License to chill: Environmental enrichment for post-vaccination recovery and social training of lumpfish (Cyclopterus lumpus)

Commercial production of lumpfish (Cyclopterus lumpus) offers a non-invasive, continuous, and environmentally friendly method for controlling sea lice (Lepeophtheirus salmonis and Caligus elongatus) in Atlantic salmon (Salmo salar) farming in the North-Atlantic region. However, rearing lumpfish in barren tanks contrasts with its natural benthic lifestyle, and may impair cognitive development, social interaction and ultimately cleaner behaviour. This study investigated the effect of environmental enrichment (EE) during post-vaccination recovery and after deployment in salmon net pens. A total of 18,000 lumpfish were distributed in six production tanks in a randomised triplicate setup with three tanks for control and three tanks with the combination of plastic shelters and salmon silicone models for seven weeks. The study continued at sea for two months using triplicate net pens, with blinded sampling protocols. Results showed that lumpfish in enriched tanks experienced a significant increase in growth, observed on day 28 post-vaccination and interacted actively with both enrichment types. Vaccination had a significant effect on brain neurotransmitters and blood cortisol but was not affected by EE. After deployment at sea, net pens with enriched lumpfish showed a trend towards reduced lice counts of adult female lice during first two weeks. However, lice consumption in both control and treatment were remarkably high, with pooled ingestion rates ranging from 30 to 36.7 % in weeks 4 and 8 post-deployment. Welfare assessments indicated good condition and no difference across treatments. These findings suggest that EE may serve as a practical strategy with the potential to mitigate post-vaccination stress, and enhance habituation to shelters and salmon cues, supporting its use as a reinforcing measure before deployment at sea.