Abstract
Abstract Fishing and translocation of marine species for use in aquaculture is widespread. Corkwing, goldsinny, and ballan wrasse (Symphodus melops, Ctenolabrus rupestris, and Labrus bergylta) are fished on the Swedish west coast for use as cleaner-fish in Norwegian salmon farms. Here, we aim to provide knowledge and recommendations to support ecosystem-based management for wrasse fisheries in Sweden. We compared fished and non-fished areas to test if current fishery levels have led to stock depletion. To gain insight on the role of wrasse in the algal belt trophic chain, we analysed the gut contents of goldsinny and corkwing using metabarcoding. Finally, we analysed the trophic interactions of wrasse and potential prey in a mesocosm study. We could not detect any signs of stock depletion or altered size structure in fished areas compared to the protected control area. Gut analyses confirmed both goldsinny and corkwing as non-specialized, omnivorous opportunists and revealed, with 189 prey taxa detected, a broader spectrum of prey than previously known. Common prey items included mesoherbivores such as small gastropods and crustaceans, but also insects and algae. We conclude that there are no visible signs of stock depletion at the current removal level of wrasses by the fishery. However, this emerging fishery should be closely monitored for potential cascading effects on the algal belt ecosystem, and our study could provide a baseline for future monitoring.
Highlights
Sea lice infestation is a major problem faced by the salmon aquaculture industry, and the use of wild caught cleaner-fish to control the lice is seen as the most environmentally-friendly option compared to other methods (Gonzalez and de Boer, 2017), and more effective than chemical treatments in the case of diseased or stressed salmon (Deady et al, 1995)
In the process of finding fished areas to compare with the marine protected area Kavra, a discussion was held with fishermen targeting wrasse in Lysekil
The corresponding value for Byxeskar was 74%, indicating that both Stora Kornoand Byxeskar are representative reference fishing grounds for wrasse compared to the marine protected area Kavra (Table 1)
Summary
Sea lice infestation is a major problem faced by the salmon aquaculture industry, and the use of wild caught cleaner-fish to control the lice is seen as the most environmentally-friendly option compared to other methods (Gonzalez and de Boer, 2017), and more effective than chemical treatments in the case of diseased or stressed salmon (Deady et al, 1995). The number of cleaner fish used in the Norwegian salmon production has increased from 1.7 million in 2008, to 61 million in 2019. This includes farmed lumpfish and ballan wrasse, but 37% is wild caught fish, of which wrasses constitute the largest share (92%) (Norwegian Directorate of Fisheries, 2019). Norwegian fisheries alone can no longer support the demand from the salmon industry, and approximately 1 million wild-caught wrasses are imported from the Swedish Skagerrak coast yearly since 2010 (Halvorsen et al, 2017a).
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