Post-release survival of Baltic salmon in the recreational trolling fishery

Background and Objective

Recreational fishing on Baltic salmon (Salmo salar) plays an important acio-economic role in angling tourism in Mecklenburg–Western Pomerania, Germany. Salmon are almost exclusively caught in Germany through trolling in the open Baltic Sea during the winter and spring around the island of Ruegen. Our data collection shows that each salmon angler spends an average of about €2,750 per year on salmon trolling in Germany, resulting in total annual expenditures of around €5 million. Approximately 40 to 50% of the total salmon catches in the Baltic Sea come from recreational fishing, with around 20,000 salmon being caught each year in the recreational trolling fishery.

The Baltic salmon populations have developed positively since the 1990s, but there are still very weak wild salmon populations in some rivers in the southern distribution area (e.g., in Latvia and Lithuania). These populations contribute little to the overall Baltic wild salmon production but are at risk of disappearing. In 2022, an EU-wide regulation was introduced, banning commercial  sea fishing south of the Åland Sea and imposing a catch limit of 1 salmon per day and angler for marine recreational fishing. Only salmon with a clipped adipose fin (former hatchery fish) may be harvested ("adipose fin regulation"). These changes have reduced the trolling fishing effort in the Ruegen area by about 60%, resulting in negative economic effects for the region. At the same time, the release rate of caught salmon increased from under 10% to nearly 60%.

However, there is still a lack of information regarding post-release mortality of salmon in trolling fisheries and the sub-lethal effects of Catch-and-Release (e.g., behavioral changes). The International Council for the Exploration of the Sea (ICES) currently estimates a mortality rate of 25% for Baltic salmon released in trolling fisheries for its stock assessments. An investigation into survival rates is urgently needed to improve the estimates of the removed salmon biomass and to assess the effectiveness of the "adipose fin regulation." Furthermore, factors that influence the mortality rate of released salmon must be identified in order to develop management measures that could reduce negative effects.

The specific goals are as follows:

• Determining the survival rates of caught and released salmon from trolling fisheries in the waters around the island of Ruegen.
• Identifying key factors that influence post-release mortality of this species.
• Developing species- and fishery-specific best-practice guidelines to reduce post-release mortality and improve fish welfare in salmon trolling fisheries.

Approach

To investigate post-release mortality rates of Baltic Sea salmon, a tagging experiment will be conducted in the waters around Rügen, Germany. The experiment will be carried out in collaboration with scientists from the Swedish University of Agricultural Sciences (SLU). The plan is to tag salmon caught during trolling in both Sweden and Germany.

For the tagging, so-called "Pop-up Satellite Archival Tags" (PSATs) will be used. These PSATs are small measurement probes that continuously measure water temperature, water depth, and light intensity in the surrounding environment over a predefined period (e.g., 30 days). Using this data, the vitality, swimming behavior, and migratory patterns of the salmon in the wild can be studied. After the defined period or in the event of the fish's death, the tag automatically detaches from the fish and floats to the water surface. Once it reaches the surface, it sends the recorded data via satellite to the owner of the tag. The advantage of PSATs compared to conventional tagging methods is that they provide data with a very high spatial and temporal resolution.

Staff from the Thünen Institute of Baltic Sea Fisheries will accompany recreational trolling boats and tag fish that are caught under realistic conditions. After capture and landing, various parameters will be documented for each salmon (e.g., date, catch location, catch time, fish length, bait and hook type, hooking position, wound bleeding). Additionally, the duration of air exposure and some weather data (e.g., air temperature) will be recorded. The salmon will then be tagged as gently as possible. The PSAT will be fixed in the back area behind the dorsal fin of the fish. After tagging, the salmon will be released back into the water, and the PSATs will begin recording data until they detach from the fish (in the case of the fish’s death or after reaching the predefined end point of data recording).

The tagging experiment is planned for the period from March to May 2024 to, among other things, investigate the effects of different water temperatures on post-release mortality. Spring is also the main season for salmon trolling anglers around Rügen.