By-Catch of Grey Seals in Fish Traps in the German Baltic Sea—From Incidents to Mitigation and Fisheries Regulation

Abstract

In autumn 2017, an unusual mortality event of Baltic grey seals (Halichoerus grypus grypus) occurred at the German Baltic coast. An alarming 23 carcasses of healthy, well-fed, male large grey seals were found in a confined area in the Greifswald Bay (Mecklenburg-Western Pomerania) in < 10 weeks from September to December. From 1997 to 2016, only a total of 30 grey seal carcasses were found along the entire coast of Mecklenburg-Western Pomerania, with an annual maximum of eight seals in 2014. Pathological investigations were conducted on 19 seals, showing acute cardiac and circulatory failure of unknown origin. However, toxicological, virological, bacteriological and other presumed causes of acute death could be excluded. These findings in combination with inquiries about fishing permits in that region indicated that the seals likely died as by-catch in a fyke net. A legal complaint was therefore filed because of the death of the legally protected grey seal. Although no offender was finally identified, this caused a series of consequences. Public prosecutors encouraged the fisheries authority to establish precautionary provisions. The use of scientific data and fishermen's expertise in by-catch prevention techniques and regulations resulted in a new provision for fyke net permits in the Greifswald Bay. Since 2020, the entrance opening size for fyke nets must now not exceed 75-cm circumference and no more comparable suspected by-catch cases of grey seals were found in this area until 2022, emphasizing the potential of technical modifications of fishing gear to mitigate marine mammal by-catch. This study shows how effective conservation measures established based on provisions, media attention, pathological assessments, scientific long-term data sets and open-minded efforts can protect a marine species from fisheries by-catch.