Dynamics of marine predators off an oceanic island and implications for management of a preventative shark fishing program

Abstract

Oceanic islands are productive ecosystems, and so have higher densities of many marine predators. We investigated the dynamics of elasmobranch and teleost predators in coastal waters off Reunion Island, Indian Ocean, using fisheries-independent data from a preventative shark fishing program from January 2014 to December 2019. We developed a moonlight index that calculates exact moonlight through incorporating lunar azimuth, elevation angle and island topography. We quantified spatial–temporal and environmental drivers of occurrence using zero-inflated mixed models and assessed species-specific catchability in the program. A consistent segregated pattern was observed with higher occurrence of all species at dusk and after-dusk associated with lower luminosity. Scalloped hammerhead sharks (Sphyrna lewini) and giant trevally (Caranx ignobilis) were found to patrol coastal waters earlier in the day than the other species. Tiger (Galeocerdo cuvier) and bull (Carcharhinus leucas) sharks showed high spatial segregation, potentially reducing competition. Teleost predators were found more frequently inside the coral reef environment of the Marine Protected Area but there was no clear pattern for sharks. Seasonality was observed for giant trevally, stingrays, bull sharks, and giant guitarfish (Rhynchobatus australiae), with higher presence during early winter periods related to turbidity, photosynthetically available radiation, and temperature. Inter-annual variation in catch rates suggested that juvenile tiger sharks might be replacing bull sharks in nearshore habitats, and the consequences for mitigation of shark bite hazard are discussed. Operational alternatives are proposed to enhance reducing the impacts of preventative shark fishing upon critically endangered species, improve their conservation and ensure local ecosystem balance.