Fishing behaviour and environmental variability influence depredation of pelagic longline catch by toothed whales

Depredation is a threat to the socioeconomic and ecological sustainability of pelagic longline fisheries, globally. Toothed whales (odontocetes) are anecdotally reported as responsible for annual revenue losses of AU$10–12 million in Australia’s Eastern Tuna and Billfish Fishery (ETBF), presenting a challenge for industry and management in developing avoidance and mitigation techniques. We trialled the use of electronic monitoring footage as a timely and cost-effective method for quantifying odontocete depredation interactions. Between March and July 2023, a mean of 5% of total catch of each set sampled (n = 27) was lost to odontocete depredation, but this was highly variable among sets (0 – 24% of catch). We used generalised additive models (GAMs) to explore the effects of operational, spatiotemporal and environmental factors on depredation rates. Our results reveal that odontocete depredation is more likely to occur when: (i) hauling process is initiated at night and completed after dawn, (ii) lines are set at shallower depths, (iii) light-sticks are used, (iv) sea-surface temperature is warmer, (v) lines are set and hauled between 24° and 27°S, and (vi) lines are set and hauled during Austral autumn. It remains difficult to disentangle the effects of spatial and temporal drivers due to our relatively small sample size, however, our work provides actionable information to guide fisher interaction-avoidance tactics and help promote fishery sustainability.