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Forensic determination of shark species as predators and scavengers of sea turtles in Florida and Alabama, USA
Abstract
Sharks are the primary predator of large immature and mature sea turtles, yet the shark species responsible for both lethal and non-lethal injuries are rarely identified. Forensic analysis of bite wounds can be used to accurately assess size and potential shark species when combined with observations on species-specific feeding behavior, geographic distribution, and habitat preference. The objective of this study was to use forensic analysis of bite damage on sea turtles to infer shark size and species. Photographs from 13 cases of documented shark predation
(n = 10) and scavenging (n = 3) attempts on sea turtles were retrospectively analyzed, including nesting, free-ranging, and/or dead stranded loggerhead Caretta caretta, green Chelonia mydas, Kemp’s ridley Lepidochelys kempii, and leatherback Dermochelys coriacea sea turtles in Florida and Alabama, USA, from 2010−2020. Mean interdental distance (IDD) and bite circumference (BC) of wound marks on sea turtles suggest that wounds were generated by white sharks Carcharodon carcharias in 3 cases, tiger sharks Galeocerdo cuvier in 3 cases, and bull shark(s) Carcharhinus leucas in one case. For 3 cases with less distinct wound patterns, 2 likely shark species were identified and thereafter narrowed down to a single species based on bite mark characteristics (e.g. punctures). Due to indistinct IDD and BC ranges of the bite patterns, a single shark species was not identified in 3 cases. Forensic analysis enables more accurate evaluations of which shark species prey on and scavenge sea turtles and is a useful technique for studying the behavioral interactions of sharks and turtles.
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Sea turtles face both anthropogenic and natural threats including boat strikes, fisheries, pollution, and predator attacks. Injuries from anthropogenic sources are more common than naturally caused injuries. The goal of this study was to determine prevalence and cause (e.g. boat strike, entanglement, hook, shark bite) of injuries on nesting loggerhead sea turtles Caretta caretta on Juno and Jupiter beaches, Florida, USA. During the 2019 and 2020 nesting seasons, 450 loggerhead females were examined for external injuries. Injuries were categorized by anatomic location, condition, and cause. We found that 24% of loggerheads had at least 1 injury. Of the 111
injuries found on 107 nesting females, 88% were healed, 9% were partially healed with some scarred tissue, and 3% were fresh injuries. Most injuries (55%) were lateral injuries on the carapace or appendages. We were able to attribute 60 injuries to a specific cause. Boat strikes accounted for 75% of the 60 injuries, shark bites accounted for 15%, fishing hooks accounted for 7%, and entanglements accounted for the remaining 3%. This study provides new insight into the prevalence of anthropogenic injuries relative to natural injuries in loggerhead sea turtles nesting in the most densely nested beach in the Western Hemisphere and can be used to improve conservation management plans through implementation of fishing and/or boating restrictions in the nesting and foraging areas most commonly frequented by sea turtles.
We present evidence of predator interactions with 8 juvenile loggerhead sea turtles Caretta caretta equipped with pop-up satellite archival transmitting tags (PSATs) following incidental capture in the Northwest Atlantic. Ingestion of PSATs occurred up to 5 mo after tagging and was identified by an abrupt change in depth distribution, a stabilization and/or increase in ambient temperature and a marked drop in light levels to near zero, with cessation of diel light level cycling. In some cases, following expulsion from the digestive tract of predators, positively buoyant PSATs descended to the sea floor or beyond the programmed release depth threshold (1800 m), indicating that they remained tethered to the indigestible carapaces of turtles and that the entire turtle was originally consumed. PSAT data, combined with the sudden termination of satellite uplinks from 2 loggerheads also equipped with platform transmitting terminals, provided additional evidence of whole-turtle predation. PSAT data indicated that both endothermic and ectothermic sharks ingested tags. Based on PSAT-logged temperature data, dive patterns and geographic distribution, the following shark species were considered as candidate predators: white, porbeagle, shortfin mako, tiger and blue. This study represents the first analysis of data collected by loggerhead turtle PSATs inside predators. The results expand the list of shark species known to prey on large juvenile loggerheads and point to the importance of acknowledging predation as an important source of mortality for loggerhead turtles in the Northwest Atlantic Ocean.
Mortality from being struck by a motorized watercraft is considerable for many aquatic vertebrates around the world, including sea turtles. We studied stranded (i.e., dead, sick, or injured) sea turtles found in Florida, USA, during 1986–2014 and identified those with sharp force or blunt force injuries indicative of a vessel strike. About a third of stranded loggerheads (Caretta caretta), green turtles (Chelonia mydas), and leatherbacks (Dermochelys coriacea) had a vessel‐strike injury (VSI). The frequency of this injury was lower but still substantial for stranded Kemp's ridleys (Lepidochelys kempii; 26.1%) and hawksbills (Eretmochelys imbricata; 14.8%). Over the study period, the annual number of stranded loggerheads, green turtles, and Kemp's ridleys with a VSI increased as did the annual number of vessels registered in Florida. Eighty‐one percent of the stranded turtles with a VSI were found in the southern half of Florida and 66% of those were found along the southeast coast. By coastal county, the proportion of stranded sea turtles with a VSI was positively related to the mean annual number of registered vessels. The percentage occurrence of a VSI was highest for adult loggerheads, green turtles, and leatherbacks, and reproductively active individuals appeared to be particularly vulnerable to these injuries. We conducted necropsies on 194 stranded sea turtles with a VSI and concluded that this injury was the cause of death or the probable cause of death in ≥92.8% of these cases. During 2000–2014, we estimate that the mean annual numbers of stranded sea turtles that died from a VSI were 142–229 loggerheads, 101–162 green turtles, 16–32 Kemp's ridleys, 4–6 leatherbacks, and 2–4 hawksbills. Considering that only about 10–20% of sea turtles that died likely washed ashore, the overall annual mortality may have been 5–10 times greater than that represented by strandings. Most of the significant clusters of stranded sea turtles with a VSI occurred at inlets or passes and the probability that a stranded sea turtle had a VSI decreased with increasing distance from inlets or passes, navigable waterways, and marinas. We suggest focusing initial management efforts on reducing watercraft‐related mortality for all sea turtle species around 8 inlets in southeast Florida, reproductively active loggerheads and green turtles along the coast of southeast Florida, and Kemp's ridleys and adult male loggerheads at passes along the coast of southwest Florida.
Identification and understanding of various patterns of injury in marine species such as cetaceans and sea turtles can elucidate corresponding threats and inform conservation efforts. Here we used standardized external injury assessments to investigate the relative importance of direct anthropogenic and natural threats to the Northwest Atlantic population of leatherback turtles Dermochelys coriacea, and to evaluate whether susceptibility to these threats varies between low-latitude nesting and high-latitude foraging regions, and by sex and size (i.e. age). We sampled leatherbacks foraging in temperate waters off Nova Scotia, Canada (n = 92), and at a nesting beach in Trinidad, West Indies (n = 137), where, combined, 62% of turtles exhibited characteristic markings associated with at least 1 injury (i.e. entanglement scarring, amputations, bites, etc.). There were no significant differences in injury rate or type between regions or between sexes or size classes. The proportion of leatherbacks exhibiting injuries of suspected anthropogenic origin (34%) was significantly higher (p < 0.001) than the proportion with injuries of suspected predatory origin (16%), but did not exceed the proportion with injuries of unknown origin (34%). Nineteen percent exhibited injuries indicative of entanglement in rope, lines, or nets, and 17% showed evidence of potential interaction with hooks. These results suggest that the Northwest Atlantic leather back population faces common and possibly widely distributed threats from both predators and fishing gear across its range and are consistent with growing evidence positing fisheries bycatch as one of the principal direct threats facing this species.
Tiger sharks, Galeocerdo cuvier, are apex predators that may structure marine communities through predation. Despite a large number of studies in other areas such as the Pacific Ocean, there are no quantitative data on the diet of tiger sharks in the northwest Atlantic Ocean and Gulf of Mexico. Diet was assessed from 169 tiger sharks by life stage, area, and environmental factors. Fifteen prey groups were identified, with teleosts, molluscs, birds, cephalopods, and reptiles being the predominant prey categories. There was an ontogenetic shift in diet, prey size and diversity. Molluscs were the most common prey in smaller sharks, while teleosts and reptiles became more important in the diet of larger sharks. Dietary overlap was significant by area (Gulf of Mexico vs Atlantic Ocean) and among all life stages except for young-of-the-year and adult tiger sharks. Juvenile tiger sharks also demonstrated selective feeding by targeting gastropod feet over ingesting the entire animal. While results were similar to feeding studies conducted on tiger sharks in other ocean basins, an understanding of area-specific trophic interactions is necessary to inform decision support tools for ecosystem-based approaches to management.
Determining patterns of migratory connectivity for highly-mobile, wide-ranging species, such as sea turtles, is challenging. Here, we combined satellite telemetry and stable isotope analysis to estimate foraging locations for 749 individual loggerheads nesting along the east central Florida (USA) coast, the largest rookery for the Northwest Atlantic population. We aggregated individual results by year, identified seven foraging hotspots and tracked these summaries to describe the dynamics of inter-annual contributions of these geographic areas to this rookery over a nine-year period. Using reproductive information for a subset of turtles (n=513), we estimated hatchling yields associated with each hotspots. We found considerable inter-annual variability in the relative contribution of foraging areas to the nesting adults. Also reproductive success differed among foraging hotspots; females using southern foraging areas laid nests that produced more offspring in all but one year of the study. These analyses identified two high priority areas for future research and conservation efforts: the continental shelf adjacent to east central Florida and the Great Bahama Bank, which support higher numbers of foraging females that provide higher rates of hatchling production. The implementation of the continuous-surface approach to determine geographic origins of unknown migrants is applicable to other migratory species.
This study presents information on the diet of two shark species, Carcharhinus leucas and Galeocerdo cuvier that inhabit the southeastern Pacific Ocean. The stomachs were collected from October 2003 to July 2005 in Ecuador. Stomachs of 41 C. leucas and six G. cuvier were analyzed. According to the index of relative importance (IRI), the most important prey for C. leucas were fishes: Ophichthidae family (13.41), Tylosurus pacificus (9.79), Katsuwonus pelamis (4.54) and fish remains (44.81). G. cuvier, for its part, consumed squids: Ancistrocheirus lesueuri (45.14), Pholidoteuthis boschmaii (7.81) and Octopoteuthis spp. (5.17), as well as turtles: Caretta caretta (9.7), Lepidochelys cf. kempii (5) and turtle remains (16.5). Results show that C. leucas (trophic level, ITR; 4.32 ± 0.13) and G. cuvier (ITR; 4.26 ± 0.09) are tertiary consumers, occupying high positions in the food chain, but also generalist predators that feed on a variety of prey. The high frequency of sea turtles in the stomachs of G. cuvier (> 300 cm) suggests that this shark species is an important predator of turtles, which are commonly found along the southeastern Pacific coasts
As upper level predators, sharks serve an important role in marine ecosystems, but are often at risk from fisheries. Successful management of these species will require detailed information about their movements and distributions. 2.Using satellite telemetry, we investigated the long-term horizontal movements and seasonal distributions of shortfin mako sharks Isurus oxyrinchus in the western North Atlantic Ocean. 3.Twenty-six sharks (14 USA, 12 Mexico) were tracked for durations of 78–527 days. Ten sharks were tracked for >1 year. Sharks displayed region-specific movements, with little distributional overlap between the Gulf of Mexico and Caribbean Sea, and the western North Atlantic. Sharks tagged off the USA ranged over a larger area, including shelf and pelagic habitats. Their core distribution was largely over the continental shelf and fluctuated seasonally, ranging from South Carolina, USA, in the winter to Nova Scotia, Canada, in the autumn, and appeared to follow seasonal productivity peaks while favouring warmer waters. Sharks tagged off Mexico displayed more restricted movements, largely confined to shelf habitats, with core activity centred year-round on the eastern Campeche Bank, Mexico. 4.Sharks moved across the jurisdictional management boundaries of 17 nations, and the proportion of tracked sharks harvested (22%) was twice that obtained from previous fisheries-dependent, conventional tagging studies. 5.Sharks also displayed considerable variability in movements, with seven sharks tagged off the USA making long-distance, highly directional southern excursions into unproductive subtropical/tropical waters before returning north. 6.Policy implications. The large-scale and region-specific movements of shortfin mako sharks underscore the need for close cooperation amongst western North Atlantic nations and implementation of regionally- and seasonally-specific management strategies. The movement patterns also provide baseline information, which could be used in spatially explicit stock assessment models. Identification of high-use areas by shortfin mako sharks provides focal areas for quantifying interactions with fisheries. The high harvest rate observed in our fisheries-independent tracking study raises questions about the true rate of fisheries mortality experienced by shortfin mako sharks, calling for a cautionary interpretation of past stock assessments used to determine management policy for this highly migratory species of conservation concern. This article is protected by copyright. All rights reserved.
The ability of predators to switch between hunting and scavenging (facultative scavenging) carries both short-term survival and long-term fitness advantages. However, the mechanistic basis for facultative scavenging remains poorly understood. The co-occurrence of tiger sharks (Galeocerdo cuvier) and green turtles (Chelonia mydas) at Raine Island (Australia), provides an opportunity to examine a top marine predator’s feeding mode in response to seasonal pulses in nesting turtles that offer both hunting and scavenging opportunities. Using satellite telemetry, we evaluated home range overlap between sharks and turtles and quantified their surfacing behavior around Raine Island during the turtle nesting season. We found core home range overlap to be highest during the nesting season. Both sharks and turtles spent significantly more time at the surface in areas of greatest range overlap closest to shore, where turtle density was highest. Both sharks and turtles showed decreased surfacing with increasing distance from Raine Island. Combined with published data on turtle demography at Raine Island, we propose the following: (1) sharks patrol the surface to increase scavenging opportunities on turtle carcasses and intercept weakened individuals after nesting; (2) healthy turtles may not perceive sharks as a major threat and/or other biological factors override anti-predatory responses; and (3) sharks during the nesting season may primarily scavenge on dead turtles individuals rather than actively hunt. Our study results and approach may be applicable to other situations in which direct observations of predator-prey interactions are limited.
Significance Statement
Every animal encounters dead or dying resources, yet the role of facultative scavenging has been difficult to study, and thus largely overlooked in marine behavioral ecological research. Movement analyses of tiger shark and green turtle movement and surfacing behavior at Raine Island (Australia) suggest that facultative scavenging may be a prevalent, yet underappreciated, feeding strategy in tiger sharks. Our integration of behavioral ecology theory with multi-species electronic tagging provided a valuable approach for investigating predator-prey interactions in situations where direct observations are limited or not possible.
Vertical movements of shortfin mako sharks Isurus oxyrinchus in the western North Atlantic Ocean are strongly influenced by temperature
Jeremy J. Vaudo*, Bradley M. Wetherbee, Anthony D. Wood, Kevin Weng, Lucy A. Howey-Jordan, Guy M. Harvey, Mahmood S. Shivji
*Email: jvaudo@nova.edu
ABSTRACT: Although shortfin mako sharks Isurus oxyrinchus are regularly encountered in pelagic fisheries, limited information is available on their vertical distribution and is primarily restricted to cooler areas of their distribution. We investigated the vertical movements of mako sharks across differing temperature regimes within the western North Atlantic by tagging 8 individuals with pop-up satellite archival tags off the northeastern United States and the Yucatan Peninsula, Mexico. Depth and temperature records across 587 d showed vertical movements strongly associated with ocean temperature. Temperatures <15° C created a lower depth limit to most diving behaviors, and shifts in depths used coincided with changes in the thermal properties of the vertical habitat. In the warmest water columns, sharks spent 36% of the daytime at depths >150 m compared to only 1% in the coldest water columns. The sharks showed diel diving behavior, with deeper dives occurring primarily during the daytime (maximum depth: 866 m). Overall, sharks experienced temperatures between 5.2 and 31.1°C. When the opportunity was available, sharks spent considerable time in waters ranging from 22 to 27° C, indicating underestimation of the previously reported upper limit of the mako sharks’ preferred temperature. The preference for higher temperatures does not support endothermy as an adaption for niche expansion in mako sharks. The strong influence of thermal habitat on movement behavior suggests potentially strong impacts of rising ocean temperatures on the ecology of this highly migratory top predator.
Assessment and management of sea turtle populations is often limited by a lack of available data pertaining to at-sea distributions at appropriate spatial and temporal resolutions. Assessing the spatial and temporal distributions of marine turtles in an open system poses both observational and analytical challenges due to the turtles' highly migratory nature. Surface counts of marine turtles in waters along the southern part of Florida's east coast were made in and adjacent to the southeast portion of the Florida Current using standard aerial surveys during 2011 and 2012 to assess their seasonal presence. This area is of particular concern for sea turtles as interest increases in offshore energy developments, specifically harnessing the power of the Florida Current. While it is understood that marine turtles use these waters, here we evaluate seasonal variation in sea turtle abundance and density over two years. Density of sea turtles observed within the study area ranged from 0.003 turtles km-2 in the winter of 2011 to 0.064 turtles km-2 in the spring of 2012. This assessment of marine turtles in the waters off southeast Florida quantifies their in-water abundance across seasons in this area to establish baselines and inform future management strategies of these protected species.
Stomachs of shortfin mako sharks Isurus oxyrinchus caught in the northeastern Atlantic by Iberian longliners were analyzed. A number of juveniles, 6 out of 96 individuals with non-empty stomachs, had consumed marine mammals. The remains (skin, fat, vertebrae and flesh with the dorsal fin) were not identifiable at species level by non-genetic methods. Portions of the mitochondrial DNA control region and of the gene coding for cytochrome b were therefore sequenced. Both the short-beaked common dolphin Delphinus delphis and possibly the striped dolphin Stenella coeruleoalba were identified. Shortfin makos are able to consume marine mammals almost as large as themselves. Well-preserved D. delphis were juveniles.
Bull Sharks Carcharhinus leucas in the Indian River Lagoon, Florida, have been documented to frequently occur in human-altered habitats, including dredged creeks and channels, boat marinas, and power plant outfalls. The purpose of this study was to examine the short-term movements of age-0 and juvenile Bull Sharks to quantify the extent to which those movements occur in altered habitats. A total of 16 short-term active acoustic tracks (2-26h) were carried out with 9 individuals, and a 10th individual was fitted with a long-term coded transmitter for passive monitoring by fixed listening stations. Movement and activity space statistics indicated high levels of area reuse over the span of tracking (hours to days). All but one shark used altered habitat at some point during tracking, such that 51% of all tracking positions occurred in some type of altered habitat. Of the sharks that used altered habitat, the mean (1 SD) percent of positions within altered habitat was 66 (+/- 40)%. Furthermore, tracks for 3 individuals indicated selection for altered habitats. The single passively monitored Bull Shark was detected in power plant outfalls almost daily over a 5-month period, providing the first indication of longer-term fidelity to thermal effluents. Use of one dredged creek was influenced by local salinity, the tracked sharks dispersing from the altered habitat when salinity declined. The affinity of young Bull Sharks to altered habitats in this system could help explain their reported accumulation of a variety of harmful contaminants, which could negatively affect their health and survival.
Despite recent advances in field research on white sharks (Carcharodon carcharias) in several regions around the world, opportunistic capture and sighting records remain the primary source of information on this species in the northwest Atlantic Ocean (NWA). Previous studies using limited datasets have suggested a precipitous decline in the abundance of white sharks from this region, but considerable uncertainty in these studies warrants additional investigation. This study builds upon previously published data combined with recent unpublished records and presents a synthesis of 649 confirmed white shark records from the NWA compiled over a 210-year period (1800-2010), resulting in the largest white shark dataset yet compiled from this region. These comprehensive records were used to update our understanding of their seasonal distribution, relative abundance trends, habitat use, and fisheries interactions. All life stages were present in continental shelf waters year-round, but median latitude of white shark occurrence varied seasonally. White sharks primarily occurred between Massachusetts and New Jersey during summer and off Florida during winter, with broad distribution along the coast during spring and fall. The majority of fishing gear interactions occurred with rod and reel, longline, and gillnet gears. Historic abundance trends from multiple sources support a significant decline in white shark abundance in the 1970s and 1980s, but there have been apparent increases in abundance since the 1990s when a variety of conservation measures were implemented. Though the white shark's inherent vulnerability to exploitation warrants continued protections, our results suggest a more optimistic outlook for the recovery of this iconic predator in the Atlantic.
Mean diving depth and mean surface time were greater during the day, but there was no significant difference in mean diving time between day and night for either turtle. Leatherback sea turtles are active divers during the internesting period; their dive patterns, depths and durations are similar in various wasy to other pelagic divers such as seals, sea lions and penguins; and there is a consistent duration of dives that may reflect the aerobic limit. -from Authors
I recorded mast production by oaks (Quercus sp.) at 12 forested sites in western Virginia for 6-12 yr and measured its impact on the abundance of small mammals, understory vegetation, and artificial-nest predation. White-tailed deer (Odocoileus virginianus) were excluded from half the 4-ha sites after at least one season of data collection. My hypothesis was that annual variation in acorn crops affected multiple species and that the strength of those interactions is mediated by white-tailed deer. The acorn crop was variable across sites and year, with some of the between-site variability explained by differences in elevation. All sites experienced at least one mast failure, and mast failure years were generally consistent across sites. White-footed mouse (Peromyscus leucopus), eastern chipmunk (Tamias striatus), and gray squirrel (Sciurus carolinensis) populations were significantly correlated with annual fluctuations in the acorn crop. The exclusion of deer had a significant impact on P. leucopus and T. striatus populations by increasing the number of animals captured following low acorn mast years. Annual fluctuations in the acorn crop, but not in rodent densities, were significantly correlated with the rates of predation on artificial nests the next summer. There was no significant interaction between predation rates and the exclusion of deer. An index from the Breeding Bird Survey (BBS) for Virginia was used to measure regional numbers for 11 common species captured at the sites. The index for two understory species was significantly negatively correlated with the mean acorn crop measured 2 yr previously. The effect of white-tailed deer on the forest community was not consistent across all conditions, as sites with large acorn crops were not strongly influenced by deer. These data are consistent with the hypothesis that mast crops from oaks serve as important determinants of community function within Appalachian forests.
We tracked 10 leatherback turtles by satellite from 2 Florida Atlantic Coast nesting beaches for a period ranging from 38 days to more than 454 days. Movement and foraging areas were often coastal, which contrasts with other satellite telemetry studies where leatherbacks are more pelagic. Using kernel home-range estimation we identified the primary internesting residence areas as well as Atlantic foraging areas. The primary internesting habitat was centered east-southeast of Cape Canaveral, Florida, from 2 to 60 km offshore and extending 215 km along the coast. Atlantic foraging areas were located primarily on the continental shelf from 30 degrees to 50 degrees N, and in an offshore area centered at 42 degrees N, 65 degrees W, as well as off Africa in the Mauritania upwelling. Seasonally, the location of these foraging areas changed, occurring on the North American continental shelf from March through November and off the shelf from December through February. One of the tracked turtles may have been killed with 17 other leatherbacks by coastal shrimp fishing located near the Georgia-Florida border. We illustrate how using remotely sensed data could be used to prevent such mortalities.
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 diet of the broadnose sevengill shark Notorynchus cepedianus was investigated over 3 years from 2 coastal locations in south-east Tasmania: the Derwent Estuary and Norfolk Bay. In general, individuals from both locations consumed the same broad dietary categories (sharks, batoids, teleosts and mammals). However, within these categories, species composition differed. Variations in chondrichthyan prey consumed matched estimations of prey abundance: Mustelus antarcticus was the primary prey in Norfolk Bay, where it was also the most abundant prey species; similarly, Squalus acanthias was an important prey and the most abundant in the Derwent Estuary. A decline in the catch rates of N. cepedianus and elasmobranch prey, in particular M. antarcticus over 3 years coincided with declines in dietary occurrence of M. antarcticus. Also, N. cepedianus and M. antarcticus abundances were both higher in Norfolk Bay than the Derwent Estuary. The correlation with diet and estimations of predator and prey relative abundance suggests N. cepedianus may move into coastal areas to exploit regular seasonal abundant resources, but they can also be versatile opportunistic predators that exploit a temporarily abundant resource.
For the past 25 years NIH Image and ImageJ software have been pioneers as open tools for the
analysis of scientific images. We discuss the origins, challenges and solutions of these two programs,
and how their history can serve to advise and inform other software projects.
Carrion use by terrestrial vertebrates is much more prevalent than conventional theory implies, and, rather than a curiosity of animal behavior, is a key ecological process that must be accounted for. Human aversion to rotted substances and difficulties associated with identifying scavenged material in studies of food habits have contributed to the relative lack of information concerning scavenging behavior in vertebrates. Several lines of evidence, however, suggest that carrion resources are more extensively used by vertebrates than has been widely assumed: 1) a substantial number of animals die from causes other than predation and become available to scavengers, 2) a wide variety of vertebrate scavengers, rather than microbes or arthropods, consume most available carcasses, and 3) intense competition exists between vertebrate scavengers and decomposers, especially in warm climates. Although vultures are best adapted to use carrion, nearly all vertebrate predators are also scavengers to some extent. The costs and benefits associated with carrion use influences the evolution of scavenging behavior in vertebrates, resulting in a continuum of facultative scavengers that use carrion to varying degrees. The realized usage of carrion by a vertebrate species is influenced by the speed and efficiency with which it forages, its visual and olfactory abilities, and its capacity for detoxifying products of decomposition. A deeper understanding of carrion use by facultative scavengers will improve our knowledge of community and ecosystem processes, especially the flow of energy through food webs.
Interactions between large marine predators and their prey are difficult to observe and little is known about the risk of predation faced by sea turtles. The frequency of predator-inflicted injuries, however, has afforded insights into the predation risk faced by many taxa. We measured the frequency of shark-inflicted injuries on green (Chelonia mydas) and loggerhead (Caretta caretta) sea turtles in Shark Bay, Western Australia with a view to determining differences between species and sex-classes in the risk of predation from tiger sharks (Galeocerdo cuvier). Furthermore, we investigated how escape ability and habitat use might influence the probability of turtles being injured by sharks. Shark-inflicted injuries were more frequent on loggerhead than on green turtles, and most frequent on adult male loggerhead turtles. Species effects could not be attributed to differences in habitat use, since green turtles were found in habitats favored by tiger sharks more often than were loggerhead turtles. Green turtles, however, were faster and maneuvered better than loggerhead turtles, suggesting that escape ability is a factor in interspecific differences in injury frequency. The sex-class difference in injury frequency of loggerhead turtles suggests that males face greater predation risk than females and may take more risks. For green turtles, the lack of a sex difference in injury frequency might be due to greater escape ability lowering overall predation risk or to no differences between sexes in the benefits of risk-taking.
Predators affect prey through mortality, and prey respond by employing antipredatory strategies. Although leatherback sea turtles Dermochelys coriacea are the largest sea turtle, they are susceptible to shark predation. Females lay multiple clutches of eggs separated by a ~10 d internesting interval at sea, during which they move many kilometers away from the nesting beach. The goal of this study was to test the hypothesis that this wide-ranging behavior was associated with predator avoidance. To accomplish this goal, we used animal-borne video and data recorders along with satellite telemetry to monitor diving behavior and track the at-sea movements of females during the internesting intervals on St. Croix, US Virgin Islands. After they departed the nesting beach, females immediately swam a mean maximum distance of 101 km from St. Croix. The mean transit distance during 10.19 d at sea was 466 km. We recorded 102 periods during which females inverted and rotated vigorously for an average of 2.7 min at or near the surface. Because females nesting on St. Croix commonly exhibit scars and fresh wounds, many of which result from shark encounters, this behavior likely represents a defensive response to predator attack. The frequency of defensive behaviors that occurred <6 km from the nesting beach (4.6 d ⁻¹ ) was 3.3-fold greater than when they were >6 km from shore (1.4 d ⁻¹ ). By moving offshore, females reduced the possibility of predator encounters by 53%.
This chapter introduces practical approaches to health assessment of free-ranging sea turtle populations. It covers topics including contextualization of health assessments for conservation efforts, study design, health analyte selection criteria, project planning and execution, noninvasive sampling and the Three Rs, potential pitfalls and troubleshooting, tips from the field, and implications for conservation. Various strategies for addressing the logistical challenges of assessing sea turtle health for different life history stages are discussed. Because sea turtle population viability is inseparable from the health of the individuals that constitute a given population, monitoring and understanding sea turtle health is intrinsically necessary for effective conservation measures.
Injuries inflicted by sharks are a frequent observation in stranded sea turtles. Sharks prey on live turtles and scavenge carcasses, which can create uncertainty as to the cause of stranding when sea turtles are found dead with shark-bite wounds. Consequently, attributing the cause of stranding to a shark attack based purely on the presence of the characteristic wounds can overestimate predation by sharks as a cause of mortality. To better characterize the timing of shark-bite wounds relative to death of sea turtles in the southeastern USA, we performed necropsies on 70 stranded turtles that were found dead in which the predominant observation was bite wounds without any grossly evident vital responses (inflammation or healing). Postmortem examination included assessment for evidence of exsanguination and histopathological evaluation of skeletal muscle comprising wound margins. We characterized wounds as antemortem, perimortem, or postmortem based on specific criteria related to the presence or absence of supravital and intravital responses. Most (80%) shark-bite wounds were postmortem, 10% were antemortem, and 10% were perimortem. We found that antemortem and postmortem wounds were similar in extent and location except for wounds that primarily involved the shell, which were never found in cases of scavenging. For sea turtles found dead in the southeastern USA, our findings suggest that most shark-bite wounds without externally evident vital responses are due to scavenging. Additionally, this scavenging can significantly damage a carcass, potentially obscuring the detection of other causes of mortality. These findings should be considered when using data derived from stranded sea turtles to conduct mortality assessments.
Tiger sharks (Galeocerdo cuvier) selectively prey on large cheloniid sea turtles. Of 404 tiger shark stomachs examined in eight previously published studies, 327 (80.9%) contained food; and 68 (20.8%) of those sharks with food also contained large cheloniid sea turtle remains. The literature indicates that tiger sharks are the only elasmobranch predator that consistantly earts large boney turtles. Tiger sharks are able to specialize in such a diet because they share the same inshore habitat as turtles, and have developed a unique masticating mechanism and feeding behavior. A rational tiger shark harvesting program is suggested to reduce predation on some endangered sea turtle populations.
A leatherback turtle (Dermochelys coriacea) nesting on St. Croix, Virgin Islands, was fitted with a satellite transmitter and released on 3 May 1989. During 18 days of tracking, the turtle traveled a minimum of 515.2 km at an overall mean speed of 1.2 km/h and nested twice more, first on Vieques Island and then on Culebra Island, where the transmitter was removed from the turtle. The turtle was abraded by the attachment harness, and the harness and transmitter had been bitten by a shark. Submergence data were obtained in continuous 12-h periods. Mean surface duration was 20.6 sec, and mean dive duration was 2.27 min per event; and in the 12-h periods, the turtle dove an average of 319 times. The mean submergence duration for 32 12-h periods was 625. 4 min (86.9%), whereas the mean surface duration was 94.6 min per 12-h period (13.1%).
This paper describes predation tactics used by the saltwater crocodile (Crocodylus porosus) on flatback (Natator depressus) and olive ridley (Lepidochelys olivacea) sea turtles on nesting beaches in northern Australia. For adult turtles, crocodiles used both a sit-and-wait tactic in which they attacked a turtle at the water's edge after it completed nesting and an active hunting strategy in which crocodiles followed turtle tracks into the dunes to attack turtles at nest sites. Saltwater crocodiles also hunted sea turtle hatchlings in the dunes and excavated a sea turtle nest and consumed the eggs. The protection of saltwater crocodiles in Australia starting in the early 1970s has led to increased population sizes and a greater proportion of larger individuals. This likely has resulted in increased predation rates on sea turtles over several decades, which should be considered as an important mortality component for some tropical nesting aggregations.
A total of 591 great white sharks Carcharodon carcharias was caught between 1974 and 1988 in the gill nets which are maintained along the Natal coast to protect bathers from shark attack. The species represented 2,7 per cent of the total catch of sharks for the period 1978–1988 and the mean catch rate was 1,0 shark·km-net−1·year−1. Specimens ranged in size from 131 to 348 cm precaudal length, with a mode of 206–210 cm for females and 216–220 cm for males. None was mature, and the sex ratio was 1 male to 1,4 females. Catch rates were highest in the southernmost nets, with a smaller peak in the extreme north. Catches peaked in midwinter, those in the warmer, northern region lagging 1–2 months behind those in the cooler southern waters. The relationship between catches and water clarity and temperature was tenuous. Elasmobranchs were the most important prey item, occurring in 41,1 per cent of stomachs containing food, followed by teleosts at 34,7 per cent and marine mammals at 29,0 per cent, although marine mammals, mainly cetaceans, were the most common prey category in sharks longer than 250 cm. Pelagic prey items were more common than demersal species and there was evidence of sharks scavenging on animals from each of the main prey groups.
We collected simultaneous dive Time Depth Recorder (TDR) data and video images from free swimming adult female leatherback turtles, Dermochelys coriacea, during the first 24 h after nesting on the beach, in order to determine relationships between dive parameters, activity, overall respiratory frequency and behaviour.We identified three different underwater locomotory activities (subsurface swimming, V-shaped dives and U-shaped dives) from video and TDR data that varied in their mean depth, duration and a number of other parameters. Overall respiratory frequency (overall fR) was significantly different between all locomotory activities, with turtles taking 1.7±0.1 breaths min−1 while subsurface swimming, 0.78 breaths min−1 after V-shaped dives and 0.57 breaths min−1 after U-shaped dives. Descent rates and ascent rates were significantly faster in U-shaped dives (descent 0.19±0.010 m s−1, ascent 0.28±0.015 m s−1) than in V-shaped dives (descent 0.10±0.008 m s−1, ascent 0.12±0.012 m s−1). Flipper stroke rates were significantly lower during the bottom component of U-shaped dives (0.18±0.02 strokes s−1) than during the descent (0.29±0.03 strokes s−1) or ascent (0.29±0.03 strokes s−1). From overall fR and flipper stroke rate data, we inferred that turtles used less energy during U-shaped dives than the other activity types. We recorded interactions between male turtles and the study females that lasted up to 11 min, during which male turtles displayed the characteristic courtship behaviour of sea turtles. It appeared that females attempted to avoid males by aborting ascent and extending dive duration to swim to the sea floor when males were present.
Understanding population status for endangered species is critical to developing and evaluating recovery plans mandated by the Endangered Species Act. For sea turtles, changes in abundance are difficult to detect because most life stages occur in the water. Currently, nest counts are the most reliable way of assessing trends. We determined the rate of growth for leatherback turtle (Dermochelys coriacea) nest numbers in Florida (USA) using a multilevel Poisson regression. We modeled nest counts from 68 beaches over 30 years and, using beach-level covariates (including latitude), we allowed for partial pooling of information between neighboring beaches. This modeling approach is ideal for nest count data because it recognizes the hierarchical structure of the data while incorporating variables related to survey effort. Nesting has increased at all 68 beaches in Florida, with trends ranging from 3.1% to 16.3% per year. Overall, across the state, the number of nests has been increasing by 10.2% per year since 1979. Despite being a small population (probably,1000 individuals), this nesting population may help achieve objectives in the federal recovery plan. This exponential growth rate mirrors trends observed for other Atlantic populations and may be driven partially by improved protection of nesting beaches. However, nesting is increasing even where beach protection has not been enhanced. Climate variability and associated marine food web dynamics, which could enhance productivity and reduce predators, may be driving this trend.
Analyses of the stomach contents of 399 male and female mako sharks (Isurus oxyrinchus) ranging from 67 to 328 cm are presented. Samples are from shark fishing tournaments held in New Jersey, New York, Rhode Island, and from longline catches taken between Cape Hatteras and the Grand Banks. Teleost remains occur in 67% of the stomachs with bluefish (Pomatomus saltatrix) constituting 77.5% of the diet by volume. Bluefish is the major inshore food item. Cephalopoda amount to 15% of the stomach contents by frequency of occurrence and are consumed primarily offshore. Consumption and diet are the same for both sexes. The average capacity of the stomach is 10% of the body weight. Estimates of daily ration average 2 kg (range from 1.4 to 2.7 kg). Makos may consume 4.3 to 14.5% of the available bluefish resource in the area between Cape Hatteras, NC, and Georges Bank.Key words: shortfin mako, Isurus oxyrinchus; elasmobranchs, food habits, stomach content analyses, daily ration, Pomatomus saltatrix, biomass
Large sharks have the potential to help structure ecosystem dynamics through top-down impacts on their prey, including sea turtles. Studies of interactions between large sharks and sea turtles, however, are practically nonexistent along the Brazilian coast. Between September 2002 and May 2011 we examined 655 sea turtles including green turtles Chelonia mydas (n = 607), olive ridleys Lepidochelys olivacea (n = 10), hawksbills Eretmochelys imbricata (n = 33), and loggerheads Caretta caretta (n = 5) that stranded on Paraiba coast, northeastern Brazil. A total of 63 green turtles (10.4%), two olive ridleys (20.0%) and one hawksbill (3.0%) had shark-inflicted bites. Most bites could not be definitively attributed to scavenging or attacks on living turtles, but the presence of healed shark bites and freshly bleeding bites suggests that some attacks occurred pre-mortem. Bite characteristics suggest that tiger sharks Galeocerdo cuvier were responsible for most bites that could be identified to a particular species. Within green turtles, the only species with sufficient sample size, the probability of carcasses having been bitten increased with carapace length but did not vary across seasons and years. However, there was spatial variation in the probability of a carcass having been bitten by sharks. Our estimates of the minimum proportion of turtles attacked while alive (similar to 4%) and bitten overall are similar to other areas where shark-turtle interactions have been studied. Turtles likely are an important food for tiger sharks in northeastern Brazil, but further studies are needed to determine the relative frequencies of scavenging and predation.
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.
Sex-biased predation rates have been documented in marine turtles, but no study to date has addressed the causes of such bias. In Shark Bay, male Loggerhead Turtles (Caretta caretta) display evidence of Tiger Shark (Galeocerdo cuvier) attack more often than do females, and sex-specific differences in swim speed or maneuverability have been posited as a possible reason for this pattern. We used simulated shark attacks (boat chases) in shallow water to test this hypothesis. Differences in speed and maneuverability between males and females were not detected, indicating that the heightened vulnerability to predation of male Loggerhead Turtles in Shark Bay requires a different explanation.
Shark predation may have been a central factor influencing the evolution of sociality in dolphins, as well as a determinant of dolphin habitat use and behavior. To understand the role of predation in driving interpopulation differences in behavior and sociality, it is important to quantify differences in predation risk among populations. This study describes the frequency of shark-inflicted scars and estimates the shark attack rate on bottlenose dolphins (Tursiops aduncus) in Shark Bay, Western Australia. Shark bite scars were found on 74.2% (95 of 128) of non-calves, and most of these scars were inflicted by tiger sharks (Galeocerdo cuvier). Although there were no differences among age/sex classes in the frequency of scarring, significantly more adult males than adult females bore multiple scars. The rate of unsuccessful shark attack was estimated to be between 11% and 13% of dolphins attacked each year. Large sharks (>3 m) were responsible for a disproportionate number of attacks. However, bites from small carcharhinid sharks on 6.2% of dolphins suggest that some of these small sharks may be dolphin ectoparasites. Both the scar frequencies and attack rate suggest that Shark Bay dolphins face a greater risk of predation than bottlenose dolphins in other locations.
Sharks are marine consumers believed to occupy top positions in marine food webs. But surprisingly, trophic level estimates for these predators are almost non-existent. With the hope of helping better define the ecological role of sharks in marine communities, this paper presents standardized diet compositions and trophic levels calculated for a suite of species. Dietary composition for each species was derived from published quantitative studies using a weighted average index that takes into account sample size in each study. The trophic level (TL) values of the 11 food types used to characterize the diet (obtained from published accounts) were then used to calculate fractional trophic levels for 149 species representing eight orders and 23 families. Sharks as a group are tertiary consumers (TL>4), and significant differences were found among the six orders compared, which were attributable to differences between orectolobiforms (TL<4) and all other orders, and between hexanchiforms and both carcharhiniforms and squatiniforms. Among four families of carcharhiniform sharks, carcharhinids (TL=4.1, n=39) had a significantly higher TL than triakids (TL=3.8, n=19) and scyliorhinids (TL=3.9, n=21), but not sphyrnids (TL=3.9, n=6). When compared to trophic levels for other top predators of marine communities obtained from the literature, mean TL for sharks was significantly higher than for seabirds (n=28), but not for marine mammals (n=97). Trophic level and body size were positively correlated (r s =0.33), with the fit increasing (r s =0.41) when the three predominantly zooplanktivorous sharks were omitted, and especially when considering only carcharhinid sharks (r s =0.55).
The importance of interactions between sharks and cetaceans has been a subject of much conjecture, but few studies have addressed these interactions. Sharks (order Selachii) have been hypothesized to be important predators on dolphins and porpoises (suborder Odontoceti). Unfortunately, there are often few data to back up claims that certain shark species are major threats to cetaceans. To help identify potential shark predators in speci®c locations, available data on interactions with odontocetes for all shark species that may include cetaceans in their diet are reviewed. Shark species are categorized into groups based on predatory interactions with dolphins and porpoises (regular predators, occasional predators, potential predators, ectoparasites and insuf®cient data). Several shark species that have been overlooked in the cetacean literature are identi®ed as potentially important predators while others that have been suspected to be important predators are probably at most occasional predators. How shark predation can in¯uence dolphin populations, habitat use, group size and behaviour is discussed. How risk of shark predation can vary with habitat attributes in both nearshore and pelagic waters is also discussed. Predator±prey interactions have been the focus of most studies of shark±dolphin interaction, but competitive interactions may also occur. The ®rst analysis of shark±dolphin dietary overlap is presented, which shows it to be signi®cant between common dolphins and several species of sharks, including species that prey upon these dolphins.