Migratory behaviour and spatial dynamics of large sharks and their conservation implications
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Determining the dynamic nature of animal movement has been an important component in a wider understanding of animal population ecology. Generally, this is because temporal change in the density of a population at a specific geographic location is not only a function of births and deaths but also of movements, including migration. The increased availability of remote telemetry and biologging systems in recent years has enabled many studies tracking marine predators, such as turtles, seabirds and marine mammals, but a general understanding of spatial dynamics in large sharks remains less well developed. This is in part due to few studies having achieved sufficiently long-term, multi-year tracks to detect changes in movement behaviour over time. Determining the timing, repeatability and potential motivations for movements of large sharks is necessary to understand the ecological and evolutionary role of such behaviour more generally in marine predators. Furthermore, given global concerns of declining shark populations, a detailed appreciation of shark movements can reveal the extent 6 of overlap with area-focused human activities (e.g. fishing), as well as inform assessments of population trends and spatial management options. In order to demonstrate how shark migratory behaviour and spatial dynamics can vary dramatically depending on the species and location, with subsequent contrasting conservation implications, the present work used longterm, remote telemetry to reveal detailed patterns in shark movement behaviour at two very different geographical scales: broad-scale movements of larger species that encompass ocean basins, versus fine-scale movements of reef-associated species at a remote atoll. First, using satellite telemetry, it was revealed for the tiger shark Galeocerdo cuvier, that adult males undertake annually repeated, roundtrip migrations of over 7,500 km in the northwest Atlantic. Second, acoustic telemetry was used to determine the fine-scale spatial dynamics of a multispecies shark assemblage at a small, remote atoll in the Seychelles, Indian Ocean, where a number of species displayed perennial residency. While the fine-scale movements of reef sharks in the Seychelles suggest an MPA of moderate size may be an effective management option, the long-distance migrations of the tiger sharks in the Atlantic reveal that conservation efforts targeting them must account for dynamic fisheries interactions over large geographical scales, potentially requiring time-area closures to be effective. Examining the long-term movement behaviour of different shark species over contrasting geographical scales has emphasised the importance of understanding spatial dynamics when informing management decisions, and has contributed to a wider understanding of the population ecology of these species.