Cleaning behavior of ballan wrasse (Labrus bergylta) studied using sea lice dummies in large scale sea cages

The management of sea lice is one of the major challenges in farming of Atlantic salmon (Salmo salar) in Norway. Different methods are in use to remove sea lice from farmed salmonids during the grow-out phase in the sea, including mechanical, medicinal, and biological treatments. The use of farmed lumpfish (Cyclopterus lumpus) and ballan wrasse (Labrus bergylta) as cleaner fish is considered less stressful for salmon compared to other treatments. Documenting cleaning behavior in commercial sea cages is difficult. In this study, we used artificial sea lice dummies and camera to study cleaning behavior of farmed ballan wrasse in commercial sea cages during summer and autumn of 2021. Artificial sea lice dummies, 11-12 mm in length, and 26-27 mm total length including egg-strings, were attached to fixed positions on a dead salmon and placed in the sea cage. A total of 74 h of video recordings from three different cages were analyzed. We recorded inspections of the salmon by ballan wrasse, and any bites made at sea lice dummies or the salmon. Ballan wrasse rapidly showed interest in the model after introduction, with most of the first observations of behavioral events occurring within the first 30 min after the salmon were placed in the sea cage. An inspection of salmon rarely resulted in a bite on sea lice dummies (8%) and ballan wrasse was also observed biting on the dead salmon (12%). Cleaning behavior (i.e., bite on sea lice dummies) was on average approximate to 1 bite hour(-1) and lice dummies placed on anterior ventral area were most susceptible to be bitten. This study uses a non-invasive method to study cleaner fish behavior, and it can in the future be used to investigate how different environmental factors impact cleaner fish behavior in large sea cages.