbatus, (n = 650), Negaprion brevirostris, (n = 314), C. leucas, (n = 253), and Sphyrna mokarran, (n = 238) were the most common species encountered. Overall, the largest shark surveyed was a 450 cm S. mokarran and the smallest shark was a 50 cm Rhizoprionodon terraenovae. At the assemblage level, relative abundance among regions and seasons were generally similar; however clear species-specific patterns of abundance, size structure, and sex-composition were detected by season, region, habitat, and management zones. Of the physical conditions evaluated, habitat type and depth emerged as the most influential parameters affecting abundances and sizes of species captured. While few species exhibited significant differences in catches by management zone, areas with the most restrictive fisheries regulations generally supported higher abundances for which differences were detected. These data serve as a baseline for future monitoring of shark populations in South Florida and assessing their response to environmental change.
... Clupper is an Ancestral Period 1 indigenous archaeological site located in the Florida Keys (Keys), USA ( Figure 1); the Keys comprise a small-island archipelago renowned for its breadth of marine biodiversity, including several elasmobranch species (Table 1). The marine and estuary habitats of the Keys are essentially made up of "Bay" and "Ocean" waters, including shallower waters (e.g., < 2 m deep) protected by mangrove islands and deeper waters (e.g., 10-30 m deep) beyond embayments, respectively (Tinari and Hammerschlag, 2021). ...
... Lemon sharks prefer shallow coastal waters, often feeding at night (Wetherbee et al., 1990) and growing to >3 m in length (Dibattista et al., 2007). Lemon sharks are abundant in the Keys and are known to inhabit inshore marine habitats, such as coral reefs or flats, as well as estuaries (Compagno, 1984;Jennings et al., 2008;Tinari and Hammerschlag, 2021;Wiley and Simpfendorfer, 2007). ...
... The blacktip is described as a stocky species, typically reaching between 1.2 to 1.9 m, and only rarely reaching 2 m in length (Castro, 1996;Compagno, 1984). While this species is present in waters of the Gulf of Mexico, southern Florida and the Keys year-round (Tinari and Hammerschlag, 2021;Wiley and Simpfendorfer, 2007), it tends to be most abundant during winter months after migrating southward from nursery grounds along the northern Gulf coast, Carolinas, and Georgia (Castro, 1996;Heithaus et al., 2007). It occurs in inshore habitats with shallower waters. ...
Introduction
Elasmobranchs, such as sharks and rays, are among the world’s most endangered vertebrates, with over 70% loss in abundance over the past 50 years due to human impacts. Zooarchaeological baselines of elasmobranch diversity, distribution, and exploitation hold great promise for contributing essential historical contexts in the assessment of contemporary patterns in their taxonomic diversity and vulnerability to human-caused extinction. Yet, the historical ecology of elasmobranchs receives relatively less archaeological attention compared to that of ray-finned fishes or marine mammals, largely due to issues of taxonomic resolution across zooarchaeological identifications.
Methods
We explore the use of Zooarchaeology by Mass Spectrometry (ZooMS) for species identification in this unstudied group, using an archaeological case study from the marine environments of the Florida Keys, a marine biodiversity hotspot that is home to an array of elasmobranch species and conservation efforts. By comparison with 39 modern reference species, we could distinguish 12 taxa within the zooarchaeological assemblage from the Clupper archaeological site (Upper Matecumbe Key) that included nine sharks, two rays and a sawfish.
Results and discussion
The results indicate that, through additional complexity of the collagen peptide mass fingerprint, obtained due to the presence of the cartilaginous type II collagen, ZooMS collagen peptide mass fingerprinting provides exceptionally high taxonomic resolution in this group, yielding species-level identifications in all cases where sufficient reference material was used. This case study also highlights the added value of ZooMS for taxa that are more difficult to distinguish in zooarchaeological analyses, such as vertebrae of the Atlantic sharpnose shark (Rhizoprionodon terraenovae) and the hammerhead sharks (Sphyrna spp.) in the Florida Keys. Therefore, the application of collagen peptide mass fingerprinting to elasmobranchs offers great potential to improve our understanding of their archaeological past and historical ecology.
... Whereas there is no evidence of blacktip shark migratory connectivity between the Gulf of Mexico and the U.S. East Coast, connectivity between Biscayne Bay, FL, and Florida Bay was recently documented (Tinari and Hammerschlag 2021). Blacktip sharks are found year-round in Biscayne Bay, which is immediately south of Miami, FL (Tinari and Hammerschlag 2021). ...
... Whereas there is no evidence of blacktip shark migratory connectivity between the Gulf of Mexico and the U.S. East Coast, connectivity between Biscayne Bay, FL, and Florida Bay was recently documented (Tinari and Hammerschlag 2021). Blacktip sharks are found year-round in Biscayne Bay, which is immediately south of Miami, FL (Tinari and Hammerschlag 2021). Most individuals that left Biscayne Bay, FL, moved southward and westward into Florida Bay (N = 3) (Tinari and Hammerschlag 2021). ...
... Blacktip sharks are found year-round in Biscayne Bay, which is immediately south of Miami, FL (Tinari and Hammerschlag 2021). Most individuals that left Biscayne Bay, FL, moved southward and westward into Florida Bay (N = 3) (Tinari and Hammerschlag 2021). Another individual relocated from Florida Bay to Biscayne Bay (Tinari and Hammerschlag 2021). ...
Global climate change affects a suite of oceanic characteristics including temperature, salinity, currents, and animal distribution. It is imperative that accurate baselines in distribution and range are used to measure changes in the behavior of marine animals that occur in response to climate change. Current knowledge on the migratory movement of the blacktip shark, Carcharhinus limbatus, off the United States East Coast is challenged by inconsistent reports. Published data on the migration and sexual segregation of the U.S. East Coast blacktip shark population can be interpreted a variety of ways and the scientific literature continues to perpetuate historical and uncertain observations on this population derived from the older literature, which carries low certainty. Here, we provide an overview of the biology and life history of the blacktip shark, consolidate the migratory pattern of the species off the U.S. East Coast, and then critically dissect the many inconsistencies perpetuated in the literature. We argue that (1) blacktip sharks that inhabit the Florida Keys may belong to the Gulf of Mexico stock, (2) the currently accepted northern extent of this species (Massachusetts) may be inaccurate, and (3) that environmental variability and/or sexual segregation of this population may play a role in the inconsistent reports. In addition, we identify future topics of research that will help to close the gaps in the current knowledge of the movement ecology of the blacktip shark off the U.S. East Coast.
... Studies on the habitat colonized by Atlantic Nurse Sharks were published between 1950 and 2021 (Appendix S2) and followed a similar pattern to the distribution studies. Most data were obtained from the Floridian ecoregion (N = 13; Gilmore et al., 1977;Smith & Herrkind, 1992;Carrier & Pratt, 1998;Castro, 2000;Wiley & Simpfendorfer, 2007;Pratt & Carrier, 2007;Adams & Paperno, 2007;Ward-Paige et al., 2010;Hannan et al., 2012;Pratt et al., 2018;Rangel, Hammerschlag & Moreira, 2021;Gutowsky et al., 2021;Tinari & Hammerschlag, 2021) The South Atlantic province had a lower variety of habitats associated with the Nurse Sharks, being composed of reefs and sandy and rocky bottoms (28.6% for each type) (Fig. 3B). Studies performed within MPAs had a greater abundance and diversity of habitat types than in locations outside these borders (Fig. 4). ...
... The habitat of neonates was addressed in only two studies (Pratt & Carrier, 2007;Garla et al., 2009;Garla, Garrone-Neto & Gadig, 2014), with occurrences in reefs, rocky, macroalgae, sandy, and seagrass at a maximum depth of 20 m, with an average temperature of 26 C and a salinity of 36 ppt. Adult sharks ranged from 215 to 380 TL cm and were recorded in all habitat categories (Castro, 2000;Chapman et al., 2005;Pratt & Carrier, 2007;DeAngelis et al., 2008;Hannan et al., 2012;Afonso, Andrade & Hazin, 2014;Shipley et al., 2018;Hansell et al., 2018;Tinari & Hammerschlag, 2021;Stoffers et al., 2021) 5). The majority of studies did not discriminate variation in size and sex per type of habitat. ...
... Sharks with larger sizes and occurrences in medium depths are more likely to be threatened due to lower population growth rates and refugees from fisheries (Dulvy et al., 2021). The Atlantic Nurse Shark is an example of a shallow-water shark that reaches up to 380 cm (Tinari & Hammerschlag, 2021). Thus, it is likely subjected to numerous coastal human impacts throughout its entire life, worsened by a lower ability to replace individuals removed from overfishing in populations, and a higher exposition to fisheries in shallow habitats (Dulvy et al., 2021). ...
Human activities have led to the loss of critical habitats for aquatic species at such an accelerated rate that habitat modification is considered a leading threat to biodiversity. Sharks and rays are considered the second most threatened group of vertebrates that have also suffered from habitat loss, especially in nursery grounds and reef-associated species. In this sense, actions toward the conservation of critical grounds for species survival are urgently needed, especially for those threatened with extinction. This study aimed to gather and provide information on the worldwide distribution and habitat association of the ‘vulnerable’ Atlantic Nurse Shark Ginglymostoma cirratum through a literature review performed at the Dimensions research database. A total of 30 studies published between 1950 and 2021 were retained since they defined at least the type of habitat in which G. cirratum was associated. Most studies covered the Floridian ecoregion, where G. cirratum is more common and abundant. Reefs, seagrass, sandy, rocky, mangrove, and macroalgae accounted for the majority of habitat associations, with a higher diversity of habitats detected within marine protected areas (MPAs). Ginglymostoma cirratum was recorded at a maximum depth of 75 m, temperatures ranging from 25 °C to 34 °C, and salinities between 31 and 38 ppt. Neonates were associated with shallower habitats (
... Sharks were captured using a series of baited drumlines, as described in Tinari & Hammerschlag (2021). Upon capture, sharks were either secured alongside a boat in the water or on top of a floatable platform and tagged with the Innovasea V16-4X internal acoustic transmitters (Amirix) as outlined in Rider et al. (2021a). ...
... The behaviors exhibited by the 3 shark species tracked here are most consistent with those considered to be 'urban adapters', exhibiting partial use of urbanized sites while largely relying on more natural areas (McKinney 2006). These space use patterns are consistent with relative abundance surveys of large coastal sharks in the region (Tinari & Hammerschlag 2021); although sharks are routinely captured in close proximity to Miami, catch rates tend to be lower here than in adjacent areas that are less urbanimpacted. These space use patterns are in stark con-trast to those generally exhibited by large terrestrial carnivores, which tend to be urban avoiders (Bateman & Fleming 2012). ...
... It is likely that a range of factors not evaluated here, such as spatial and temporal variations in temperature, salinity, depth, habitat type, nutrients, and dissolved oxygen concentrations, directly or indirectly influence shark residency patterns. Such factors have previously been shown to affect relative abundance rates of great hammerheads as well as bull and nurse sharks in Biscayne Bay (Phenix et al. 2019, Tinari & Hammerschlag 2021, Rider et al. 2021a). Therefore, we recommend that future research seek to incorporate such environmental and water quality factors into assessments of shark space use and residency patterns in Biscayne Bay and how these parameters interact with proximity to the North Bay Urban zone. ...
Understanding and ultimately predicting how marine organisms will respond to urbanization is central for effective wildlife conservation and management in the Anthropocene. Sharks are upper trophic level predators in virtually all marine environments, but if and how their behaviors are influenced by coastal urbanization remains understudied. Here, we examined space use and residency patterns of 14 great hammerheads Sphyrna mokarran , 13 bull sharks Carcharhinus leucas , and 25 nurse sharks Ginglymostoma cirratum in proximity to the coastal metropolis of Miami, Florida, using passive acoustic telemetry. Based on the terrestrial urban carnivore literature, we predicted sharks would exhibit avoidance behaviors of areas close to Miami, with residency patterns in these urban areas increasing during periods of lower human activity, such as during nocturnal hours and weekdays, and that dietary specialists (great hammerhead) would exhibit comparatively lower affinity towards highly urbanized areas relative to dietary generalists (bull and nurse shark). However, we did not find empirical support for these predictions. Space use patterns of tracked sharks were consistent with that of ‘urban adapters’ (species that exhibit partial use of urban areas). Modeling also revealed that an unmeasured spatial variable was driving considerable shark residency in areas exposed to high urbanization. We propose several hypotheses that could explain our findings, including food provisioning from shore-based activities that could be attracting sharks to urban areas. Ultimately, the lack of avoidance of urban areas by sharks documented here, as compared to terrestrial carnivores, should motivate future research in the growing field of urban ecology.
... Even though this area comprises a small portion of the species' US range, adult C. limbatus use this area extensively (Kajiura and Tellman 2016). Although many individuals migrate northward out of the region annually in the spring, some remain in southern FL waters throughout the year (Tinari and Hammerschlag 2021). Previous authors have hypothesised that these yearround individuals could represent a resident subpopulation (Kajiura and Tellman 2016;Tinari and Hammerschlag 2021). ...
... Although many individuals migrate northward out of the region annually in the spring, some remain in southern FL waters throughout the year (Tinari and Hammerschlag 2021). Previous authors have hypothesised that these yearround individuals could represent a resident subpopulation (Kajiura and Tellman 2016;Tinari and Hammerschlag 2021). It is crucial to study the reproductive biology of C. limbatus in this region, as reproductive characteristics have been shown to vary spatially within other species (i.e. ...
... Carcharhinus limbatus individuals were captured off the coast of southern FL (Fig. 1) throughout the year, from 2012 to 2017, by the University of Miami's Shark Research and Conservation Program (Miami, FL, US) as part of a longterm monitoring study (Tinari and Hammerschlag 2021), by using drumline fishing methods described in Gallagher et al. (2014). Each individual was sexed and measured for total length (TL). ...
Accurate reproductive information is crucial for the effective management of sharks. Here, we used non-lethal methods to evaluate reproductive hormone profiles of blacktip sharks (Carcharhinus limbatus) off the coast of southern Florida (FL) as part of a preliminary investigation of this species’ reproductive patterns in the region. Circulating testosterone concentrations in males ranged from 0.04 to 320.7 ng mL⁻¹, and concentrations peaked during months previously shown to represent peak spermatogenic activity. In females, circulating concentrations of oestradiol (E2) ranged from 12.5 to 941.4 pg mL⁻¹, and progesterone (P4) ranged from 30.1 to 821.2 pg mL⁻¹. E2 peaked during months with peak vitellogenic and ovulatory activity in previous studies; P4 displayed less variability among months. In general, changes in monthly hormone concentrations align well with the timing of reproductive events in C. limbatus in other regions. However, because of low monthly sample sizes and a lack of reproductive status confirmation, the female profiles raise interesting questions about this population’s reproductive cycle that could be explored in future studies. Additional studies should collect larger monthly samples and incorporate supplementary data to provide more insight into the reproductive cycle of this species in southern FL.
... Florida Bay has high biomass of prey fishes (including planktivorous and low-level predatory fishes like Engraulidae, Blenniodei, Gobiidae and Clupeidae, and mesopredatory fishes from the family Sciaenidae, Thayer et al., 1999). Due to high levels of productivity, this region supports a robust and diverse predatory community, composed of numerous large-bodied shark species (Tinari and Hammerschlag, 2021). The northern portion of the Bay is protected within Everglades National Park. ...
... Sampling occurred during wet season (November -Abril) from 2011 to 2018 as part of ongoing coastal shark surveys (see Tinari and Hammerschlag, 2021). All sharks were captured using circle-hook drumlines, a minimally invasive technique that allows the captured sharks to swim (Gallagher et al., 2014). ...
The synergistic effects of coastal urbanization have dramatically impacted biological communities. Yet, few studies have investigated how urbanization can influence the diet quality and trophic ecology of coastal sharks. In a preliminary study, we examined for spatial variation in the nutritional ecology of a highly active marine predator, the blacktip sharks (Carcharhinus limbatus) exposed to regional differences in coastal urbanization in southeast Florida. We used medium-term nutritional indicators (i.e., body condition; whole blood stable isotopes [δ¹⁵N and δ¹³C]) and short-term dietary markers (i.e., plasma fatty acid profiles) to test the hypothesis that the nutritional ecology of marine predators would differ in areas exposed to increased urbanization. Our initial results showed that blacktip sharks sampled in high urbanized area (hereafter, ‘urban sharks’) exhibited relatively higher body condition, blood δ¹⁵N levels, and percentages of saturated fatty acids compared to sharks sampled in low urbanized area (hereafter ‘non-urban sharks’). Collectively, these results suggest a possible positive alteration in the amount of food consumed by sharks and/or in the caloric value of their prey. We also found lower percentages of bacterial markers and higher values of dinoflagellate markers in urban sharks. Compared to more resident species evaluated in the region, we did not detect a reduction in diet quality (in terms of essential fatty acids) in this highly active species exposed to urbanization. Therefore, it is possible that the lifestyle and feeding behavior have an influence on the quality of food consumed by urban sharks, and maybe the impacts of urbanization are more pronounced in resident, sedentary and benthic species.
... In males, the breeding season extends from February to June and mating period from April to August (Castro 1996;Baremore and Passerotti 2013). While blacktips tagged off northeast Florida display seasonal migrations (Kajiura and Tellman 2016), the population off southeast Florida may be year-round residents (Tinari and Hammerschlag 2021). The blacktip shark is harvested in both commercial and artisanal fisheries worldwide, and is currently listed as Near Threatened by the International Union for the Conservation of Nature Red List (IUCN, Burgess and Branstetter 2009). ...
... Sampling occurred opportunistically between 2015 and 2019 within Biscayne Bay, Florida, USA (25.61°N, 80.17°W) as part of an ongoing standarized shark survey (Tinari and Hammerschlag 2021). Sharks were captured using circlehook drumlines, a passive fishing technique that allows the captured sharks to swim (as described by Gallagher et al. 2014). ...
Energetic condition is one of the most important factors that influence fitness and reproductive performance in vertebrates. Yet, we lack evidence on how energetic states change in response to reproduction in large marine vertebrates. In the present study, we used a non-lethal approach to assess relationships among reproductive stage, circulating steroid hormones (testosterone and relative corticosteroid levels), plasma fatty acids, and the ketone body β-hydroxybutyrate in male sharks of two species with divergent ecologies, the benthic nurse shark (Ginglymostoma cirratum) and the epipelagic blacktip shark (Carcharhinus limbatus). We found higher relative corticosteroid levels in adult nurse sharks during the pre-mating period and in blacktip sharks during the mating period. Higher levels of β-hydroxybutyrate were found in adult nurse sharks during the mating period, but concentrations of this ketone body did not significantly vary across reproductive stages in blacktip sharks. We also detected reduced percentages of essential fatty acids during the mating period of both nurse and blacktip sharks. Taken together, our findings suggest that nurse and blacktip sharks differ in their energetic strategy to support reproduction, however, they likely rely on physiologically important fatty acids during mating, to support spermatogenesis.
... A total of 251 sharks were caught off the coast of Miami, Florida, USA, as part of ongoing ecological shark surveys as described in Tinari and Hammerschlag (2021). Sampling occurred from 2015 to 2018, across both wet (May-October) and dry (November-April) seasons. ...
Serum proteins found in the blood have been used as diagnostic markers in both human and animal medicine. Evaluating proteins in the blood of sharks may be a valuable way to assess shark health. The purpose of this study was to perform serum protein electrophoresis from non-lethal blood samples of wild sharks and establish baseline values for six species off South Florida: blacknose (Carcharhinus acronotus), blacktip (Carcharhinus limbatus), bull (Carcharhinus leucas), lemon (Negaprion brevirostris), sandbar (Carcharhinus plumbeus), and tiger shark (Galeocerdo cuvier). Two hundred and fifty-one serum samples were collected between 2015-2018 along with sex determination and morphometric measurements. Agarose gel analysis was performed using mammalian protein fraction delimits albumin, alpha 1-globulins, alpha 2-globulins, beta globulins, and gamma globulins alternatively defined as fractions 1-5. Reference electrophoretic profiles were established for each species, revealing the dominance of protein fractions 3 and 4. The fraction 3:4 ratio was determined for each shark and reference intervals for total solids, protein fractions, and fraction 3:4 ratio are reported for each species. There were no significant differences in total solids or protein fractions between males and females of any species. In both blacktip sharks and bull sharks, total solids was positively correlated with body size, but not in the other species evaluated. In blacknose sharks, the fraction 3:4 ratio was positively correlated with body condition. Results suggest that there may be differences in total solids values across dry and wet seasons for some species. Newly established reference intervals can now be utilized in future research to evaluate the health of captive sharks and to investigate the health status of individuals in the wild as well as relate health measures to environmental conditions.
... The severity of shark population declines within the Northwest Atlantic and Gulf of Mexico has been debated (Baum et al., 2005;Burgess, Beerkircher, Cailliet, Carlson, Cortes, et al., 2005b;Burgess, Beerkircher, Cailliet, Carlson, Cortés, et al., 2005a;Jiao et al., 2009); however, in some regions, conservation practices are succeeding and coastal shark populations are increasing (Carlson et al., 2012;Curtis et al., 2014;Pondella & Allen, 2008). Though populations have likely not returned to historic abundance levels in the Florida Keys (Froeschke et al., 2012;Tinari & Hammerschlag, 2021;Ward-Paige et al., 2010), increasing shark populations in conjunction with increased human use of the ocean has the potential to lead to increased human-wildlife conflict and highlights the need to proactively manage human behavior to account for larger predator populations (Carlson et al., 2019). Indeed, within our study area and throughout the state of Florida, some fishing boat captains, guides, and anglers have noted increased conflict with sharks and believe that they are a threat to their catch (Drymon & Scyphers, 2017. ...
Interspecific interactions can play an essential role in shaping wildlife populations and communities. To date, assessments of interspecific interactions, and more specifically predator–prey dynamics, in aquatic systems over broad spatial and temporal scales (i.e., hundreds of kilometers and multiple years) are rare due to constraints on our abilities to measure effectively at those scales. We applied new methods to identify space‐use overlap and potential predation risk to Atlantic tarpon (Megalops atlanticus) and permit (Trachinotus falcatus) from two known predators, great hammerhead (Sphyrna mokarran) and bull (Carcharhinus leucas) sharks, over a 3‐year period using acoustic telemetry in the coastal region of the Florida Keys (USA). By examining spatiotemporal overlap, as well as the timing and order of arrival at specific locations compared to random chance, we show that potential predation risk from great hammerhead and bull sharks to Atlantic tarpon and permit are heterogeneous across the Florida Keys. Additionally, we find that predator encounter rates with these game fishes are elevated at specific locations and times, including a prespawning aggregation site in the case of Atlantic tarpon. Further, using machine learning algorithms, we identify environmental variability in overlap between predators and their potential prey, including location, habitat, time of year, lunar cycle, depth, and water temperature. These predator–prey landscapes provide insights into fundamental ecosystem function and biological conservation, especially in the context of emerging fishery‐related depredation issues in coastal marine ecosystems.
... In the light of that, understanding their ecology and distribution, especially as a response to environmental factors, is essential and urgently needed knowledge. Such factors include but are not limited to abiotic ones like temperature, salinity, dissolved oxygen and season (Drymon et al., 2020;Tinari & Hammerschlag, 2021), and biotic ones like foraging (Andrzejaczek et al., 2019;Vedor et al., 2021), but also include anthropogenic ones like fishing, aquaculture and global warming Queiroz et al., 2019;Sguotti et al., 2016). ...
Using national and international research survey data and applying a combination of models and mapping tools, this study revealed temperature and depth as the crucial environmental drivers of both the distribution and the abundance of four benthopelagic chondrichthyans inhabiting Norwegian and Icelandic waters: rabbitfish (Chimaera monstrosa), velvet‐belly lanternshark (Etmopterus spinax), blackmouth catshark (Galeus melastomus) and spurdog (Squalus acanthias). C. monstrosa and E. spinax seem to prefer similar spatial and ecological habitats, that is deep and cold waters. In contrast, G. melastomus and S. acanthias both prefer similar ecological habitats, that is warmer and shallower waters; nonetheless, they exhibit a different spatial distribution pattern. The species' varied habitat and spatial preferences may lead to different levels of exposure to fishing activities and associated by‐catch risks. Findings of the species' spatial distributions and their driving forces are expected to inform the sustainable management of these species and the ecosystems they inhabit.
... Tiger sharks were predominantly tagged near Tiger Beach in Grand Bahama (n = 41), but several individuals that frequent Tiger Beach were tagged in Florida (n = 2) and South Carolina (n = 5) using the same methodology. Sharks were captured using standardized circle-hook drum-lines following Tinari and Hammerschlag (2021), and acoustic transmitters (Vemco V16, 69 kHz, 68 × 16 mm, 60-90 s nominal delay) were surgically implanted in the body cavity of tiger sharks as per Hammerschlag et al. (2017) while the sharks were in tonic immobility (Kessel and Hussey, 2015). Individual sex was recorded, total length (TL) was measured, and reproductive status of each individual at the time of tagging was determined from a combination of ultrasonography and hormone analysis of blood samples (see Sulikowski et al., 2016 for details). ...
Shark dive ecotourism is a lucrative industry in many regions around the globe. In some cases, sharks are provisioned using bait, prompting increased research on how baited dives influence shark behavior and yielding mixed results. Effects on patterns of habitat use and movement seemly vary across species and locations. It is unknown, however, whether wide-ranging, marine apex predators respond to provisioning by changing their patterns of grouping or social behavior. We applied a tiered analytical approach (aggregation-gregariousness-social preferences) examining the impact of provisioning on the putative social behavior of tiger sharks (Galeocerdo cuvier) at a dive tourism location in The Bahamas. Using network inference on three years of acoustic tracking data from 48 sharks, we tested for non-random social structure between non-provisioned and provisioned monitoring sites resulting in 12 distinct networks. Generally considered a solitary nomadic predator, we found evidence of sociality in tiger sharks, which varied spatiotemporally. We documented periods of both random (n = 7 networks) and non-random aggregation (n = 5 networks). Three of five non-random aggregations were at locations unimpacted by provisioning regardless of season, one occurred at an active provisioning site during the dry season and one at the same receivers during the wet season when provision activity is less prevalent. Aggregations lasted longer and occurred more frequently at provisioning sites, where gregariousness was also more variable. While differences in gregariousness among individuals was generally predictive of non-random network structure, individual site preferences, size and sex were not. Within five social preference networks, constructed using generalized affiliation indices, network density was lower at provisioning sites, indicating lower connectivity at these locations. We found no evidence of size assortment on preferences. Our data suggest that sociality may occur naturally within the Tiger Beach area, perhaps due to the unusually high density of individuals there. This study demonstrates the existence of periodic social behavior, but also considerable variation in association between tiger sharks, which we argue may help to mitigate any long-term impacts of provisioning on this population. Finally, we illustrate the utility of combining telemetry and social network approaches for assessing the impact of human disturbance on wildlife behavior.
The bull shark (Carcharhinus leucas Valenciennes, 1839) is a large, primarily coastally distributed shark famous for its ability to penetrate far into freshwater bodies in tropical, subtropical, and warm-temperate climates. It is a cosmopolitan species with a geographical range that includes the coastlines of all major ocean basins (Atlantic Ocean, Indian Ocean, Pacific Ocean). As a consequence, freshwater occurrences of C. leucas are possible everywhere inside its geographic range. Carcharhinus leucas is a fully euryhaline, amphidromous species and possibly the widest-ranging of all freshwater tolerating elasmobranchs. This species is found not only in river systems with sea access that are not interrupted by human impediments but in hypersaline lakes as well. Rivers and estuaries are believed to be important nursery grounds for C. leucas, as suggested by observations of pregnant females in estuaries and neonates with umbilical scars in rivers and river mouths. Due to the physical capability of this species to enter riverine systems, the documentation of its occurrence in fresh and brackish water is essential for future conservation plans, fishery inspections, and scientific studies that focus on the link between low salinity habitats, shark nurseries, and feeding areas. The author’s review of the available literature on C. leucas revealed the absence of a comprehensive overview of fresh and brackish water localities (rivers and associated lakes, estuaries) with C. leucas
records. The purpose of this literature review is to provide a global list of rivers, river systems, lakes, estuaries, and lagoons with records and reports of this species, including a link to the used references as a base for regional, national, and international conservation strategies. Therefore, the objective of this work is to present lists of fresh and brackish water habitats with records of C. leucas as the result of an extensive literature review and analysis of databases. This survey also took into account estuaries and lagoons, regarding their function as important nursery grounds for C. leucas. The analysis of references included is not only from the scientific literature, but also includes semi-scientific references and the common press if reliable. The result of 415 global fresh and brackish water localities with evidence of C. leucas highlights the importance of these habitats for the reproduction of this species. Moreover, gaps in available distribution maps are critically discussed as well as interpretations and conclusions made regarding possible reasons for the distribution range of C. leucas, which can be interpreted as the result of geographic circumstances, but also as a result of the current state of knowledge about the distribution of this species. The results of the examination of available references were used to build a reliable and updated distribution map for C. leucas, which is also presented here.
Bacteria are known to have explicit roles within the microbiomes of host tissues, therefore examining these communities may prove useful in assessing host health and responses to environmental change. The present study contributes to the emerging, yet understudied, field of microbiome research in elasmobranchs. We provide a screening of the culturable bacteria communities found on multiple tissue sites on the body surface of blacktip (Carcharhinus limbatus), bull (Carcharhinus leucas), and tiger (Galeocerdo cuvier) sharks near Miami, Florida. Tissue sites include mouth, gills, skin, and any visible wounds. The study adds to our understanding of the diversity of bacteria present on sharks in comparison to their natural environment. We also compare bacterial groups found within wounds in shark skin to healthy tissue sites on the same individual. Results indicate that wounds on an individual may allow for opportunistic bacteria to invade or overgrow where they would not normally be found, which may have potential health consequences for sharks that become wounded due to fishing practices. Identified bacteria belonged to the Actinobacteria, Firmicutes, and Proteobacteria phyla, known to be prominent bacterial groups associated with marine organisms. Results indicate shark species-specific differences in bacterial communities, including the presence of bacteria belonging to Planococcaceae exclusively on the skin of tiger sharks. To our knowledge, this is the first report of this family in any elasmobranch. While most tissue sites displayed commensal bacteria identified in similar studies, known pathogens belonging to Vibrionaceae and Staphylococcaceae were identified in the wounds of blacktip and bull sharks. Some bacteria may be normal residents, but the loss of protective dermal denticles due to a wound may allow colonization by pathogens. Continued research is needed to explore microbial communities associated with sharks and their influence on host health.
The rapid expansion of human activities threatens ocean-wide biodiversity. Numerous marine animal populations have declined, yet it remains unclear whether these trends are symptomatic of a chronic accumulation of global marine extinction risk. We present the first systematic analysis of threat for a globally distributed lineage of 1,041 chondrichthyan fishes-sharks, rays, and chimaeras. We estimate that one-quarter are threatened according to IUCN Red List criteria due to overfishing (targeted and incidental). Large-bodied, shallow-water species are at greatest risk and five out of the seven most threatened families are rays. Overall chondrichthyan extinction risk is substantially higher than for most other vertebrates, and only one-third of species are considered safe. Population depletion has occurred throughout the world's ice-free waters, but is particularly prevalent in the Indo-Pacific Biodiversity Triangle and Mediterranean Sea. Improved management of fisheries and trade is urgently needed to avoid extinctions and promote population recovery.
Understanding the movement ecology of marine species is important for conservation management and monitoring their responses to environmental change. In this study, adult and subadult bull sharks (Carcharhinus leucas; n = 16) were acoustically tagged in Biscayne Bay, Florida (USA), where they were tracked locally via an array of 40 passive acoustic receivers, as well as regionally via cooperative acoustic telemetry networks, with individuals tracked up to 4.5 years. Detection data were used to assess philopatry, regional connectivity, and environmental correlates of shark habitat use. Spatial range varied per individual; however, most individuals displayed high residency to Biscayne Bay, exhibiting strong philopa-tric behavior to the tagging area. A generalized linear mixed model revealed a seasonal pattern in habitat use, with mature females displaying high residency in Biscayne Bay during the colder dry season (November to February) and lower residencies during the warmer wet season (June to October). These seasonal patterns were supported by catch data from long-term fishery-independent shark surveys in the study area. During summer months when residencies of C. leucas declined in Biscayne Bay, their residencies increased in other regions (e.g., Florida Gulf Coast), demonstrative of seasonal migrations. Connectivity between areas of high use (Biscayne Bay and Florida Gulf Coast) was demonstrated by some individuals traveling between these areas. Results from generalized additive mixed models suggest that these movement patterns could be partially driven by seasonal changes in environmental variables as well as an individual's life stage, including reproductive status.
Overfishing is the primary cause of marine defaunation, yet declines in and increasing extinction risks of individual species are difficult to measure, particularly for the largest predators found in the high seas. Here we calculate two well-established indicators to track progress towards Aichi Biodiversity Targets and Sustainable Development Goals: the Living Planet Index (a measure of changes in abundance aggregated from 57 abundance time-series datasets for 18 oceanic shark and ray species) and the Red List Index (a measure of change in extinction risk calculated for all 31 oceanic species of sharks and rays). We find that, since 1970, the global abundance of oceanic sharks and rays has declined by 71% owing to an 18-fold increase in relative fishing pressure. This depletion has increased the global extinction risk to the point at which three-quarters of the species comprising this functionally important assemblage are threatened with extinction. Strict prohibitions and precautionary science-based catch limits are urgently needed to avert population collapse, avoid the disruption of ecological functions and promote species recovery.
Marine protected areas (MPAs) have emerged as potentially important conservation tools for the conservation of biodiversity and mitigation of climate impacts. Among MPAs, a large percentage has been created with the implicit goal of protecting shark populations, including 17 shark sanctuaries which fully protect sharks throughout their jurisdiction. The Commonwealth of the Bahamas represents a long-term MPA for sharks, following the banning of commercial longlining in 1993 and subsequent designation as a shark sanctuary in 2011. Little is known, however, about the long-term behavior and space use of sharks within this protected area, particularly among reef-associated sharks for which the sanctuary presumably offers the most benefit. We used acoustic telemetry to advance our understanding of the ecology of such sharks, namely Caribbean reef sharks (Carcharhinus perezi) and tiger sharks (Galeocerdo cuvier), over two discrete islands (New Providence and Great Exuma) varying in human activity level, over 2 years. We evaluated which factors influenced the likelihood of detection of individuals, analyzed patterns of movement and occurrence, and identified variability in habitat selection among species and regions, using a dataset of 23 Caribbean reef sharks and 15 tiger sharks which were passively monitored in two arrays with a combined total of 13 acoustic receivers. Caribbean reef sharks had lower detection probabilities than tiger sharks, and exhibited relatively low habitat connectivity and high residency, while tiger sharks demonstrated wider roaming behavior across much greater space. Tiger sharks were associated with shallow seagrass habitats where available, but frequently transited between and connected different habitat types. Our data support the notion that large MPAs afford greater degrees of protection for highly resident species such as Caribbean reef sharks, yet still may provide substantial benefits for more migratory species such as tiger sharks. We discuss these findings within the context of species-habitat linkages, ecosystem services, and the establishment of future MPAs.
As coastal urbanization increases globally, the subsequent effects on marine animals, especially endangered species, inhabiting nearshore waters have become a research priority. The smalltooth sawfish Pristis pectinata , once abundant in US waters, now only persists in a few parts of its former range, including South Florida. Many areas utilized by smalltooth sawfish are estuarine systems or other shallow coastal habitats, making this species particularly vulnerable to threats associated with coastal development. To date, P. pectinata has been understudied in the waters in and around Biscayne Bay, Florida, a coastal waterway subjected to the urbanization of adjacent Miami-Dade County. Here, we summarize data from reported smalltooth sawfish encounters dating as far back as 1895 (N = 90) and detail opportune recordings (incidental catches, acoustic detections, and baited remote underwater videos) of sub-adults and adults (N = 14 individuals) in Biscayne Bay and the adjacent reef tract. These data demonstrate historical and increased contemporary use of the study area by this imperiled species, suggesting potential local and regional recovery. Most documented sawfish occurrences were near the urban center, indicating a need to understand the effects of coastal urbanization on sawfish and on the species' recovery potential. We suggest priorities for future research on P. pectinata in the study area that will assist in addressing regional management goals and contribute to understanding the ecology of smalltooth sawfish under environmental change.
Understanding the factors that influence species’ distributions is crucial for implementing effective management and conservation practices, yet difficult for highly vagile species like sharks. Many shark species demonstrate either spatial and/or temporal sexual segregation, further confounding accurate quantification of habitat suitability. Given the importance of understanding spatiotemporal patterns in the distribution of coastal shark assemblages, we sought to quantify sex-specific abiotic factors that influence seasonal variation in a coastal shark assemblage using data from a long-term fisheries-independent bottom longline program in the northern Gulf of Mexico. Catch data (individuals/100 hooks/hour) were coupled with a suite of potentially predictive variables: surface and bottom values for temperature (°C) and salinity (psu), sea surface height (m), three-dimensional surface and bottom current velocity (u, v, w, in m/s), bottom dissolved oxygen (mg/l), depth (m), substrate grain size (mm), daylength (min), and distance from shore (km). Data were analyzed using boosted regression trees (BRT) to describe the relationships between catch data and environmental factors potentially influencing sex-specific species distribution and abundance. Between May 2006 and November 2018, we conducted 1,226 bottom longline sets and caught 13,742 individuals encompassing 67 species. The majority of the animals captured (74%) were elasmobranchs, primarily sharks. Two species from each of the following three categories were selected for further analyses: small coastal sharks (Atlantic sharpnose shark Rhizoprionodon terraenovae and blacknose shark Carcharhinus acronotus), large coastal sharks (blacktip shark C. limbatus and sandbar shark C. plumbeus), and shelf-associated sharks (smoothhound sharks, Mustelus spp. and scalloped hammerhead Sphyrna lewini). Depth and distance from shore were the strongest predictors of distribution and relative abundance, followed by longitude and bottom salinity; other factors (e.g., temperature, daylength, substrate grain size) were less predictive. For the six species examined, predictive factors were often the same for males and females, although the range of preferred values varied. Surprisingly, the importance of these predictors varied little across seasons. Collectively, our findings demonstrate that sexual segregation is the norm for sharks in the north-central Gulf of Mexico. Long-term fishery-independent monitoring to further quantify these sex-based differences in habitat use should be prioritized, particularly in light of impending climate change.
Abstract The recovery of communities of predatory fishes within a no‐take marine reserve after the eradication of illegal fishing provides an opportunity to examine the role of sharks and other large‐bodied mesopredatory fishes in structuring reef fish communities. We used baited remote underwater video stations to investigate whether an increase in sharks was associated with a change in structure of the mesopredatory fish community at Ashmore Reef, Western Australia. We found an almost fourfold increase in shark abundance in reef habitat from 0.64 hr−1 ± 0.15 SE in 2004, when Ashmore Reef was being fished illegally, to 2.45 hr−1 ± 0.37 in 2016, after eight years of full‐time enforcement of the reserve. Shark recovery in reef habitat was accompanied by a two and a half‐fold decline in the abundance of small mesopredatory fishes (≤50 cm TL) (14.00 hr−1 ± 3.79 to 5.6 hr−1 ± 1.20) and a concomitant increase in large mesopredatory fishes (≥100 cm TL) from 1.82 hr−1 ± 0.48 to 4.27 hr−1 ± 0.93. In contrast, near‐reef habitats showed an increase in abundance of large mesopredatory fishes between years (2.00 hr−1 ± 0.65 to 4.56 hr−1 ± 1.11), although only smaller increases in sharks (0.67 hr−1 ± 0.25 to 1.22 hr−1 ± 0.34) and smaller mesopredatory fishes. Although the abundance of most mesopredatory groups increased with recovery from fishing, we suggest that the large decline of small mesopredatory fish in reef habitat was mostly due to higher predation pressure following the increase in sharks and large mesopredatory fishes. At the regional scale, the structure of fished communities at Ashmore Reef in 2004 resembled those of present day Scott Reefs, where fishing still continues today. In 2016, Ashmore fish communities resembled those of the Rowley Shoals, which have been protected from fishing for decades.
Few studies have assessed whether patterns of resource partitioning among sympatric marine predators vary between different habitats. This type of data is important for understanding food web functioning as well as for supporting an ecosystem-based fisheries management plan. In this study, we used d 13 C and d 15 N stable isotope analysis to assess the relative isotopic niche area (i.e., which species has the largest and smallest isotopic niche area within a study site) and core isotopic niche overlap between 299 sharks of 11 co-occurring shark species that vary in size, diet and behavior in three coastal study areas in South Florida. Overall, results show that the relative isotopic niche area and patterns of core isotopic niche overlap between some sympatric shark species varied between sites, suggesting plasticity in resource use and competitive interactions between shark species (e.g., 85% of blacktip shark Carcharhinus limbatus standard ellipse area overlapped with the blacknose shark C. acronotus ellipse in Biscayne Bay, but there was no overlap between these species' ellipses in nearby Florida Bay). Therefore, patterns of resource use and trophic interactions documented among species from one site may not be applicable to a neighboring location. These findings may have implications for food web models that incorporate trophic data from different areas for a species when local data are unavailable.
Nurse sharks have not previously been known to migrate. Nurse sharks of the Dry Tortugas (DRTO) mating population have a highly predictable periodic residency cycle, returning to the Dry Tortugas Courtship and Mating Ground (DTCMG) annually (males) or bi- to triennially (females) during the June/July mating season. For 23 years we have followed the movements of 76 recaptured adults of a total of 115 tagged adults. Telemetry detections of 40 females tagged with acoustic transmitters show that most tagged and presumably post-partum females are continuously present in the DRTO in the fall, winter and early spring following the June mating season but these females depart in late March to early May. Detections reveal these females avoid the DTCMG completely during the next mating season, returning from late summer to fall. Telemetry records of nine of 17 adult males that co-habited with these females in the DTCMG depart DRTO waters every July. Both sexes may overwinter in the DRTO. Between 2011 and 2016 three males and five females with transmitters were detected to move up the west coast of Florida outside of the mating season as far north as the waters off Tampa Bay (335 km). Six others were only detected in the lower Florida Keys (292 km). Nine sharks returned to DRTO; one returned six times. Some overwintered and some resumed courtship in June, demonstrating both resident and migratory contingents within their population, partial migration and an ability to navigate with high spatial and temporal precision.
Predation is one of the most fundamental and unifying concepts in ecology, and we are beginning to obtain a more complete understanding of how predators drive community structure and ecosystem function through their impacts on prey. We know considerably less about how predators affect each other through intraguild interactions, which is surprising considering predators often occur simultaneously and may compete for resources while avoiding being killed themselves. In the present study, we examined aspects of inter- and intra-specific resource use among three species of large-bodied predatory sharks (blacktip, bull, lemon) co-occurring within a subtropical, protected bay in the southeastern USA. Specifically, we inferred relative trophic position, isotopic niche overlap, and patterns of resource use of sharks using stable isotope analysis of carbon-13 and nitrogen-15 from blood and fin cartilage samples. We also combined these approaches with estimates of abundance and occurrence from empirical shark surveys to consider whether these species may exhibit resource partitioning in space and time. We found that all three species overlapped in space, and there was some isotopic niche overlap between the species. We also found evidence of temporal isotopic niche stability, suggesting that co-occurring shark species may compete for available prey resources, but individuals of those species may have similar patterns of resource use over time. We discuss our findings as they relate to the ecologies of the species in question and how sound conservation and management of ecosystems can allow for predator diversity, sympatry, and stable use of resources at the top of the food chain.
Relative abundance of many shark species in the Atlantic is assessed by compiling data from several independently conducted, but somewhat spatially limited surveys. Although these localized surveys annually sample the same populations, resulting trends in yearly indices often conflict with one another, thereby hindering interpretation of abundance patterns at broad spatial scales. We used delta-lognormal generalized linear models (GLMs) to generate indices of abundance for seven Atlantic coastal shark species from six fishery-independent surveys along the US east coast and Gulf of Mexico from 1975 to 2014. These indices were further analysed using dynamic factor analysis (DFA) to produce simplified, broad-scale common trends in relative abundance over the entire sampled distribution. Effects of drivers including the North Atlantic Oscillation index, the Atlantic Multidecadal Oscillation index, annually averaged sea surface temperature and species landings were evaluated within the DFA model. The two decadal oscillations and species landings were shown to affect shark distribution along south-east US coast. Estimated common trends of relative abundance for all large coastal shark species showed similar decreasing patterns into the early 1990s, periods of sustained low index values thereafter and recent indications of recovery. Small coastal shark species exhibited more regional variability in their estimated common trends, such that two common trends were required to adequately describe patterns in relative abundance throughout the Gulf of Mexico and Atlantic. Overall, all species’ (except the Gulf of Mexico blacknose shark) time series concluded with an increasing trend, suggestive of initial recovery from past exploitation.
A thorough understanding of movement patterns of a species is critical for designing effective conservation and management initiatives. However, generating such information for large marine vertebrates is challenging, as they typically move over long distances, live in concealing environments, are logistically difficult to capture and, as upper-trophic predators, are naturally low in abundance. Large-bodied, broadly distributed tropical shark typically restricted to coastal and shelf habitats, the great hammerhead shark Sphyrna mokarran epitomizes such challenges. Highly valued for its fins (in target and incidental fisheries), it suffers high bycatch mortality coupled with fecundity conservative life history, and as a result, is vulnerable to over-exploitation and population depletion. Although there are very little species-specific data available, the absence of recent catch records give cause to suspect substantial declines across its range. Here, we used biotelemetry techniques (acoustic and satellite), conventional tagging, laser-photogrammetry, and photo-identification to investigate the level of site fidelity/residency for great hammerheads to coastal areas in the Bahamas and U.S., and the extent of movements and connectivity of great hammerheads between the U.S. and Bahamas. Results revealed large-scale return migrations (3030 km), seasonal residency to local areas (some for 5 months), site fidelity (annual return to Bimini and Jupiter for many individuals) and numerous international movements. These findings enhance the understanding of movement ecology in great hammerhead sharks and have potential to contribute to improved conservation and management.
Coastal shark populations have been subject to increasing anthropogenic pressure over the past two decades. This study focused on two lemon shark (Negaprion brevirostris) populations, the site-attached maturing sub-adults at the Island of Bimini, Bahamas, threatened by a large-scale resort development, and adults forming winter aggregations off the coast of Jupiter, Florida, subject to direct fishing pressure. For the sub-adult population, analysis was carried out on the long-term temporal patterns in abundance and population structure, relative to potential driving factors, and the influence of variables affecting longline catch-rates used as the basis for stock assessment. For the aggregating adult population, life-history aspects of population structure and distribution were investigated for relative implications on the species' vulnerability status. The following analysis and methodologies were utilised to investigate the two populations: longline catch records from 1982 – 2008; monitoring of variables potentially affecting longline catch-rates; documentation of shark behavioural interactions with longline equipment using underwater video surveillance; aerial surveying for abundance estimates; comparison of spatial utilisation patterns with longline catch locations; external tagging; the utilisation of archival satellite tags; passive tracking with Vemco acoustic monitoring system and research collaborations with other scientific groups utilising the same acoustic monitoring system. The key findings of this study were that in the northwest Atlantic, N. brevirostris populations are experiencing considerable anthropogenic pressure at all life-stages. In Bimini, the effects of a large-scale resort development have resulted in a significant decline in abundance, to a level (~52 individuals) well below the temporal average (~158). On the U.S. east coast, seasonal aggregating behaviour has further increased Steven Kessel Ph.D Thesis ii vulnerability through increased catchability, beyond the highly vulnerable status already attributed to this species, and targeted N. brevirostris fisheries appear to be currently operating at unsustainable levels. Shark longline catchability was noted to be significantly influenced by multiple shark presence, resulting in greater susceptibility for N. brevirostris (and other similar species) that naturally exhibit group behaviour. Incidental encounterability and predation risk significantly influenced longline catch-rates. Adult N. brevirostris exhibited large-scale seasonal migrations on the U.S. east coast, which, in addition to documented international transitions, supports existing evidence for genetic mixing across the distribution. Water temperature was found to be a significant driver of N. brevirostris behaviour at all life-stages, with an apparent adult temperature preference of ~24°C. This study represents the first long-term abundance assessment for sub-adult N. brevirostris, and the first in-depth study to focus on an adult N. brevirostris population. The results provide essential life-history information, revealing that at all life-stages N. brevirostris appear to be highly sensitive to anthropogenic activities, relative to other species, and therefore require enhanced management for species protection. It is therefore highly recommended that N. brevirostris be added to the U.S. prohibited species list.
The response to capture is important in fisheries because it can reveal potential threats to species beyond fishing mortalities resulting from direct harvest. To date, the vast majority of studies assessing shark stress responses have used physiology or biotelemetry to look at sensitivity after capture, leaving a gap in our understanding of the behaviours of sharks during capture. We examined the behavioural responses of sharks to capture by attaching accelerometers to fishing gear and measuring the immediate and prolonged forces they exerted while on the line. We recorded acceleration vectors and derived the rate of intense fighting behaviours of 23 individual sharks comprising three species. Results suggest that blacktip sharks (Carcharhinus limbatus) exhibited intense bouts of fighting behaviour at the onset of hooking, while nurse (Ginglymostoma cirratum) and tiger sharks (Galeocerdo cuvier) displayed more subdued acceleration values during capture. We also obtained plasma lactate from a subset of individuals and detected a strong correlation with maximum acceleration. These results align with previously published values and suggest that shark movement during fisheries capture is an important factor during bycatch and catch-and-release interactions.
Evidence suggests the great hammerhead shark, Sphyrna mokarran,
is vulnerable to a variety of anthropogenic stressors, and is an
understudied species of shark due to its cryptic nature and wideranging
movements. While recognized as both a pelagic-coastal and
a highly mobile predator, minimal anecdotal evidence exist describing
shallow water habitat use by this species. This report describes six
cases in which a great hammerhead shark utilizes an inshore shallow
water flats environment (<1.5 m in depth), five of which involve prey
capture. These observations permitted identification of two novel
behaviors that may allow great hammerheads to inhabit these
shallow habitats: a (1) prey-capture technique termed ‘grasp-turning’
that involves burst swimming at tight turning angles while grasping
prey and (2) a post-predation recovery period whereby the shark
maintains head-first orientation into the current that may facilitate
respiration and prey consumption. These behavioral observations
provide insights into the natural history of this species.
Southeast Florida witnesses an enormous seasonal influx of upper trophic level marine predators each year as massive aggregations of migrating blacktip sharks (Carcharhinus limbatus) overwinter in nearshore waters. The narrow shelf and close proximity of the Gulf Stream current to the Palm Beach County shoreline drive tens of thousands of sharks to the shallow, coastal environment. This natural bottleneck provides a unique opportunity to estimate relative abundance. Over a four year period from 2011-2014, an aerial survey was flown approximately biweekly along the length of Palm Beach County. A high definition video camera and digital still camera mounted out of the airplane window provided a continuous record of the belt transect which extended 200 m seaward from the shoreline between Boca Raton Inlet and Jupiter Inlet. The number of sharks within the survey transect was directly counted from the video. Shark abundance peaked in the winter (January-March) with a maximum in 2011 of 12,128 individuals counted within the 75.6 km-2 belt transect. This resulted in a maximum density of 803.2 sharks km-2. By the late spring (April-May), shark abundance had sharply declined to 1.1% of its peak, where it remained until spiking again in January of the following year. Shark abundance was inversely correlated with water temperature and large numbers of sharks were found only when water temperatures were less than 25°C. Shark abundance was also correlated with day of the year but not with barometric pressure. Although shark abundance was not correlated with photoperiod, the departure of the sharks from southeast Florida occurred around the vernal equinox. The shark migration along the United States eastern seaboard corresponds spatially and temporally with the spawning aggregations of various baitfish species. These baseline abundance data can be compared to future studies to determine if shark population size is changing and if sharks are restricting their southward migration as global water temperatures increase.
Study aim and location: Many populations of highly mobile marine fishes, including large sharks, are experiencing declines. The benefits of spatial management zones, such as marine protected areas (MPAs), for such animals are unclear. To help fill this knowledge gap, we examined core habitat use areas (CHUAs) for bull (Carcharhinus leucas), great hammerhead (Sphyrna mokarran) and tiger sharks (Galeocerdo cuvier) in relation to specific MPAs and exclusive economic zones (EEZs) in the western North Atlantic Ocean.
Methods: Bull, great hammerhead and tiger sharks (N = 86 total) were satellite tagged and tracked in southern Florida and the northern Bahamas between 2010 and 2013. Filtered and regularized positions from Argos locations of tag transmissions were used to generate CHUAs for these sharks. Overlaps of CHUAs with regional protected areas and exclusive economic management zones were quantified to determine the proportion of each tracked shark’s CHUA under spatial protection from exploitation.
Results: A total of 0%, 17.9% and 34.7% of the regional CHUAs for tracked bull, great hammerhead and tiger sharks, respectively, were fully protected from exploitation in the study area.
Main conclusions: Expansion of protected areas to include U.S. territorial waters would effectively protect 100% of the CHUAs for all tracked sharks in the study area. This finding is particularly significant for great hammerhead sharks, which are currently overfished, vulnerable to bycatch mortality and are the focus of strident regional conservation efforts. These findings also provide a
means to inform decision makers and marine conservation planning efforts as to the types of management actions available and potential efficacy of spatial protections for these marine predators.
Non-consumptive or risk effects imposed by predators can influence prey behaviour over different spatio-temporal scales. Prey vulnerability to predation can also be dependent on abiotic conditions, such as tidal height. We conducted direct field observations of juvenile lemon sharks Negaprion brevirostris in a tidally influenced mangrove-inlet. We also used acoustic tracking to determine the movement patterns of juvenile lemon sharks and their predators (sub-adult lemon sharks) across the tidal cycle. Results showed that greater numbers of juvenile lemon sharks used the mangrove-inlet for longer time periods at deeper and warmer high tide depths. This coincided with an increased presence of potential predators (sub-adult lemon sharks) in the surrounding areas. Furthermore, in accordance with body-size dependent anti-predatory investment, smaller juvenile lemon sharks visited the mangrove inlet more often, spent longer there and left latest on average. Our acoustic tracking data also revealed a tidally-influenced pattern, with both juvenile and sub-adult lemon sharks detected at locations inshore over the high tide and offshore during the low tide. We concluded that the mangrove lake served as a 'refuge' for juvenile lemon sharks over the high tide, providing safe habitat when inshore areas become accessible to large predators, such as sub-adult lemon sharks. We suggest that these decisions are updated through ontogeny and also with daily fluctuations in abiotic factors, such as water depth. This study provides evidence for how intra-specific predator-prey interactions in a top predator species influence juvenile habitat selection, with potential implications for population structure and regulation.
Understanding how and why animals are distributed through time and space has always been a fundamental component of ecology and is an essential prerequisite for effective conservation and/or management. However, for highly mobile K-selected species, behavioural predictability is rarely considered over appropriate scales relative to life history. To address this point, a multidisciplinary approach combining telemetry, external tagging, physical assessment, environmental monitoring and genetic analysis was adopted to determine a spatial framework for the movements of adult lemon sharks Negaprion brevirostris at multiple spatial and temporal scales from 2007 to 2011. Lemon sharks (n = 83) were tracked with passive acoustic telemetry, revealing a winter residency in the southeast Florida region. Detections from individuals recorded within the core winter habitat for >20 d (n = 56) were incorporated into generalized linear mixed-effects models to investigate the influence of water temperature, photoperiod, moon phase, month and year on presence. The findings of this study suggest a temperature driven ‘migration-residency’ model for mature lemon shark distribution across the USA eastern seaboard. Lemon sharks are distributed across a wide geographical area in the summer months and migrate south concentrating off southeast Florida in the winter, with this pattern repeated each year. From comparative genetic analysis and the absence of any evidence of mating behaviour during the winter residency period, mating and parturition most probably occur in May/June between northern Florida and the Carolinas. This study highlights the importance of determining the specific dynamics and proximate causes of animal movement and distribution over appropriate spatial and temporal scales relative to life history.
Resolving the geographic extent and timing of coastal shark migrations, as well as their environmental cues, is essential for refining shark management strategies in anticipation of increasing anthropogenic stressors to coastal ecosystems. We employed a regional-scale passive acoustic telemetry array encompassing 300 km of the east Florida coast to assess what factors influence site fidelity of juvenile lemon sharks (Negaprion brevirostris) to an exposed coastal nursery at Cape Canaveral, and to document the timing and rate of their seasonal migrations. Movements of 54 juvenile lemon sharks were monitored for three years with individuals tracked for up to 751 days. While most sharks demonstrated site fidelity to the Cape Canaveral region December through February under typical winter water temperatures, historically extreme declines in ocean temperature were accompanied by rapid and often temporary, southward displacements of up to 190 km along the Florida east coast. From late February through April each year, most sharks initiated a northward migration at speeds of up to 64 km day(-1) with several individuals then detected in compatible estuarine telemetry arrays in Georgia and South Carolina up to 472 km from release locations. Nineteen sharks returned for a second or even third consecutive winter, thus demonstrating strong seasonal philopatry to the Cape Canaveral region. The long distance movements and habitat associations of immature lemon sharks along the US southeast coast contrast sharply with the natal site fidelity observed in this species at other sites in the western Atlantic Ocean. These findings validate the existing multi-state management strategies now in place. Results also affirm the value of collaborative passive arrays for resolving seasonal movements and habitat preferences of migratory coastal shark species not easily studied with other tagging techniques.
The American Fisheries Society (AFS) recommends that regulatory agencies give shark and ray management high priority because of the naturally slow population growth inherent to most sharks and rays, and their resulting vulnerability to overfishing and stock collapse. Fisheries managers should be particularly sensitive to the vulnerability of less productive species of sharks and rays taken as a bycatch in mixed-species fisheries. The AFS encourages the development and implementation of management plans for sharks and rays in North America. Management practices including regulations, international agreements and treaties should err on the side of the health of the resource rather than short-term economic gain. The AFS encourages the release of sharks and rays taken as bycatch in a survivable condition. Regulatory agencies should mandate full utilization of shark carcasses and prohibit the wasteful practice of finning. Multilateral agreements among fishing nations, or management through regional fisheries management organizations are sorely needed for effective management of wide ranging shark stocks. The AFS encourages its members to become involved by providing technical information needed for protection of sharks and rays to international, federal, state, and provincial policy makers so decisions are made on a scientific, rather than emotional or political, basis.
We conducted a two-year gillnet survey of the relative abundance, species diversity, and reproductive condition of sharks from southeastern North Carolina coastal waters. Sixteen shark species were recorded of which Rhizoprionodon terraenovae (Richardson, 1836) was the most common, representing >80% of the total catch in both years. Large coastal species such as Carcharhinus limbatus (Müller and Henle, 1839) and Carcharhinus plumbeus (Nardo, 1827) were caught infrequently, representing 1% of the total catch. Juvenile, or sub-adult size classes in the total catch were dominated by Carcharhinus brevipinna (Müller and Henle, 1839), Carcharhinus obscurus (LeSueur, 1818), C. plumbeus, and Carcharias taurus Rafinesque, 1810. Sub-adult, or adult size classes in the total catch were dominated by Sphyrna tiburo (Linnaeus, 1758), C. limbatus, Carcharhinus isodon (Müller and Henle, 1839), and Carcharhinus acronotus (Poey, 1860). The smallest mature specimen of R. terraenovae, C. limbatus (female) and C. isodon were smaller than previously documented minimum sizes at maturity. Based on the association of gravid females present in catches with neonates, or small juveniles of the same species, southeastern North Carolina was identified as a primary and/or secondary nursery ground for R. terraenovae , C. brevipinna, C. acronotus and C. obscurus during the months of investigation. Our data also suggest that C. limbatus and Sphyrna lewini (Griffith and Smith, 1834) use southeastern North Carolina as a nursery ground, although more infrequently. Similarly, C. plumbeus uses southeastern North Carolina as an over-wintering ground and neonates are reported infrequently. Almost 100% mature female R. terraenovae and S. tiburo were gravid; most were caught from AugustOctober. Embryos of S. tiburo, based on growth rate extrapolation, were at documented parturition size by September.
The elasmobranch fauna of Everglades National Park was studied using longline, gillnet, and rod and reel surveys. Thirteen species of elasmobranchs were identified including three species not previously reported in the park. Species richness was highest in the areas with the greatest influence of marine waters from the Gulf of Mexico and lower in estuarine areas and those subject to periods of hypersalinity. Most elasmobranch species were recorded as juveniles, with at least three species occurring as neonates, and there were few adults of any species. Electivity indices for salinity, temperature, and depth were calculated for Carcharhinus leucas (Valenciennes, 1841), Carcharhinus limbatus (Valenciennes, 1841), Ginglymostoma cirratum (Bonnaterre, 1788), and Negaprion brevirostris (Poey, 1868), and indicated possible habitat partitioning based on these environmental characteristics. Tag-recapture data suggested that N. brevirostris may have a high level of residency and probably remains inside the park for long periods, while all other species showed movements into and out of the park on a regular basis. Results demonstrate the utility of the park for the conservation and management of elasmobranch species and the need to consider how future changes to the environment will affect this important group of predators.
Predators can impact ecosystems through trophic cascades such that differential patterns in habitat use can lead to spatiotemporal variation in top down forcing on community dynamics. Thus, improved understanding of predator movements is important for evaluating the potential ecosystem effects of their declines.
We satellite-tagged an apex predator (bull sharks, Carcharhinus leucas) and a sympatric mesopredator (Atlantic tarpon, Megalops atlanticus) in southern Florida waters to describe their habitat use, abundance and movement patterns. We asked four questions: (1) How do the seasonal abundance patterns of bull sharks and tarpon compare? (2) How do the movement patterns of bull sharks and tarpon compare, and what proportion of time do their respective primary ranges overlap? (3) Do tarpon movement patterns (e.g., straight versus convoluted paths) and/or their rates of movement (ROM) differ in areas of low versus high bull shark abundance? and (4) Can any general conclusions be reached concerning whether tarpon may mitigate risk of predation by sharks when they are in areas of high bull shark abundance?
Despite similarities in diet, bull sharks and tarpon showed little overlap in habitat use. Bull shark abundance was high year-round, but peaked in winter; while tarpon abundance and fishery catches were highest in late spring. However, presence of the largest sharks (>230 cm) coincided with peak tarpon abundance. When moving over deep open waters (areas of high shark abundance and high food availability) tarpon maintained relatively high ROM in directed lines until reaching shallow structurally-complex areas. At such locations, tarpon exhibited slow tortuous movements over relatively long time periods indicative of foraging. Tarpon periodically concentrated up rivers, where tracked bull sharks were absent. We propose that tarpon trade-off energetic costs of both food assimilation and osmoregulation to reduce predation risk by bull sharks.
We evaluate if the distribution and abundance ofThalassia testudinum, Syringodium filiforme, andHalodule wrightii within Biscayne Bay, Florida, are influenced by salinity regimes using, a combination of field surveys, salinity exposure
experiments, and a seagrass simulation model. Surveys conducted in June 2001 revealed that whileT. testudinum is found throughout Biscayne Bay (84% of sites surveyed),S. filiforme andH wrightii have distributions limited mainly to the Key Biscayne area.H. wrightii can also be found in areas influenced by canal discharge. The exposure of seagrasses to short-term salinity pulses (14 d,
5–45‰) within microcosms showed species-specific susceptibility to the salinity treatments. Maximum growth rates forT testudinum were observed near oceanic salinity values (30–40‰) and lowest growth rates at extreme values (5‰ and 45‰).S. filiforme was the most susceptible seagrass species; maximum growth rates for this species were observed at 25‰ and dropped dramatically
at higher and lower salinity.H. wrightii was the most tolerant, growing well at all salinity levels. Establishing the relationship between seagrass abundance and
distribution and salinity is especially relevant in South Florida where freshwater deliveries into coastal bays are influenced
by water management practices. The seagrass model developed by Fong and Harwell (1994) and modified here to include a shortterm
salinity response function suggests that freshwater inputs and associated decreases in salinity in nearshore areas influence
the distribution and growth of single species as well as modify competitive interactions so that species replacements may
occur. Our simulations indicate that although growth rates ofT. testudinum decrease when salinity is lowered, this species can still be a dominant component of nearshore communities as confirmed by
our surveys. Only when mean salinity values are drastically lowered in a hypothetical restoration scenario isH. wrightii able to outcompeteT. testudinum.
Biscayne Bay is a naturally clear-water bay that spans the length of Miami-Dade County, Florida, USA. It is bordered on the
east by barrier islands that include Miami Beach and is an almost completely urban bay in the north and a relatively natural
bay in the south. Planned water management changes in the next few years may decrease freshwater flows to the bay from present
sources, while offering reclaimed wastewater in return. In addition, a project is planned to restore the former diffuse freshwater
flow to the bay through many small creeks crossing coastal wetlands by redistributing the water that now flows into the bay
through several large canals. To guide a science-based, adaptive-management approach to water-management planning, a conceptual
ecological model of Biscayne Bay was developed based upon a series of open workshops involving researchers familiar with Biscayne
Bay. The CEM model relates ecological attributes of the bay to outside forcing functions, identified as water management,
watershed development, and sea-level rise. The model depicts the effects of these forcing functions on the ecological attributes
of the bay through four stressors. The hypothesized pathways of these effects include salinity patterns, water quality, sediment
contaminant concentrations, and physical impacts. Major research questions were identified with regard to uncertainties explicit
in the model. The issues addressed include, for example (1) the quantitative relationship between upstream water management,
rainfall, and flow into Biscayne Bay; (2) the salinity gradient required to restore the historical estuarine fish community;
(3) the potential effect of freshwater inputs on benthic habitats; (4) the effect of introduced nutrient and contaminant loads,
including the effects of reclaimed wastewater.
In recent decades, large pelagic and coastal shark populations have declined dramatically with increased fishing; however, the status of sharks in other systems such as coral reefs remains largely unassessed despite a long history of exploitation. Here we explore the contemporary distribution and sighting frequency of sharks on reefs in the greater-Caribbean and assess the possible role of human pressures on observed patterns.
We analyzed 76,340 underwater surveys carried out by trained volunteer divers between 1993 and 2008. Surveys were grouped within one km2 cells, which allowed us to determine the contemporary geographical distribution and sighting frequency of sharks. Sighting frequency was calculated as the ratio of surveys with sharks to the total number of surveys in each cell. We compared sighting frequency to the number of people in the cell vicinity and used population viability analyses to assess the effects of exploitation on population trends. Sharks, with the exception of nurse sharks occurred mainly in areas with very low human population or strong fishing regulations and marine conservation. Population viability analysis suggests that exploitation alone could explain the large-scale absence; however, this pattern is likely to be exacerbated by additional anthropogenic stressors, such as pollution and habitat degradation, that also correlate with human population.
Human pressures in coastal zones have lead to the broad-scale absence of sharks on reefs in the greater-Caribbean. Preventing further loss of sharks requires urgent management measures to curb fishing mortality and to mitigate other anthropogenic stressors to protect sites where sharks still exist. The fact that sharks still occur in some densely populated areas where strong fishing regulations are in place indicates the possibility of success and encourages the implementation of conservation measures.
Marine fish movement plays a critical role in ecosystem functioning and is increasingly studied with acoustic telemetry. Traditionally, this research has focused on single species and small spatial scales. However, integrated tracking networks, such as the Integrated Tracking of Aquatic Animals in the Gulf of Mexico (iTAG) network, are building the capacity to monitor multiple species over larger spatial scales. We conducted a synthesis of passive acoustic monitoring data for 29 species (889 transmitters), ranging from large top predators to small consumers, monitored along the west coast of Florida, USA, over 3 yr (2016-2018). Space use was highly variable, with some groups using all monitored areas and others using only the area where they were tagged. The most extensive space use was found for Atlantic tarpon Megalops atlanticus and bull sharks Carcharhinus leucas . Individual detection patterns clustered into 4 groups, ranging from occasionally detected long-distance movers to frequently detected juvenile or adult residents. Synchronized, alongshore, long-distance movements were found for Atlantic tarpon, cobia Rachycentron canadum , and several elasmobranch species. These movements were predominantly northbound in spring and southbound in fall. Detections of top predators were highest in summer, except for nearshore Tampa Bay where the most detections occurred in fall, coinciding with large red drum Sciaenops ocellatus spawning aggregations. We discuss the future of collaborative telemetry research, including current limitations and potential solutions to maximize its impact for understanding movement ecology, conducting ecosystem monitoring, and supporting fisheries management.
Many elasmobranch species utilize estuaries as nurseries, parturition areas, and foraging grounds. Florida’s Indian River Lagoon (IRL), an “estuary of national significance,” has experienced many anthropogenic impacts in recent decades, such as habitat degradation and declining water quality, and there is a substantial data gap surrounding the status of elasmobranchs in this system. A fishery-independent survey (longline/gillnet) was implemented to characterize the elasmobranch community and understand distribution patterns and habitat use in the IRL (Sebastian to St. Lucie Inlet). From July 2016 to June 2018, 630 individuals of 16 species were caught and tagged, including two critically endangered smalltooth sawfish Pristis pectinata. Bull sharks Carcharhinus leucas and Atlantic stingrays Hypanus sabinus were the two most common species collected (47% of the total catch), and size differences by region were observed. The longline catch exhibited a significant difference in species composition among regions while the gillnet catch composition significantly varied among seasons. Although dependent on survey gear type, there was evidence of combinations of abiotic parameters (e.g., depth, salinity, water clarity, distance to a freshwater source, distance to an inlet) driving elasmobranch species composition. Bull sharks and Atlantic stingrays dominated areas with frequently low salinities while more diverse assemblages of species were apparent towards inlet passes. This study provides the first in-depth analysis of the elasmobranch community in the IRL and develops capacity to understand how these species may respond to further environmental changes in this highly impacted estuary.
Arguments for the need to conserve aquatic predator (AP) populations often focus on the ecological and socioeconomic roles they play. Here, we summarize the diverse ecosystem functions and services connected to APs, including regulating food webs, cycling nutrients, engineering habitats, transmitting diseases/parasites, mediating ecological invasions, affecting climate, supporting fisheries, generating tourism, and providing bioinspiration. In some cases, human-driven declines and increases in AP populations have altered these ecosystem functions and services. We present a social ecological framework for supporting adaptive management decisions involving APs in response to social and environmental change. We also identify outstanding questions to guide future research on the ecological functions and ecosystem services of APs in a changing world.
Highly migratory species (e.g. sharks, tunas, turtles, cetaceans) present unique conservation
management challenges due to their wide-ranging movements. Consequently, the
extent to which management areas protect habitats for highly migratory species is often unknown. Within the southeast region of the USA’s exclusive economic zone, highly migratory sharks are target and/or bycatch species in pelagic and bottom longline fisheries. Here, we developed maximum entropy habitat suitability models for great hammerhead sharks Sphyrna mokarran, tiger sharks Galeocerdo cuvier, and bull sharks Carcharhinus leucas within the southeast region based on satellite tag (n = 96) and remotely sensed environmental data. Modeled highly suitable habitats were compared to longline gear management areas to determine what proportion of these habitats are protected from, and vulnerable to, longline fisheries. The percentages of highly suitable habitats overlapping with longline management areas varied by species and season (78% warm, 36% cool season for great hammerhead sharks; 48% warm, 79% cool for tiger sharks; and 2% warm, 100% cool for bull sharks). Highly suitable great hammerhead and tiger shark habitats were relatively well protected from pelagic longline fisheries yet vulnerable to bottom longline fisheries. Additionally, both species were vulnerable to pelagic and bottom longline fisheries off southwestern Florida; thus, extending gear restrictions to this area may benefit both species. Bull shark highly suitable habitats were only well protected from longline gear during the cool season. These results demonstrate how habitat suitability modeling can be used to help assess the efficacy of spatial management strategies and inform conservation plans for highly migratory species.
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.
In both commercial and recreational fisheries, sharks are captured and released alive to comply with regulations or due to low economic value or voluntary conservation ethic. As a result, understanding the physiological and behavioural responses of sharks to capture stress is important for determining subsequent effects of fisheries interactions on a species-specific basis, as well as for identifying factors that influence mortality. Here, we employed a suite of conventional blood physiology endpoints (glucose, lactate, and haematocrit) integrated with assessments of reflex impairment on blacktip (Carcharhinus limbatus), great hammerhead (Sphyrna mokarran), nurse (Ginglymostoma cirratum) and sand-bar sharks (Carcharhinus plumbeus) captured via experimental drumline gear. We documented a wide range of species-specific differences in all parameters assessed, with nurse sharks consistently having the lowest relative levels of physiological disturbance and reflex impairment; and with great hammerheads exhibiting the highest level of physiological disturbance and reflex impairment, suggesting higher vulnerability to fishing. In general, increases in lactate were positively associated with hook time and correlated with reflex impairment assessment. Moreover, reflex indices showed significant impairment with hook time, with the "jaw" reflex emerging as the most potential predictor of disturbance. Our study results connect previously reported species-specific at-vessel and post-release mortality rates to their physiological disturbance and reflex impairment.
There is increasing concern for the conservation of sharks among scientists, environmental conservation advocates, and the interested public, but misunderstanding among policy non-specialists about which conservation and management policies are available, and which might work best for certain situations, persists. Here we present a review of fisheries management and conservation literature relating to sharks. Policies are broadly divided into target-based policies that aim for sustainable fisheries exploitation (e.g. fisheries quotas) and limit-based policies that aim to prevent all fisheries exploitation of entire taxa (e.g. marine reserves). A list of the pros and cons of each policy is included, as is a decision tree to aid in selection of the most appropriate policy. Our goal is that this paper will allow policy non-specialists, including scientists without policy training, environmental activists, and concerned citizens, to make informed decisions when advocating for shark conservation.
In many fisheries, some component of the catch is usually released. Quantifying the
effects of capture and release on fish survival is critical for determining which practices are sustainable,
particularly for threatened species. Using a standardized fishing technique, we studied
sublethal (blood physiology and reflex impairment assessment) and lethal (post-release mortality
with satellite tags) outcomes of fishing stress on 5 species of coastal sharks (great hammerhead,
bull, blacktip, lemon, and tiger). Species-specific differences were detected in whole blood
lactate, partial pressure of carbon dioxide, and pH values, with lactate emerging as the sole
parameter to be significantly affected by increasing hooking duration and shark size. Species-specific
differences in reflex impairment were also found; however, we did not detect any significant
relationships between reflex impairment and hooking duration. Taken together, we ranked each
species according to degree of stress response, from most to least disturbed, as follows: hammerhead
shark > blacktip shark > bull shark > lemon shark > tiger shark. Satellite tagging data
revealed that nearly 100% of all tracked tiger sharks reported for at least 4 wk after release, which
was significantly higher than bull (74.1%) and great hammerhead (53.6%) sharks. We discuss
which mechanisms may lead to species-specific differences in sensitivity to fishing and suggest
that observed variation in responses may be influenced by ecological and evolutionary phenomena.
Moreover, our results show that certain species (i.e. hammerhead sharks in this study) are
inherently vulnerable to capture stress and mortality resulting from fisheries interactions and
should receive additional attention in future conservation strategies.
The decomposition of the mangrove Rhizophora mangle and the seagrass Thalassia testudinum was examined using litterbags along a natural gradient in nutrient availability. Seagrass leaves had a higher fraction of their biomass in the labile pool (57%), compared to mangrove leaves (36%) and seagrass rhizomes (29%); the overall decomposition rates of the starting material reflected the fractionation into labile and refractory components. There was no relationship between the N or P content of the starting material and the decomposition rate.Nutrient availability had no influence on decomposition rate, and mass was lost at the same rate from litterbags that were buried in the sediment and litterbags that were left on the sediment surface. The dynamics of N and P content during decomposition varied as a function of starting material and burial state. N content of decomposing mangrove leaves increased, but seagrass rhizomes decreased in N content during decomposition while there was no change in seagrass leaf N content. These same general patterns held for P content, but buried seagrass leaves increased in P content while surficial leaves decreased. δC and δN changed by as much as 2‰ during decomposition.
Sharks are top predators in many marine ecosystems. Despite recent concerns over declines in shark populations, studies of shark communities in coastal habitats are limited. We used drumlines and longlines to determine shark community composition and habitat affinities in the Florida Keys, USA. Community composition varied among habitats. Catch rates of smaller sharks were highest in protected shallow waters, while large sharks were more abundant in deep channels. Overall probabilities of catching large sharks on drumlines did not vary with water temperature, while catches of small sharks on longlines increased with increasing water temperature. Individual species differed in their responsiveness to variation in water temperatures and habitat. Bait type affected catch rates of some species, suggesting that fishing methods should be considered explicitly in studies describing shark communities or temporal trends in abundance. Catch rates of large-bodied sharks were higher in a remote and protected location compared with similar habitats near inhabited Keys. Also, historical accounts of a shark fishery in the Study area during the 1920s suggest substantial declines in large shark abundance and shifts in community composition. By implication, ecosystem impacts of changes in the large shark community may be dramatic and likely occurred before adequate baselines were established.
Florida Bay is a unique subtropical estuary at the southern tip of the Florida peninsula. Recent ecological changes (seagrass
die-off, algal blooms, increased turbidity) to the Florida Bay ecosystem have focused the attention of the public, commercial
interests, scientists, and resource managers on the factors influencing the structure and function of Florida Bay. Restoring
Florida Bay to some historic condition is the goal of resource managers, but what is not clear is what an anthropogenically-unaltered
Florida Bay would look like. While there is general consensus that human activities have contributed to the changes occurring
in the Florida Bay ecosystem, a high degree of natural system variability has made elucidation of the links between human
activity and Florida Bay dynamics difficult. Paleoecological analyses, examination of long-term datasets, and directed measurements
of aspects of the ecology of Florida Bay all contribute to our understanding of the behavior of the bay, and allow quantification
of the magnitude of the recent ecological changes with respect to historical variability of the system.
Ecological risk assessments (ERAs) are employed to quantify and predict the vulnerability of a particular species, stock or population to a specific stressor, e.g. pollution, harvesting, climate change, by-catch. Data generated from ERAs are used to identify and prioritize species for implementation of effective conservation and management strategies. At this time, ERAs are of particular importance to elasmobranchs, given the ecological importance and documented global population declines of some elasmobranch species. Here, ERAs as a tool for elasmobranch conservation and management are reviewed and a theoretical roadmap provided for future studies. To achieve these goals, a brief history of ERAs and approaches used within them (in the context of elasmobranchs) are given, and a comprehensive review conducted of all ERA studies associated with elasmobranchs published between 1998 and 2011. The hazards assessed, species evaluated and methodological approaches taken are recorded. Chronological and geographical patterns suggest that this tool has grown in popularity as a commercial fishery management instrument, while also signalling a recent precautionary approach to elasmobranch management in commercial fisheries globally. The analysis demonstrates that the predominant parameters incorporated in previous ERAs are largely based on life-history characteristics, and sharks have received the majority of attention; batoids (including skates) have received less attention. Recreational fishing and habitat degradation are discussed as hazards which warrant future investigation through ERA. Lastly, suggestions are made for incorporating descriptive ecological data to aid in the continued development and evolution of this management tool as it applies to future elasmobranch conservation.
Bray–Curtis similarity is widely employed in multivariate analysis of assemblage data, for sound biological reasons. This paper discusses two problems, however, with its practical application: its behaviour is erratic (or even undefined) for the vanishingly sparse samples that may be found as an end-point to a severe impact gradient, or a start-point in colonisation studies; and, in common with all similarity measures on species-level data, it is sensitive to inconsistency of taxonomic identification through time. It is shown that the latter problem is ameliorated by application of ‘taxonomic dissimilarity’ coefficients, a natural extension of the concept of taxonomic distinctness indices. Two previous suggestions for use with presence/absence data, denoted here by Γ+ and Θ+, are noted to be simple generalisations of the Bray–Curtis and Kulczynski measures, respectively. Also seen is their ability to permit ordinations of assemblages from wide geographic scales, with no species in common, and for which Bray–Curtis would return zero similarity for all pairs of samples.