Assessing the Impact of Pingers and Fishery-related Factors on Seal and Porpoise Bycatch: A study on marine mammal bycatch in Swedish gillnet fisheries

Incidental bycatch in gillnet fisheries is considered one of the main threats to harbor porpoise populations worldwide. Such bycatch is depending on several factors, including biotic factors such as porpoise distribution and behaviour, as well as factors related to the characteristics of the fisheries.

One porpoise bycatch mitigation method proven to be effective in gillnet fisheries are acoustic deterrent devices (pingers). In an ICES’ advice from 2020, pingers were recommended to be used in the Baltic to reduce the bycatch of the endangered Baltic harbour porpoise. However, type of pinger was not specified, most likely due to the fact that pingers available on the market have not been tested in fishing trials or were audible to seals. In the Baltic, the large and increasing grey seal (Halichoerus grypus) population is causing a serious conflict with the coastal fisheries, due to economic losses. Pingers audible to seals may lead to the “dinner bell” effect, i.e., seals associating the pinger sounds with easily accessible food, leading to increased depredation and seal bycatch.

In this study we evaluate several factors, including two types of pingers which have not been tested in a long-term fishery trial before, potentially affecting the bycatch of harbour porpoises and harbour seals in Swedish gillnet fisheries. Fishery trials were carried out in collaboration with 14 fishermen, operating along the Swedish west coast from 2018 until 2023. The fishing boats were equipped with camera systems documenting the fishing operation. The fishermen operated the onboard systems along with filling out protocols with information such as net mesh size, height, and length and soak time. Data from 960 fishing days, 2550 net links emptied was evaluated. A Generalized Additive Model was used to analyze the bycatch of porpoises and seals in relation to environmental and fisheries variables. The results show that pingers significantly reduced harbour porpoise bycatch, one pinger type being more effective than the other. Mesh size, effort (soak time * net length), year and number of bycaught harbour seals also significantly affected porpoise bycatch. Seal bycatch on the other hand was not affected by pinger use. Seal bycatch was affected by net mesh size and height, effort, and number of bycaught porpoises. These results give us a quantitative understanding of the variables involved in porpoise and seal bycatch, which can help inform management on how to mitigate bycatch in the gillnet fisheries, and on what fisheries needs to be targeted.