Quantifying shark depredation in a recreational fishery in the Ningaloo Marine Park and Exmouth Gulf, Western Australia

Shark depredation, where a shark consumes a hooked fish before it can be retrieved to the fishing vessel, can occur in recreational fisheries. This may cause higher mortality rates in target fish species, injuries to sharks from fishing gear and negatively impact the recreational fishing experience. This study quantified spatial variation and frequency of shark depredation in a recreational fishery in the Ningaloo Marine Park and Exmouth Gulf, Western Australia, by surveying 248 fishing boats at west coast boat ramps and 155 boats at Exmouth Gulf boat ramps from July 2015 to May 2016. Shark depredation occurred on 38.7% of fishing trips from west coast boat ramps and 41.9% of trips from Exmouth Gulf boat ramps. The mean (+/- 95% CI) shark depredation rate per trip was 13.7 +/- 3.3% for demersal fishing (n = 185) and 11.8 +/- 6.8% for trolling (n = 63) for west coast boat ramps, compared to 11.5 +/- 2.8% (n = 128) and 7.2 +/- 8.4% (n = 27) for Exmouth Gulf ramps. Depredation rates varied spatially, with higher depredation in areas which received greater fishing pressure. A novel application of Tweedie generalised additive mixed models indicated that depth, the number of other boats fishing within 5 km and survey period influenced depredation rates for fishing trips from west coast boat ramps. For the Exmouth Gulf ramps, fishing pressure and decreasing latitude positively affected the number of fish depredated. These results highlight the important influence of spatial variation in fishing pressure. The occurrence of higher depredation rates in areas which receive greater fishing pressure may indicate the formation of a behavioural association in the depredating sharks. This study is the first quantitative assessment of shark depredation in an Australian recreational fishery, and provides important insights that can assist recreational fishers and managers in reducing depredation.