Relative abundance and movement ecology of tiger sharks Galeocerdo cuvier in the waters surrounding Bimini, the Bahamas

Abstract

Shark populations are under increased anthropogenic pressures around the world. Large-bodied shark species are upper trophic level predators, and therefore there is concern that declining numbers of sharks may have significant implications for ecosystems. In response to these conservation concerns there has been increased focus on the implementation of regulations, fishery management plans, national plans of action, and marine management zones in some nations. However, large sharks are often highly mobile and can move readily across jurisdictional boundaries. Therefore, there is a need to monitor long-term relative abundances within regions and simultaneously examine the duration to which species remain in these areas. The Bahamas was established as a 'Shark Sanctuary' in 2011, but long-term abundance and movement data are lacking for many species. The goal of this study is to determine the relative abundance and movement ecology for tiger sharks Galeocerdo cuvier in the waters around Bimini, The Bahamas. Fishery independent ongline surveys from 1984 to 2019 suggest that the local relative abundance remained stable during this time. Gear selectively prevented direct comparisons of catch rates to other surveys, but for context abundance trend were analyzed from the directed shark fishery in the nearby southeastern USA, which interacts with the same population of tiger sharks. This analysis showed an increasing abundance trend in USA waters during the late 1990s and early 2000s, after which abundances stabilized. Conventional tagging and passive acoustic and satellite telemetry were used to determine residency, regional movements, and long-term philopatry of young-of-the-year to mature tiger sharks. Overall data for juveniles were sparse, likely due to constraints of energetic condition and high natural mortality. Large juvenile and mature tiger sharks displayed seasonal local residency that was negatively correlated with water temperature, but also dispersed widely throughout the region, and spent significant time outside of Bahamas territorial waters. Taken together, these results highlight that localized conservation measures offer some level of protection for tiger sharks, however they do spend time in multiple jurisdictions and regional cooperation on management plans is important.