Survival of mackerel and saithe escaping through sorting grids in purse seines

Abstract

Technology for efficient size-selection of mackerel and saithe in purse seines has been developed. A critical constraint for application of the technology is the survival of escaping fish. We have conducted a series of mesoscale experitnents where penned fish were forced through rigid grids and into new storing net pens to quantify the survival of mackerel and saithe. These experiments showed insignificant mortality of both mackerel and saithe. Full scale experiments during mackerel purse seining off western Norway and saithe purse seining in fjords in western and northern Norway have also been conducted. During these experiments control groups were established by transferring parts of the catch gently from the purse seine to attached net pens. Experiment groups were established by collecting fish escaping through the selection grids in attached net pens. The net pens were then towed up to 30 nautical miles for anchoring inshore. In the mackerel experiments, up to 95% of the fish in the control groups and up to 60% in the experiment groups were alive one month after the experiments. This indicates that the size selection process in mackerel purse seining induce too high mortality to be recommended in commercial fishing. On the other hand, the mortality in the saithe experiments was insignificant, and the use of size selection grids in saithe purse seines can therefore be recommended.