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Insight into cumulative intra-guild and intra-specific depredation among sharks
Abstract
Intra-guild predation (IGP), depredation of hooked sharks and cannibalism by large individuals on smaller conspecifics have been documented for both tiger shark Galeocerdo cuvier and bull shark Carcharhinus leucas. In this paper we report a case study of cumulative inter and intra-depredation from the Indian and the Pacific Ocean involving large G. cuvier and C. leucas adults at the final stage. These findings further the hypothesis that IGP and cannibalism among adult sharks occurs in the wild and potentially influences predator-prey relationships in tropical marine ecosystems.
... In addition to the chemical stimuli provided by the bait, any catch struggling on a fishing line is likely to attract sharks from large distances (Kritzler and Wood, 1961;Gardiner et al., 2014;Mitchell et al., 2018). Few studies have directly addressed this topic, however Clua et al. (2014) described two cases where a shark was caught on a drumline and subsequently eaten by a larger shark, with the latter then being eaten by an even larger shark. Based on these two cases, the authors stated that fishing gear such as SDL could attract more sharks into the area than would otherwise be present. ...
... This isolated incident constituted only 1.75 % of the total catch and was clearly the exception rather than the rule, as there was no significant difference in the number of tagged sharks present or their residence time when modelled against the number of SDL catches. Clua et al. (2014) conducted their study in a remote, pristine island in the Seychelles archipelago with high densities of sharks, which cannot be directly compared to the heavily exploited coastal waters of Reunion Island. Finally, the use of SDL instead of conventional drumlines significantly decreased the fight time (Guyomard et al., 2019) thereby lowering the duration of the potentially attractive struggles of the catch (Mitchell et al., 2018). ...
Following a series of shark attacks, local authorities in Reunion Island developed an experimental shark control programme using innovative fishing gear, namely Shark Management Alert in Real Time (SMART) drumlines (SDL). From January to November 2014, four SDL were deployed 24 h per day, four days per week to target bull (Carcharhinus leucas) and tiger (Galeocerdo cuvier) sharks and to test the fishing efficacy of the SDL. Presence and residence time of 19 acoustically tagged bull and 19 tiger sharks, which had been tagged up to two years before the SDL deployment, were modelled against different SDL configurations, which included bait type and presence of bait or catches on the hooks, as well as environmental parameters before, during and after SLD deployment. There was insufficient acoustic data from the tiger sharks for any analyses. Bull sharks spent less time in nearshore waters when drumlines were deployed, and their presence was influenced by sea surface temperatures (SST), rainfall and time of day. There was no difference in the number of bull sharks detected in the SDL deployment area compared with surrounding sites. As SDL catch rates were only poorly correlated with presence of tagged sharks, their efficacy in catching sharks present in the area could not be accurately determined. Overall, the results show that SDL, deployed without chum and baited with small, whole, low-fat fish cannot be considered as "shark magnets", which could attract dangerous bull sharks inshore where they would pose a threat to the safety of surfers and other sea users. The detection of tagged bull sharks moving into and out of the SDL fishing area indicates that these fishing devices do not provide an impenetrable barrier to the passage of this potentially dangerous species.
... Depredation of fisheries catch is where a predator partially or completely consumes an animal caught by fishing gear before it can be retrieved to the fishing vessel (IOTC 2007;Gilman et al. 2008;MacNeil et al. 2009), and it is a source of mortality which occurs widely in commercial (Bullis 1955;Hirayama 1976;Gilman et al. 2007;Mandelman et al. 2008;Romanov et al. 2013;Muñoz-Lechuga et al. 2016) and recreational fisheries (Sumner et al. 2002;Labinjoh 2014;Mitchell et al. 2018) around the world. Depredating taxa can include sharks, cetaceans, pinnipeds, large teleosts, squid, crabs, octopus and seabirds (Meyer et al. 1992;Donoghue et al. 2003;Brock et al. 2006;IOTC 2007;Gilman et al. 2008Gilman et al. , 2013Remeslo et al. 2015), which may consume teleost and elasmobranch fishes, cetaceans, crabs, squid, and marine turtles caught by fishing gear (Dudley and Cliff 1993;Noke and Odell 2002;Gilman et al. 2007;Romanov et al. 2007;Clua et al. 2014;Cruz et al. 2014). Depredation can occur in hook and line (Fig. 1a, b), net and trap fisheries (Cliff et al. 1989;Ebert 1991;Noke and Odell 2002;Brock et al. 2006;Gilman et al. 2007;IOTC 2007;Rafferty et al. 2012;Uhlmann and Broadhurst 2015). ...
... Competitive behaviour was observed between the tiger shark (Galeocerdo cuvier, Carcharhinidae), great hammerhead shark (Sphyrna mokarran, Sphyrnidae), C. leucas and C. perezi, and only the G. cuvier and C. leucas actually fed on the hooked shark, because they were able to control the resource and prevent the other sharks from feeding (O'Shea et al. 2015). Furthermore, Clua et al. (2014), reported not only a single depredation event but a process of 'cumulative' depredation, where a grey reef shark (Carcharhinus amblyrhynchos, Carcharhinidae) was consumed by a C. leucas, which was then itself depredated by a larger G. cuvier, on drumlines deployed in the Seychelles and New Caledonia. Depredation also occurred on drumlines deployed in KwaZulu-Natal, South Africa, which were tested as part of a shark control program (Dudley et al. 1998). ...
Shark depredation, where a shark partially or completely consumes an animal caught by fishing gear before it can be retrieved to the fishing vessel, occurs in commercial and recreational fisheries worldwide, causing a range of negative biological and economic impacts. Despite this, it remains relatively understudied compared to other fisheries issues. This is the first review of the literature relating to shark depredation, which also includes an overview of the potential mechanisms underlying its occurrence and options for mitigation. Furthermore, this review highlights key research gaps that remain to be investigated, thereby providing impetus for future research. In total, 61 studies have been published between 1955 and 2018, which include information on shark depredation. These studies recorded quantitative rates of depredation between 0.9 and 26% in commercial and recreational fisheries and during research fishing, identified 27 shark species from seven families that were responsible for depredation and discussed potential factors influencing its occurrence. Information from research into bycatch mitigation and the testing of shark deterrent approaches and technologies is also presented, in the context of applying these approaches to the reduction of shark depredation. This review presents an holistic overview of shark depredation in fisheries globally and, in doing so, provides a central resource for fisheries researchers and managers focusing on this topic to stimulate further collaborative research on this important fisheries issue.
... In high latitudes, sperm whales ( Physeter macrocephalus ) and killer whales ( Orcinus orca ) are the two species most documented depredating in demersal longline fisheries (Yano and Dalheim 1995, Kock et al. 2006, Sigler et al. 2008, Peterson et al. 2014, Guinet et al. 2015. In tropical and sub-tropical regions, pelagic longlines targeting tuna ( Thunnus spp.) and swordfish ( Xiphias gladius ) are generally depredated by pelagic sharks such as the oceanic whitetip shark ( Carcharhinus longimanus ) in the Indian, Atlantic, and Pacific oceans (Gilman et al. 2007, Hamer et al. 2015, Madigan et al. 2015, Mitchell et al. 2018, the bull shark ( Carcharhinus leucas ), and the tiger shark ( Galeocerdo cuvier ) in Seychelles and New Caledonia (Clua et al. 2014, Rees et al. 2018, all of which are generalist opportunistic feeders taking advantage of any food source when they happen upon it (Clua et al. 2013, Dicken et al. 2017, Trystram et al. 2017. Depredation is also observed in these regions by odontocetes such as false killer whales ( Pseudorca crassidens ) and short-finned pilot whales ( Globicephala macrorhynchus ) (Nishida and Tanio 2001, Dalla Rosa and Secchi 2007, Hernandez-Milian et al. 2008, Forney et al. 2011, Rabearisoa et al. 2012, Fader et al. 2021. ...
Large marine predators feeding on fish caught on fishing gear, referred to as ‘depredation’, occur in a wide range of fisheries worldwide. Depredation can result in negative ecological and socio-economic impacts, leading to conflict between fishers and depredating species. However, depredation remains understudied in many fisheries, and this hampers the development of effective mitigation solutions. In this study, 21 years of fishing data (2002–2022) were used to assess shark and odontocete depredation in the pelagic tuna longline fishery of New Caledonia. Using generalized linear models, the year, season, effort, soaking time, and vessel were identified as variables significantly influencing the probability of depredation to occur. Results showed that while shark depredation occurred more frequently than odontocete depredation (58.5% vs. 9.2% of the longline sets), they damaged a lower proportion of fish (3.9% vs. 12.3%) over the study period. Unlike sharks, odontocetes selectively depredate tuna, with their highest occurrence during periods of high tuna catch rates, suggesting a co-occurrence with fishing activities. Together, these results indicate that depredation in the New Caledonian fishery is high compared to other regions and provide essential information on the dynamics and impacts of the issue as a basis for considering management and mitigation options.
... This presents an important knowledge gap, particularly because river mouths do not appear to be functionally equivalent as nursery habitats due to geographic differences in freshwater inflow, leading to differing environmental conditions at each river mouth (Matich et al. 2020a). River mouths with greater freshwater inflow were preferred by YOY bull sharks (Matich et al. 2020a), which may be related to their exclusion of large sharks like adult spinner sharks Carcharhinus brevipinna and blacktip sharks that pose a threat to small sharks (Lowry et al. 2009, Clua et al. 2014, Matich et al. 2017, Plumlee et al. 2018. These larger sharks are physiologically restricted to higher salinity habitats, and larger bull sharks tend to utilize higher salinity habitats within most estuaries, making it likely that predation risk is positively correlated with salinity (Matich et al. 2020a, TinHan 2020). ...
Estuarine nurseries are hypothesized to offer juvenile bull sharks Carcharhinus leucas refuge from predators, with reduced mortality compared to adjacent marine habitats. Many estuaries in the Gulf of Mexico serve as nurseries for this species, yet the estuary refuge hypothesis has largely been untested, and it is unclear what factors make estuarine habitats suitable for young-of-the-year (YOY) bull sharks. Using drumline sampling and long-term gillnet monitoring data, this study investigated how predation risk and abiotic factors influenced densities of YOY bull sharks in 2 Texas estuaries. The more saline San Antonio Bay had higher densities of predatory sharks (>150 cm total length) that pose a risk to juvenile sharks, with predatory sharks caught in higher salinity regions proximate to the Gulf of Mexico. Consequently, densities of YOY bull sharks were highest in low-risk habitats proximate to river mouths in San Antonio Bay. In Sabine Lake, both predatory sharks and YOY bull sharks occurred most frequently in less saline habitats. However, only 2 predatory sharks were sampled in this hyposaline estuary, suggesting that encounters with predators are rare. Results support the estuary refuge hypothesis and indicate that YOY bull sharks avoid risky habitats in ecosystems where predators are prevalent (e.g. San Antonio Bay), while some estuaries function as refuges throughout their waters due to hyposaline conditions largely excluding predators (e.g. Sabine Lake). As freshwater inflows into estuaries change in response to anthropogenic and climatic influences, understanding how alterations in salinity affect predation risk and YOY bull shark distributions will aid in predicting the role that estuarine habitats play as shark nurseries.
... Low salinity habitats and estuary nursery grounds seem to provide low-mortality environments for young C. leucas such as neonates, young-of-the-year, and juveniles ( heuPel & simPfendorfer 2011). The penetration of freshwater systems by juvenile C. leucas can be understood as an evolutionary strategy to decrease the predation risk of immature C. leucas by large coastal sharks in marine habitats, especially by apex predators that are known to be intensive elasmobranch consumers, e.g., Galeocerdo cuvier, Sphyrna mokarran Rüppell, 1837 (great hammerhead shark), and also adult C. leucas itself ( comPaGno 1984;clua et al. 2014). Carcharodon carchar ias and predatory marine mammals such as Orcinus orca Linnaeus, 1758 (orca or killer whale) may also include C. leucas in their diet (comPaGno 2001). ...
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.
... Proposed drivers of habitat use among bull sharks include predation risk, competition, physiological tolerances and energetic needs (e.g. Hueter and Tyminski 2007;Heupel and Simpfendorfer 2008;Werry et al. 2011;Clua et al. 2014). Some of these drivers are relatively widespread; for example, cooling winter water temperatures in many estuaries trigger seasonal emigrations among bull sharks in pursuit of more suitable conditions (Jones and Grace 2002;McCandless et al. 2007;Yeiser et al. 2008;Curtis et al. 2011;Bangley et al. 2018). ...
The life histories of estuarine species are often adapted to the environmental variability they experience. However, estuaries are increasingly vulnerable to natural and anthropogenic changes, necessitating an understanding of how shifting conditions affect the survival, behaviour and population structure of estuarine-dependent animals. In this study we used data from fisheries-independent surveys collected across six estuaries with variable salinity regimes in Texas, USA, from 1975 to 2016 to investigate the role sources of freshwater inflow play in shaping juvenile bull shark Carcharhinus leucas size structure. High frequencies of co-occurrence with similarly sized conspecifics (59% of capture events) suggest bull sharks segregated within Texan estuaries based on body size. Bull shark sizes increased with distance to the nearest source of freshwater inflow, although effect sizes were small and access to freshwater habitats may be more important in shaping size-dependent distribution patterns. River mouths were disproportionately used by smaller juveniles (<90-cm total length, TL) and avoided by larger juveniles (>135 cm TL). However, the use of river mouths decreased in estuaries characterised by limited freshwater inflow and greater variability in salinities at river mouths, highlighting geographic differences in the functions these habitats provide as potential environmental and predator refugia. Young-of-the-year (i.e. age-0) sharks also increased their use of river mouths throughout the 40-year study period, revealing the growing importance of river mouths as potential nursery habitats.
... Although no irrefutable evidence exists thus far, the possibility that the parata warriors practiced cannibalism is evoked both in tradition and in European historical sources (Dening 1982), and existed in the Tuamotuan language under Bkai tagata^(man eating), although this could be a figurative expression used as an insult. Cannibalism is well known among sharks, particularly for large species (Clua et al. 2014) including the oceanic whitetip shark (Poisson 2007). Whatever the extent and reality of the analogy with oceanic whitetip shark behaviour, the negative notoriety of the 'Anaa warriors created an understandable terror, killing men and forcing women and children to leave their native atolls, ultimately resulting in the desertion of 38 atolls and including the population of western Tuamotu (Mihiroa people) who took refuge along the Tahitian peninsula (Ottino 1965;Nolet 2006;Torrente 2012). ...
Polynesians’ detailed observations of shark behaviour encompass the notion of a divinity, the fleeting image of a sky god, as well as potential source of food and valued tools. Due to prevailing cosmogony, sharks benefited from being a taboo species, historically limiting their exploitation. We examine how the reputedly fierce warriors of ‘Anaa (an atoll in Tuamotu archipelago, French Polynesia) came to be symbolically identified with a marine predator, being called “Parata,” the vernacular name of the oceanic whitetip shark Carcharinus longimanus. Both sharks and indigenous cultures are currently under threat in the East Pacific and we propose that an understanding of these sacred relationships could be used to help protect them.
... underwater, no more than six sharks were seen together at the same time during the 2010 field trip. Through both missions our findings of average abundances of 2.0 and 2.2 sharks per dive are consistent with those of graham et al. (2010) in remote atolls of the indian Ocean where the number of sharks observed per scientific dive declined over 90% from a mean of 4.2 in the 1970s to 0.4 in 2006. it is however relevant to mention the contrasting situation found around avon north islet, where a very high density of grey reef sharks was encountered, leading to intra-specific depredation among this species, as already described in such situations ( Clua et al., 2014). ...
The decline of meso-predators such as reef sharks is a concern as such species can have important
ecological roles in maintaining reef ecosystem resilience. Two field trips conducted in August 2010 and November
2011 to the Chesterfield archipelago (Coral Sea) allowed us to assess the abundances and average sizes of
medium-bodied Carcharhinidae with a specific focus on grey reef shark (Carcharhinus amblyrhynchos), through
fishing (46 hours of accumulated effort) and underwater visual censuses (25 hours of accumulated effort). We
found low abundance and small average total length (TL) for all reef shark species, and in the case of the grey
reef shark, an average abundance of 2.1 individuals/dive with the majority of animals less than 110 cm TL. We
compared our findings with historical data and, given our low sampling effort, we so far hypothesise that a general
strong decline in the reef shark populations may have occurred in this area, probably due to recent overfishing.
The enforcement of conservation measures is strongly recommended among these remote reefs as well as
complementary studies for confirming this hypothesis
A total of 772 bull sharks Carcharhinus leucas was caught in Natal's protective "shark nets" between 1978 and 1990. Confusion in distinguishing C. leucas from C. amboinensis resulted in their catch data being combined from 1966, when data collection began, to 1977. The catch rate of the species pair declined until 1977, recovered until the mid 1980s, but subsequently declined again. The trend in catch rate of C. leucas alone for the period 1978-1990 was similar, with minima of 0,70 sharks·km-net−1·year−1 in 1978 and 0,95 in 1990, and a maximum of 2,08 in 1986. Recaptures of six tagged sharks suggest that the species is not highly migratory. Catches, particularly of immature sharks, were highest at the northernmost beaches. Most bull sharks were caught in summer and in turbid water (mean water clarity 2,0 m). The sex ratio of the catch was 1 male to 1,3 females. Sizes ranged from 74 to 213 cm precaudal length, with modes of 141-145 cm (males) and 171-175 cm (females). Size at maturity for both sexes was between 180 and 190 cm. The mating season was prolonged but with a summer peak. Seven gravid females were examined; the mean litter size was 8,7 embryos and size at birth was approximately 55 cm. Fluke infections were observed on 9 per cent of animals examined. As size increased there was a shift in diet, in terms of frequency of occurrence, from teleost to elasmobranch predominance. There was a high incidence of benthic and demersal species in the stomachs. Minor prey groups included mammals, birds, turtles, molluscs and crustaceans. Scavenging appeared to be important.
The study of spatial patterns in the distribution of organisms is a central issue in ecology. Here we address the question of whether predator-prey interactions can induce nonuniform distributions. We study how diffusion affects the stability of predator-prey coexistence equilibria and show a new difference between ratio- and prey-dependent models. Recently, Peter Abrams and Lev Ginzburg reviewed the controversial issue of what kind of function better describes the rate of prey consumption by an average predator. the so-called "predator functional response." Here, we focus on reaction-diffusion predator-prey models with and without predator dependence in the functional response. We show that classical prey-dependent models cannot give rise to spatial structures through diffusion-driven instabilities: however. predator-dependent models with the same degree of complexity can. The origin of predator dependence in the rate of prey consumption is the mutual interference between predators. Therefore, we show that this mechanism can generate patchiness in it homogeneous environment under certain conditions of trophic interaction and predator-prey relative diffusion.
One of the central issues in ecology is the study of spatial pattern in the distribution of organisms. Thus, in this paper,
spatial pattern of a predator–prey system with predator cannibalism is considered. By mathematical analysis, we obtain the
condition for emerging Turing pattern formation. Furthermore, numerical simulations reveal that large variety of different
spatiotemporal dynamics emerge as the consequence of the interaction of Holling typeII with predator cannibalism. The obtained
results show predator cannibalism has great influence on the spatial pattern formation. In other words, the regular pattern
is induced by predator cannibalism. Moreover, we find that although the environment is heterogeneous, the system still exhibits
Turing pattern, which means the pattern is self-organized. It may help us better understand the dynamics of predator–prey
interaction in a real environment.
Bite damage patterns have long been used to estimate shark species and body size, with somewhat limited success. The lack
of fit between damage patterns and shark size is partially due to variation in tooth size and shape within an individual.
The ability to accurately predict body size from bite patterns is important for better understanding the ecological and behavioral
underpinnings of shark bites/attacks on marine organisms, humans, and submarine equipment. To this end, we measured interdental
distance (IDD) between the most labial teeth in the first six tooth files on both the upper and lower jaws, as well as the
circumference of the portion of each jaw that bears teeth, for prepared jaw sets from fourteen shark species and regressed
these data against total length. IDD is allometric as well as an accurate predictor of total length in all species examined,
except Carcharhinus acronotus. Tooth-bearing circumference is also allometric and predictive of total length in all species. Though considerable overlap
exists in IDD and circumference ranges among species for the total length ranges examined, Carcharodon carcharias and Isurus sp. can be differentiated from Carcharhinus limbatus, Carcharhinus brevipinna, and C. acronotus based on these values alone. When combined with knowledge of species-specific feeding behavior, geographic distribution,
and habitat preferences, these simple measures from bite damage patterns allow quick, accurate assessment of shark size and
potential species.
Substantial ecological, economic and social problems result from shark interactions in pelagic longline fisheries. Improved understanding of industry attitudes and practices towards shark interactions assists with managing these problems. Information on fisher knowledge and new strategies for shark avoidance may benefit sharks and fishers. A study of 12 pelagic longline fisheries from eight countries shows that incentives to avoid sharks vary along a continuum, based on whether sharks represent an economic disadvantage or advantage. Shark avoidance practices are limited, including avoiding certain areas, moving when shark interaction rates are high, using fish instead of squid for bait and deeper setting. Some conventionally employed fishing gear and methods used to target non-shark species contribute to shark avoidance. Shark repellents hold promise; more research and development is needed. Development of specifically designed equipment to discard sharks could improve shark post release survival prospects, reduce gear loss and improve crew safety. With expanding exploitation of sharks for fins and meat, improved data collection, monitoring and precautionary shark management measures are needed to ensure that shark fishing mortality levels are sustainable.
MacNeil, M. A., Carlson, J. K., and Beerkircher, L. R. 2009. Shark depredation rates in pelagic longline fisheries: a case study from the Northwest Atlantic. – ICES Journal of Marine Science, 66: 708–719.
A suite of modelling approaches was employed to analyse shark depredation rates from the US Atlantic pelagic longline fishery. As depredation events are relatively rare, there are a large number of zeroes in pelagic longline data and conventional generalized linear models (GLMs) may be ineffective as tools for statistical inference. GLMs (Poisson and negative binomial), two-part (delta-lognormal and truncated negative binomial, T-NB), and mixture models (zero-inflated Poisson, ZIP, and zero-inflated negative binomial, ZINB) were used to understand the factors that contributed most to the occurrence of depredation events that included a small proportion of whale damage. Of the six distribution forms used, only the ZIP and T-NB models performed adequately in describing depredation data, and the T-NB and ZINB models outperformed the ZIP models in bootstrap cross-validation estimates of prediction error. Candidate T-NB and ZINB model results showed that encounter probabilities were more strongly related to large-scale covariates (space, season) and that depredation counts were correlated with small-scale characteristics of the fishery (temperature, catch composition). Moreover, there was little evidence of historical trends in depredation rates. The results show that the factors contributing to most depredation events are those already controlled by ships' captains and, beyond novel technologies to repel sharks, there may be little more to do to reduce depredation loss in the fishery within current economic and operational constraints.
The bull shark, Carcharhinus leucas, is the most common shark in the brackish Indian River lagoon system on the central east coast of Florida. The biology of the lagoon population was studied between May 1975 and May 1979. There was substantial spatial and seasonal variation in catch rates with gill nets. Bull sharks were usually most abundant in the low-salinity lagoon basins. Catch rates were generally highest in the spring and fall and were always higher at night than day. No specimens were netted during the winter although bull sharks are known to be present during that season. The permanent lagoon population was composed entirely of newborn young and juveniles up to 202 cm TL. As they approach maturity, the subadults leave the estuary. Pregnant adult females return to lagoon waters in late spring and summer to give birth. One pregnant female 249 cm TL was captured during this study. Juvenile bull sharks in the lagoon system fed primarily on stingrays and marine catfishes.
The increased exploitation of pelagic sharks by longline fisheries raised questions about changes in the food webs that include
sharks as apex predators. We used a version of Ecopath/Ecosim models to evaluate changes in trophic interactions due to shark
exploitation in the Central North Pacific. Fisheries targeted on blue sharks tend to produce compensatory responses that favor
other shark species and billfishes, but they have only modest effects on the majority of food web components. Modest levels
of intraguild predation (adult sharks that eat juvenile sharks) produce strong, nonlinear responses in shark populations.
In general, analysis of the Central North Pacific model reveals that sharks are not keystone predators, but that increases
in longline fisheries can have profound effects on the food webs that support sharks.
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