Escape mortality of trawl caught Baltic cod (Gadus morhua) - the effect of water temperature, fish size and codend catch

Experiments were conducted in the Baltic Sea to measure mortality of cod that escape through trawl codend meshes under commercial fishing conditions. Three codend types were tested: a 120 mm diamond mesh codend, a Danish type 105 mm escape window codend, and a 105 mm square mesh top-panel codend (Bacoma-window). In total, 30 tows were carried out in three different experiments. Mean tow duration was 3 h and average codend catch 536 kg (range 47-2592 kg). Escapees were collected during the last 20 min of each haul by a caging method. These fish were then held in cages anchored on the seabed and checked daily by divers. Average number of escapees in cage was 133. Average caging duration was 9.5 days. The mortality of escapees was low in normal water temperatures (<10 °C) in all codend types. Higher mortalities were observed when cages were held in temperatures above 15 °C. The majority of these deaths occurred during the first day after the tow. Apparently, these fish experienced strong thermal stress when they were towed to the cage release site through the thermal stratification layer. In commercial trawling the tow is usually not directed to shallow waters. A somewhat higher mortality was predicted for larger fish that escaped from the 120 mm diamond mesh codend while in the 105 mm escape window codend the effect was reverse. In the Bacoma-window codend the number of dead escapees was very low in all length groups. We did not find any clear difference in skin injury between the three codend types. The predicted escape mortality was somewhat higher with higher codend catches. We conclude that the mortality of Baltic cod codend escapees is low in the normal water temperature (3-9 °C).