Welfare and survival of lumpfish (Cyclopterus lumpus) in Norwegian commercial Atlantic salmon (Salmo salar) production

Lumpfish are currently used as a part of the overall strategy for the prevention and control of sea lice in Atlantic salmon farming in Norway. Their use as biological delousers is advantageous as it exposes salmon to very little stress and injuries. However, there is a growing concern and criticism about the welfare and survival of lumpfish in salmon farms. This study monitored the welfare and survival of lumpfish from four commercial salmon farms for 6 to 12 months in 2019???2021. Environmental and production conditions were also registered. Lumpfish welfare decreased during the study period, mainly related to eye and skin injuries. Mortality rate was low in the first few months after transfer to sea, but increased thereafter. Mechanical delousing affected the welfare and survival most negatively of all monitored external parameters. Even though diseases are often pointed out as the main factor for mortality of lumpfish in sea cages, it was not the case in the present study. However, there were indications that diseases are more likely to occur when lumpfish is already weakened by other factors such as mechanical delousing. One to two thirds of lumpfish were assessed as underweighted or emaciated at all time during the monitoring period, suggesting that the nutritional requirements of lumpfish are not fully met in sea cages.