Minimising dolphin-induced bait depredation in a bather protection shark management program

Abstract

Context The New South Wales (NSW) shark management bather protection program deploys 305 shark-management-alert-in-real-time (SMART) drumlines daily across 20 locations to catch, tag and release target sharks by using baited hooks. To optimise program effectiveness, maximising in-water fishing effort is essential. Fishing effort is reduced and program costs increased by bait depredation and false alarms, which can be caused by non-target species such as dolphins (Tursiops spp.). Bait depredation by dolphins may be reduced through operational changes to the program. Aims The effectiveness of acoustic pingers that emit sound waves to reduce bait depredation on drumlines remains unexplored, despite being used for decades on bather protection nets. Bait depredation may also be reduced by changing teleost bait to shark bait. Methods We present observational data from an operational bather protection program that used two pinger types (STM products (Dolphin dissuasive device (DDD), random 5–500 kHz, 165 decibels); and Future Oceans (Netshield, 70 kHz, 175 decibels)) off Tweed Heads in northern NSW, Australia, where bait depredation by dolphins (Tursiops spp.) was a growing issue. We also provide 3 years of operational data on bait depredation after bait was switched from teleost to shark. Key results Pingers did not appear to reduce bait depredation or visual observations of dolphin within 50 m of SMART drumlines at deployment or retrieval. However, bait depredation decreased after replacing teleost bait with shark bait. The catch rates of target sharks appeared to be maintained when using shark bait, however, the factors of season and bait type could not be separated in this data set. Conclusions Our results highlighted the challenge of mitigating bait depredation by dolphins by using pingers in bather protection programs, which rely on bait to catch target sharks, and indicated that using alternative bait may be a solution. Implications Such practicalities from the perspective of catching target sharks must also be weighed up against the other dual objective of ‘non-lethal’ programs of minimising harm to other marine life.