Purse seining is an efficient and profitable fishing technique, however unwanted catches are common and releasing (“slipping”) badly can cause mortality. During slipping fish experience several stressors (crowding, hypoxia and injury) that may affect their welfare and survival. To promote the sustainability of this fishery, good-welfare of the catch must be maintained during capture. Individual swimming activity (tail beat frequency) has potential as an informative stress/welfare indicator. This thesis aimed to assess whether swimming activity was an informative stress/welfare indicator in Atlantic mackerel (Scomber scombrus) during crowding. In net cages, schools of mackerel fish (n: ~150 fish) were crowded (by reducing cage volume) at different densities (57, 145, 180 kg/m3), which induced hypoxia (to 65%saturation). Using video analysis tail beat frequency (TBF), along with “feeding behaviour” and “tail beat type”, were assessed as potential stress/welfare indicators. TBF increased with crowding density (increasing 45%, 70%, 131%, respectively), which was also correlated with mortality and injuries. TBF also changed throughout the crowding due to exercise exhaustion and this change rate was related to mortality and injuries. We conclude that TBF was a reliable indicator, with high potential as a catch welfare indicator for mackerel to minimise mortality of slipped catch.
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