Effect of the trap net emptying method on release mortality of Atlantic salmon estimated by a Bayesian system model

We evaluated the effect of emptying techniques of a pontoon trap on mortality of Atlantic salmon on their spawning migration along the Finnish coast of the northern Baltic Sea by tagging and releasing fish from commercial trap nets. Two methods currently used, a traditional emptying chute and a lifting bag, were compared. The lifting bag has been developed primarily for minimising injuries for fish during emptying. A custom-built Bayesian system model was used for mortality estimation. We also explored external injuries of salmon and compared the probability of getting an injury between the emptying methods. The mean probability of salmon dying due to release from a trap with an emptying chute was 24% (90% HDI 3%–43%), whereas that for salmon released from a trap with a lifting bag was 13% (90% HDI 1%–24%). Results showed a higher probability of injuries and longer air exposure time in salmon caught with a trap with an emptying chute, which may explain the observed difference in mortality. Manual handling of a lifting bag appears to pose a clear work safety risk for fishers. It is noteworthy that the role of release mortality for northern Baltic salmon stocks is small because the number of salmon released in coastal fishery is currently low.