High-frequency pingers do not increase catch loss owing to seals in the Baltic Sea

The Baltic Proper harbour porpoise population is listed as Critically Endangered by IUCN, and only a few hundred animals remain. One of the most serious threats to this population is bycatch in static net fisheries. To minimize bycatch, ICES has proposed that acoustic deterrent devices, pingers, should be used on static nets within the population's range.However, there have previously been indications that grey seals learn to associate the pinger sounds with the presence of nets with easily accessible prey, and that pinger use therefore can increase the risk of seal depredation in static net fisheries.In recent years, pingers operating at higher frequencies have been made commercially available, the idea being that harbour porpoises will still be able to hear the sound at 50-120 kHz while this will be above or at the limit of seal hearing. This would mean that such high-frequency pingers would not induce this so-called dinner-bell effect.In this study, data were collected from active commercial static net fisheries in the Baltic Proper, using two types of high-frequency pingers operating at 50-120 and 70 kHz, respectively. Pingers were attached to nets in accordance with manufacturers' guidelines, and fishers were asked to estimate the catch loss owing to seals in nets with and without pingers.Data were received from 646 deployments made between May 2019 and September 2020. Four generalized mixed effect models were developed with relevant variables.The results suggest using high-frequency pingers does not increase seal depredation on static nets. It is concluded that pingers could be used as an interim measure to minimize bycatch of the Critically Endangered Baltic Proper harbour porpoise population.