Survival of undersized plaice (Pleuronectes platessa) discarded in the bottom otter trawl and Danish seine mixed fisheries in Skagerrak

The landing obligation of the new Common Fisheries Policy includes a “high survival”-exemption, which allows to continue discarding of fish with scientifically proven high chances to survive the process of capture, handling and release. This study presents survival rates of undersized plaice (Pleuronectes platessa) caught by two commercial fishing vessels targeting plaice in Skagerrak during the summer, a bottom otter trawler and a Danish anchor seiner. Instead of being discarded, the fish were assessed and monitored in on-land observation tanks for 14 days. For each gear, a Weibull mixture model was fitted to the data to estimate survival probabilities, including the uncertainties from the haul selection and conditions of the captivity experiment, and to determine which biological, environmental and operational factors influence survival. Generally, fish caught by the trawler showed lower survival (44%, 95%-confidence interval: 37–52%) than those caught by the seiner (78%, 95%-confidence interval: 67–87%). Air exposure was found to be the most important factor in determining survivability of fish discarded from the trawler. For the seiner, bottom temperature was identified as an additional important determinant.