Shark depredation and behavioural interactions with fishing gear in a recreational fishery in Western Australia

Shark depredation, whereby a shark consumes an animal caught by fishing gear, can cause higher mortality for target species, injury to sharks and the loss of catch and fishing gear. A critical first step towards potential mitigation is understanding this behaviour and the shark species involved, because the identity of depredating shark species is unknown in many fisheries, and behavioural dynamics of shark interactions with fishing gear are not well understood. We used line-mounted video cameras in a recreational fishery in the Ningaloo region of Western Australia to: (1) identify shark species responsible for depredation, (2) investigate behavioural interactions with fishing gear, (3) identify the prevalence of retained fishing gear in sharks and (4) quantify the influence of environmental variables and fishing methods on shark abundance during demersal fishing at 92 locations. The shark depredation rate was 9.1%, and sicklefin lemon Negaprion acutidens, blacktip/Australian blacktip Carcharhinus limbatus/tilstoni, grey reef C. amblyrhynchos and spottail C. sorrah sharks were observed depredating lethrinid and epinephelid fishes. Five additional shark species from 4 families were recorded but were not responsible for depredation. Sharks frequently investigated baited hooks and other fishing gear components and were observed following the fishing gear as it was retrieved. The relative abundance of sharks at each fishing location was influenced by longitude, sea surface temperature and total number of fish hooked. By identifying the shark species responsible for depredation, and investigating their behavioural interactions with fishing gear, this study provides important insights that have broader significance to other fisheries, particularly for understanding impacts on sharks and for developing effective deterrents to mitigate shark depredation.