Does the mortality of released Sardinops sagax increase if rolled over the headline of a purse seine net?

Abstract

Mortality of Sardinops sagax released after being forced over the headline of a purse seine net was investigated on three occasions at Bremer Bay, Western Australia. A control sample of fish (∼500 individuals) was collected from the loosely bunted pocket of a purse seine net using a watertight, plastic-lined brail net and transferred into a 1000-l holding tank onboard a commercial fishing vessel. A commercial quantity of fish was then removed from the purse seine net and transferred into bins on deck. A small quantity of Sardinops (∼500 individuals) was retained in the net and the process of tightly bunting the net (aggregating the fish at high densities) and rolling excess fish over the headline of the purse seine net was simulated; these fish were similarly transferred to an onboard tank. Mortality rates of Sardinops were monitored over a 3–4-day period in a partitioned ocean cage. Significant differences in mortality between the control and the treatment groups were observed for all experiments. Mortality of Sardinops transported before the net was tightly bunted (i.e. control) and held in the floating cage ranged between 8 and 20%, which were significantly lower than the mortality observed for the treatment group (11–55%). Based on our results, we recommend that purse seine fisheries incorporate a net design which allows quick release of any unwanted catch.