Assessing mackerel behaviour following crowding-induced stress in purse seine fisheries

The practice of slipping in purse seine fisheries has been shown to cause high levels of delayed mortality in released fish. This unaccounted mortality could lead to bias in stock assessments, and brings the sustainability of these fisheries into question. Behavioural stress responses of individual mackerel (Scomber scombrus L.) and mackerel schools were analysed using visual and acoustic methods under non-lethal crowding and hypoxic conditions in purse seine simulations. Metrics observed included tail beat frequency and amplitude, and school vertical distribution and density. Tail beat frequency and school density were the best potential stress indicators for welfare in mackerel during purse seine fisheries - with significant increases in tail beat frequencies and densities of up to 60 fish m-3 with crowding, as well as evidence of adaption and recovery over treatment time. The addition of hypoxia shows an interaction of effects on these metrics, showing no additive effect to the crowding treatment, and suggests a behavioural trade-off in mackerel between the maintenance of school structure and oxidative stress. Further study into the sole effect of hypoxia on mackerel behaviour is recommended