Cleaner fish growth, welfare and survival in Atlantic salmon sea cages during an autumn-winter production

Cleaner fish used as a biological control agent against salmon lice is rapidly increasing in Atlantic salmon aquaculture. However, concerns have been raised about the welfare and mortality of cleaner fish in salmon cage systems, which could in turn affect their performance in controlling salmon lice. In a 4-month autumn-winter study, we monitored growth, welfare, mortality and daytime depth distribution of the most commonly used cleaner fish, farmed ballan wrasse and lumpfish, in six salmon production sea cages where thermo- and haloclines were present. Ballan wrasse did not grow (SGR: small: −0.01% day−1, large: −0.06% day−1), while lumpfish significantly doubled in size (SGR: 0.87% day−1) during the study. High losses (registered mortality + unregistered loss) were observed in both species (57 and 27% of ballan wrasse and lumpfish, respectively). The welfare status of remaining individuals generally improved over the study period, regardless of species. Brief daytime camera observations at hides found ballan wrasse were typically deeper at warmer (median 12.4 °C) more saline (median 31.7 ppt) depths, where salmon were expected to reside during day periods, compared to lumpfish generally occupying colder (median 7.3 °C), brackish (median 18.9 ppt) water in surface layers. Considerable mortalities, minimal feeding (inferred from ceased growth) by ballan wrasse and a possible mismatch in lumpfish and salmon depths (inferred from limited daytime camera observations) suggest that cleaner fish may have low long-term effectiveness against salmon lice in stratified salmon sea cages over autumn-winter. Similar studies across seasons, locations and cage types (e.g. depth-based cage technologies) are vital to understand the extent of these issues in salmon aquaculture more broadly.publishedVersio