Mortality of mackerel (Scomber scombrus L.) after pursing and slipping from a purse seine

Abstract A new method was used to study the effect of crowding and subsequent slipping from a purse seine on the mortality of Atlantic mackerel ( Scomber scombrus L.). Mackerel were allowed to swim from a purse seine through a transfer channel into two identical large floating net-pens. One pen was used as a control and was left floating in the sea without further treatment. The other was used to simulate crowding and slipping. The volume of the pen was gradually decreased by hoisting the bottom of the pen using a crane until the fish started to show panic reactions, and this volume was maintained for 15 min (2006) or 10 min (2007). The volume was then allowed to return to normal and the net-pens were left to drift freely in the open sea for 3–6 days. Five repeat experiments were performed, all of which showed that crowding has a major effect on survival rates. In all five experiments, mortality was higher among the crowded fish (80–100% mortality) than the controls (0.1–46% mortality), and the difference was significant ( p  = 0.01). The experiments demonstrate that excessive crowding before slipping mackerel from purse seines should be avoided, if possible, in order to avoid massive fish kills.