Escape response of juvenile fish in a shaking codend determined from tank experiments

A new method to increase escape rate of juvenile fish from trawl codend was developed. The method encourages fish to approach the netting by generating an active shaking motion using a cap-like canvas at the end of the codend. Three types of model codend were used: a traditional codend, a fluttering flag-like netting panel and a double conical rope array used as an active stimulating device (ASD). The escape responses of juvenile bastard halibut (Paralichthys olivaceus) and juvenile red seabream (Pagrus major) were observed in a circulating water channel. The amplitude ratio of the shaking distance was 0.6-0.8 of the codend diameter and its period was 3-4 s. The escape rate was 22-30% higher in the shaking codends than in steady codends. The relative lateral moving speeds of the escaping fish were 1.4-1.7 times higher when shaking occurred than when no shaking occurred. The sieving effect of the shaking codend operated to actively drive fish to the netting panels and weakened the optomotor response of fish, possibly by generating erratic movement, allowing more fish to escape, and resulting in a reduction of the juvenile by-catch.