Environmental enrichment of lumpfish (Cyclopterus lumpus) during rearing increases sea lice consumption in commercial Atlantic salmon (Salmo salar) farming

Cleaner fish such as the lumpfish (Cyclopterus lumpus) are used in salmon farming for controlling infestations of sea lice. Previous laboratory studies suggest that environmental enrichment (EE) can be used to habituate lumpfish before encountering live salmon and thus promote earlier onset of cleaning behaviour. This study applied EE in commercial production, and reports findings from four trials conducted from 2020 to 2022. Juvenile lumpfish were exposed to life-like salmon models for seven days at the production facility prior to deployment in salmon net pens. During this period, the incidence of physically attaching to the salmon models significantly increased. Lumpfish did not express any outward signs of stress during exposure to the models. However, alterations in serotonergic and dopaminergic activity in the telencephalon was detected. This was potentially related to adjustments for social engagement as observed in other cleaner fish species. Stomach contents (n = 523) were analysed on day two and day seven after deployment and revealed a significantly higher proportion of EE lumpfish with lice in their stomachs compared to control, on day two but not on day seven. Furthermore, weight loss post deployment was observed only in the control group. Finally, serotonergic activity in the telencephalon of the enriched group was found to be lower than in the control group on day seven after deployment, which could be related to a lower level of anxiety/stress. These findings suggest that EE influences brain regulation related to social interaction with indirect positive effects on lice consumption. It is concluded that EE may encourage lumpfish to begin consuming lice more quickly after deployment in salmon net pens, and that the effect is temporal.