A Comparison of the Seasonal Movements of Tiger Sharks and Green Turtles Provides Insight into Their Predator-Prey Relationship (original) (raw)
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Biology of sea turtles under risk from tiger sharks at a foraging ground
Marine Ecology Progress Series, 2005
It is important to understand the population structure and space use of sea turtles because of their potential effects on the dynamics of nearshore ecosystems. Much of our current understanding is skewed because the ecology of sea turtles on their foraging grounds is known best from areas where there are, or have been, major human impacts. We studied green Chelonia mydas and loggerhead Caretta caretta sea turtles on the relatively pristine feeding ground of Shark Bay, Western Australia, where tiger sharks Galeocerdo cuvier are a major mortality agent. Recapture rates were lower for green than for loggerhead turtles, possibly reflecting a larger population rather than low site fidelity for greens. The sex ratio of loggerhead turtles was not significantly different from 1:1, whereas green turtles showed a strong female bias. Size distributions of both species were skewed towards larger (and presumably older) individuals relative to a human-impacted feeding ground in eastern Australia. Body condition varied temporally for green turtles, but not for loggerhead turtles, possibly due to longer distances traveled to nesting beaches by green turtles. Rates of shark-inflicted injuries were higher for loggerhead turtles, especially males, than for greens. Sublethal effects of these injuries were not evident. There are notable differences between sea turtles threatened primarily by tiger sharks in Shark Bay and populations on feeding grounds where historical and current mortality causes are anthropogenic. We conclude that without baseline data from relatively pristine habitats our understanding of human impacts on sea turtle populations suffers from a limited scope.
Biology of green and loggerhead turtles under risk from tiger sharks at a foraging ground
Marine Ecology Progress Series, 2005
It is important to understand the population structure and space use of sea turtles because of their potential effects on the dynamics of nearshore ecosystems. Much of our current understanding is skewed because the ecology of sea turtles on their foraging grounds is known best from areas where there are, or have been, major human impacts. We studied green Chelonia mydas and loggerhead Caretta caretta sea turtles on the relatively pristine feeding ground of Shark Bay, Western Australia, where tiger sharks Galeocerdo cuvier are a major mortality agent. Recapture rates were lower for green than for loggerhead turtles, possibly reflecting a larger population rather than low site fidelity for greens. The sex ratio of loggerhead turtles was not significantly different from 1:1, whereas green turtles showed a strong female bias. Size distributions of both species were skewed towards larger (and presumably older) individuals relative to a human-impacted feeding ground in eastern Australia. Body condition varied temporally for green turtles, but not for loggerhead turtles, possibly due to longer distances traveled to nesting beaches by green turtles. Rates of shark-inflicted injuries were higher for loggerhead turtles, especially males, than for greens. Sublethal effects of these injuries were not evident. There are notable differences between sea turtles threatened primarily by tiger sharks in Shark Bay and populations on feeding grounds where historical and current mortality causes are anthropogenic. We conclude that without baseline data from relatively pristine habitats our understanding of human impacts on sea turtle populations suffers from a limited scope.
Behavioral evidence suggests facultative scavenging by a marine apex predator during a food pulse
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:
Marine and Freshwater Research, 2012
Large marine vertebrates are particularly susceptible to anthropogenic threats because they tend to be long-lived, late to mature and wide-ranging. Loggerhead sea turtles (Caretta caretta) are characterised by such life history traits and are listed as ‘Endangered’ by The World Conservation Union. Although juvenile movements and at-sea behaviour of adult females are relatively well studied, little is known about the movements of males and their subsequent exposure to threats. Shark Bay, Western Australia, is home to the largest breeding population of loggerhead turtles in Australia. We assessed the large-scale movements of nine adult male loggerhead turtles, with the goal of aiding conservation and management policies. During 7 months outside the breeding season, all nine turtles stayed within the Shark Bay World Heritage Area, with most showing fidelity to small coastal foraging areas. Several turtles, however, showed relatively large movements between core foraging areas. None of ...
The 'landscape of fear' model has been proposed as a unifying concept in ecology, describing, in part, how animals behave and move about in their environment. The basic model predicts that as an animal's landscape changes from low to high risk of predation, prey species will alter their behavior to risk avoidance. However, studies investigating and evaluating the landscape of fear model across large spatial scales (tens to hundreds of thousands of square kilometers) in dynamic open aquatic systems involving apex predators and highly mobile prey are lacking. To address this knowledge gap, we investigated predator-prey relationships between tiger sharks (Galeocerdo cuvier) and loggerhead turtles (Caretta caretta) in the North Atlantic Ocean. This included the use of satellite tracking to examine shark and turtle distributions as well as their surfacing behaviors under varying levels of home range overlap. Our findings revealed patterns that deviated from our a priori predictions based on the landscape of fear model. Specifically, turtles did not alter their surfacing behaviors to risk avoidance when overlap in shark-turtle core home range was high. However, in areas of high overlap with turtles, sharks exhibited modified surfacing behaviors that may enhance predation opportunity. We suggest that turtles may be an important factor in determining shark distribution, whereas for turtles, other life-history trade-offs may play a larger role in defining their habitat use. We propose that these findings are a result of both biotic and physically-driven factors that independently or synergistically affect predator-prey interactions in this system. These results have implications for evolutionary biology, community ecology and wildlife conservation. Further, given the difficulty in studying highly migratory marine species, our approach and conclusions may be applied to the study of other predator-prey systems.
Shark-inflicted injury frequencies, escape ability, and habitat use of green and loggerhead turtles
Marine Biology, 2002
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.
Stable isotope data are useful for inferring foraging and niche variation in marine taxa but can be difficult to interpret, in part because different foraging patterns may result in similar isotopic values. Here, we integrate stable isotope analysis (δ 13 C and δ 15 N) with behavioral data to investigate the foraging ecology of loggerhead turtles Caretta caretta on a feeding ground in Shark Bay, Western Australia. Large loggerhead turtles showed little among-individual isotopic variance in skin samples, suggesting similar foraging or habitat use patterns over several months or more. Analysis of loggerhead foraging in video data, and comparison with isotopic variance for sympatric green turtles Chelonia mydas, suggest that low isotopic variance among large loggerheads reflects a similar, highly generalized diet within individuals. Higher isotopic variance among smaller turtles may reflect variation in diet, timing of recruitment to neritic habitat or use of food webs varying along other isotopic gradients. Loggerheads showed strong fidelity to the study site over many years, and individuals recaptured frequently showed remarkable affinity for very small geographic areas, often < 5 km 2 . Thus, a substantial proportion of the Shark Bay loggerhead population comprises site specialists, with larger adults appearing to be diet generalists. Our results also suggest that among-individual isotopic variation found at some loggerhead nesting locations may reflect the isotopic characteristics of preferred migratory or foraging grounds owing to long-term site fidelity and less likely reflects prey specialization by individuals within specific feeding areas.
Environmental predictors of foraging and transit behaviour in flatback turtles Natator depressus
Endangered Species Research, 2017
Sea turtles migrate between nesting beaches and foraging grounds, but little is known about the cues they use to direct these migrations, and the habitats that define their foraging grounds. Here, we used satellite telemetry to follow the movements of 11 flatback turtles Natator depressus after nesting on islands in the waters off the coast of the Kimberley region of northern Australia. State-space models were used to objectively define inter-nesting, migration and foraging behaviour during the 327 ± 315 d (mean ± SD) that the turtles were tracked. These animals migrated along the coast in water depths of 63 ± 5 m to foraging grounds on the mid-Sahul Shelf in the Timor Sea in average water depths of 74 ± 12 m, 135 ± 35 km from shore. Distribution modelling showed that flatback turtles preferred foraging and transiting in clear waters (suspended material < 0.06 g m −3), 60 to 90 m deep and in association with complex, benthic geomorphology (banks, shoals, terraces, deep holes and valleys) thought to support a high abundance of sessile invertebrates, the likely targets of their foraging. Distance to the tidal front was also a strong predictor of turtle migratory behaviour, with the animals potentially following tidal fronts along the Kimberley coast. Our study identified both critical habitats for this species and the environmental variables that predict their migration and foraging. This information is important to aid spatial planning of conservation for this data-deficient species that is endemic to northern Australia.
Methods in Ecology and Evolution, 2012
During aerial or boat-based surveys for large-bodied diving taxa (e.g. marine mammals and marine turtles), a proportion of animals present will be missed because they are submerged and out of view, leading to 'availability bias' in abundance indices. Information on dive-surfacing patterns can improve corrections for availability bias. However, as dive data are typically limited, availability correction factors are often based on poorly resolved dive and surface times, and diving heterogeneity is not considered. 2. We collected dive records for green turtles Chelonia mydas, Linnaeus 1758, and loggerhead turtles Caretta caretta, Linnaeus 1758, on a foraging ground in Shark Bay, Western Australia to quantify dive-surfacing patterns and assess potential correlations with easily measured environmental features: habitat depth and water temperature. Bayesian regression models were used to predict dive and surface interval durations across temperature-depth gradients and assess their uncertainty. We used these predictions to quantify variation in availability correction factors, which were multipliers designed, in this case, to adjust surface sightings data to incorporate diving animals. 3. Dive and surface interval durations for both species varied positively with depth and negatively with temperature, consistent with a priori expectations, although temperature effects were not always significant. Dive metrics were predictable, although uncertainty increased in deeper habitat with few observed dives. 4. Availability correction factors were highly heterogeneous, with larger corrections necessary in colder, deeper conditions (long-diving, infrequent surfacing behaviour) and smaller corrections required in warmer, shallower conditions (short-diving, frequent-surfacing behaviour). 5. Predictable variation in the diving behaviour of chelonid sea turtles across environmental gradients on a foraging ground reveals that site-specific knowledge of dive-surfacing patterns can be important to mitigate the effects of availability bias during population surveys. Accounting for such trends may improve the reliability of ecological inferences (e.g. spatiotemporal distribution trends) and the efficacy of applications (e.g. conservation planning) based on survey data.
Habitat use and foraging behavior of tiger sharks ( Galeocerdo cuvier ) in a seagrass ecosystem
Marine Biology, 2002
Understanding the foraging behavior and spatial distribution of top predators is crucial to gaining a complete understanding of communities. However, studies of top predators are often logistically dicult and it is important to develop appropriate methods for identifying factors in¯uencing their spatial distribution. Sharks are top predators in many marine communities, yet no studies have quanti®ed the habitat use of large predatory sharks or determined the factors that might in¯uence shark spatial distributions. We used acoustic telemetry and animal-borne video cameras (``Crittercam'') to test the hypothesis that tiger shark (Galeocerdo cuvier) habitat use is determined by the availability of their prey. We also used Crittercam to conduct the ®rst investigation of foraging behavior of tiger sharks. To test for habitat preferences of sharks, the observed proportion of time in each habitat for each individual was compared to the predicted values for that individual based on correlated random walk and track randomization methods. Although there was individual variation in habitat use, tiger sharks preferred shallow seagrass habitats, where their prey is most abundant.