State-dependent risk-taking by green sea turtles mediates top-down effects of tiger shark intimidation in a marine ecosystem

1. A predictive framework of community and ecosystem dynamics that applies across systems has remained elusive, in part because non-consumptive predator effects are often ignored. Further, it is unclear how much individual-level detail community models must include. 2. Previous studies of short-lived species suggest that state-dependent decisions add little to our understanding of community dynamics. Body condition-dependent decisions made by long-lived herbivores under risk of predation, however, might have greater community-level effects. This possibility remains largely unexplored, especially in marine environments. 3. In the relatively pristine seagrass community of Shark Bay, Australia, we found that herbivorous green sea turtles (Chelonia mydas Linnaeus, 1758) threatened by tiger sharks (Galeocerdo cuvier Peron and LeSueur, 1822) select microhabitats in a condition-dependent manner. Turtles in poor body condition selected profitable, high-risk microhabitats, while turtles in good body condition, which are more abundant, selected safer, less profitable microhabitats. When predation risk was low, however, turtles in good condition moved into more profitable microhabitats. 4. Condition-dependent use of space by turtles shows that tiger sharks modify the spatio-temporal pattern of turtle grazing and their impacts on ecosystem dynamics (a trait-mediated indirect interaction). Therefore, state-dependent decisions by individuals can have important implications for community dynamics in some situations. 5. Our study suggests that declines in large-bodied sharks may affect ecosystems more substantially than assumed when non-lethal effects of these top predators on mesoconsumers are not considered explicitly

Biology of sea turtles under risk from tiger sharks at a foraging ground

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

The relative importance of reproduction and survival for the conservation of two dolphin populations

It has been proposed that in slow-growing vertebrate populations survival generally has a greater influence on population growth than reproduction. Despite many studies cautioning against such generalizations for conservation, wildlife management for slow-growing populations still often focuses on perturbing survival without careful evaluation as to whether those changes are likely or feasible. Here, we evaluate the relative importance of reproduction and survival for the conservation of two bottlenose dolphin (Tursiops cf aduncus) populations: a large, apparently stable population and a smaller one that is forecast to decline. We also assessed the feasibility and effectiveness of wildlife management objectives aimed at boosting either reproduction or survival. Consistent with other analytically based elasticity studies, survival had the greatest effect on population trajectories when altering vital rates by equal proportions. However, the findings of our alternative analytical approaches are in stark contrast to commonly used proportional sensitivity analyses and suggest that reproduction is considerably more important. We show that 1. in the stable population reproductive output is higher, and adult survival is lower; 2. the difference in viability between the two populations is due to the difference in reproduction; 3. reproductive rates are variable, whereas survival rates are relatively constant over time; 4. perturbations on the basis of observed, temporal variation indicate that population dynamics are much more influenced by reproduction than by adult survival; 5. for the apparently declining population, raising reproductive rates would be an effective and feasible tool to reverse the forecast population decline; increasing survival would be ineffective. Our findings highlight the importance of reproduction – even in slow-growing populations – and the need to assess the effect of natural variation in vital rates on population viability. We echo others in cautioning against generalizations based on life-history traits and recommend that population modeling for conservation should also take into account the magnitude of vital rate changes that could be attained under alternative management scenarios

Social Network Analysis Reveals the Subtle Impacts of Tourist Provisioning on the Social Behavior of a Generalist Marine Apex Predator

Shark dive ecotourism is a lucrative industry in many regions around the globe. In some cases, sharks are provisioned using bait, prompting increased research on how baited dives influence shark behavior and yielding mixed results. Effects on patterns of habitat use and movement seemly vary across species and locations. It is unknown, however, whether wide-ranging, marine apex predators respond to provisioning by changing their patterns of grouping or social behavior. We applied a tiered analytical approach (aggregation-gregariousness-social preferences) examining the impact of provisioning on the putative social behavior of tiger sharks (Galeocerdo cuvier) at a dive tourism location in The Bahamas. Using network inference on three years of acoustic tracking data from 48 sharks, we tested for non-random social structure between non-provisioned and provisioned monitoring sites resulting in 12 distinct networks. Generally considered a solitary nomadic predator, we found evidence of sociality in tiger sharks, which varied spatiotemporally. We documented periods of both random (n = 7 networks) and non-random aggregation (n = 5 networks). Three of five non-random aggregations were at locations unimpacted by provisioning regardless of season, one occurred at an active provisioning site during the dry season and one at the same receivers during the wet season when provision activity is less prevalent. Aggregations lasted longer and occurred more frequently at provisioning sites, where gregariousness was also more variable. While differences in gregariousness among individuals was generally predictive of non-random network structure, individual site preferences, size and sex were not. Within five social preference networks, constructed using generalized affiliation indices, network density was lower at provisioning sites, indicating lower connectivity at these locations. We found no evidence of size assortment on preferences. Our data suggest that sociality may occur naturally within the Tiger Beach area, perhaps due to the unusually high density of individuals there. This study demonstrates the existence of periodic social behavior, but also considerable variation in association between tiger sharks, which we argue may help to mitigate any long-term impacts of provisioning on this population. Finally, we illustrate the utility of combining telemetry and social network approaches for assessing the impact of human disturbance on wildlife behavior

Large sharks exhibit varying behavioral responses to major hurricanes

Under global climate change, storm events are predicted to increase in strength and frequency. Although aquatic animals can be affected by acute natural disturbances, information on the immediate consequences of these weather systems on the behavioral ecology of highly mobile aquatic predators remains limited. Here we examine the spatial distributions, activity spaces, and ecological change-points of four large shark species (mean: 193 cm fork length ± 70 SD) via passive acoustic telemetry in two different locations in the subtropical Atlantic (Little Bahama Bank, Bahamas, and Biscayne Bay, Florida, USA) in relation to two separate major hurricane events (category 4 and 5). We tested whether sharks would evacuate shallow coastal habitats (and thus exit the acoustic arrays) during the hurricanes and exhibit comparable size of activity spaces pre- and post-storms, as has been previously found for smaller (50–150 cm fork length) sharks elsewhere. Located on the northwest edge of the Little Bahama Bank, Bahamas, an acoustic array consisting of 32 acoustic telemetry receivers tracking tiger sharks Galeocerdo cuvier sustained a direct hit from Hurricane Matthew in 2016. Daily detections of tagged tiger sharks within the array were consistent before and during the hurricane. Immediately following the storm, daily tiger shark detections approximately doubled. Size and extent of tiger shark activity space within the array were consistent pre- and post-storm. Located off Miami, within Biscayne Bay, an array of 32 acoustic receivers tracking bull Carcharhinus leucas, nurse Ginglymostoma cirratum, and great hammerhead sharks Sphyrna mokarran was exposed to tropical-storm-force winds from Hurricane Irma in 2017. As the eye of the storm passed 140 km to the west, most sharks previously present in the array were no longer detected, while two nurse sharks remained at receivers near Miami. Numbers of tagged bull sharks declined following Hurricane Irma, whereas other species did not. Ecological change point analyses indicated that seasonal changes - rather than storm conditions - cannot be ruled out as the primary driver of post-storm shark behavior. Unlike smaller shark species which have previously been found to evacuate shallow water habitats during storms, we found variable responses of large sharks to storm events. •The behavioral effects of tropical storms have not been evaluated for large shark species.•Acoustic telemetry was used to study the behavior of large sharks at two sites hit by hurricanes.•Species-specific responses were observed.•Several species did not evacuate shallow water as reported in small species.•Interspecific behavior, including why some sharks failed to evacuate stormimpacted areas, is discussed

Spatial variations in the concentrations of mercury and persistent organic pollutants in free-ranging bottlenose dolphins from South Florida

The bottlenose dolphin (Tursiops truncatus) is an abundant top predator found in nearshore waters of South Florida including heavily developed and remote coastal waters. The objective of this study was to quantify total mercury (T-Hg) and persistent organic pollutant (POPs: NDL-PCBs, PBDEs, DDT, HCH, HCB, PCDD/Fs and DL-PCBs) levels in bottlenose dolphins found off the highly populated island of Key West (n = 27) and from the undeveloped Florida Coastal Everglades (FCE; n = 20). T-Hg and POPs were analyzed in skin and blubber tissues, respectively, using a Direct Mercury analyzer (for T-Hg), GC-ECD (POPs) and GC-HRMS (DLCs). The 7 ICES PCBs were the main compounds found in bottlenose dolphins from Key West (8229 ng.g-1 lipids) and the FCE (2289 ng.g-1 lipids), while the concentrations of PCDD/Fs remained low (Key West: 104 pg.g-1 lipids, FCE: 102 pg.g-1 lipids). POP concentrations were higher in individuals from Key West compared to those from the FCE. However, POP concentrations in Key West dolphins were lower than those from other locations in Florida and around the world. Unlike organic pollutants, T-Hg concentrations were significantly higher in FCE dolphins (Key West: 2941 ng.g-1 dw versus, FCE: 9314 ng.g-1 dw), with the highest concentrations reported from the southeastern US. PCB concentrations remained under previously suggested threshold for adverse health effects (including immunosuppression) in marine mammals of 17 000 ng/kg lipid. To conclude, sources of T-Hg and POPs differed between Key West and the FCE as reflected by their concentrations in skin and blubber of free-ranging bottlenose dolphins highlighting their role as sentinels of their environment