Shark depredation in a commercial trolling fishery in sub-tropical Australia

Shark depredation, whereby hooked fish are partially or completely consumed before they can be retrieved, occurs globally in commercial and recreational fisheries. Depredation can damage fishing gear, injure sharks, cause additional mortality to targeted fish species and result in economic losses to fishers. Knowledge of the mechanisms behind depredation is limited. We used a 13 yr dataset of fishery-dependent commercial daily logbook data for the Mackerel Managed Fishery in Western Australia, which covers 15° of latitude and 10000 km of coastline, to quantify how fishing effort and environmental variables influence depredation. We found that shark depredation rates were relatively low in comparison with previous studies and varied across the 3 management zones of the fishery, with 1.7% of hooked fish being depredated in the northern Zone 1, 2.5% in the central Zone 2 and 5.7% in the southern Zone 3. Generalized additive mixed models found that measures of commercial fishing activity and a proxy for recreational fishing effort (distance from town centre) were positively correlated with shark depredation across Zones 1 and 2. Depredation rates increased during the 13 yr period in Zones 2 and 3, and were higher at dawn and dusk, suggesting crepuscular feeding in Zone 1. This study provides one of the first quantitative assessments of shark depredation in a commercial fishery in Western Australia, and for a trolling fishery globally. The results demonstrate a correlation between fishing effort and depredation, suggesting greater fishing effort in a concentrated area may change shark behaviour, leading to high rates of depredation.