Effect of three different codend designs on the size selectivity of juvenile cod in the Barents Sea shrimp trawl fishery

Abstract

Shrimp trawlers often use a Nordmøre sorting grid ahead of a small mesh codend to avoid bycatch of juvenile fish while allowing shrimps to be efficiently caught. However, small fish can pass through the grid to enter the codend and risk being retained. The risk of retention for fish of different sizes depends the size selection in the Nordmøre grid and the size selection in the subsequent codend, which makes the process complex and often results in a bell-shaped size selection curve. In the Barents Sea shrimp fishery, cod (Gadus morhua) is one of the species of concern because of its great commercial value. We studied the size selection of juvenile cod when the trawl was equipped with the compulsory gear in the fishery: a 19 mm Nordmøre grid followed by a 35 mm diamond mesh codend. As expected, the size selection curve showed a bell-shaped signature, with a certain size range of juveniles having high retention risk, while the risk for smaller and bigger cod was smaller. The retention risk was highest for cod between 12 and 20 cm in length. We also tested two alternative designs in the aft section of the gear: a codend with 35 mm square mesh panels and a square mesh sorting cone section. Neither of these designs affected the size selection in the trawl significantly.