Abstract
BACKGROUND: Cleaner fish play a crucial role in controlling caligid sea lice in Atlantic salmon aquaculture across the North Atlantic. However, their delousing performance varies due to multiple factors, including water temperature and cleaner fish size. Anecdotal reports of less-pigmented sea lice have raised concerns that these variants may emerge as a response to cleaner fish selection pressure. Given the high adaptation potential of sea lice and their resistance to chemical treatments, these colour variants could challenge cleaner fish delousing strategies. A series of tank experiments cohabiting lice-infected salmon with farmed cleaner fish investigated the effect of water temperature, hypopigmented 'cryptic' lice and the size of lumpfish on delousing. Delousing rates were calculated from changes in lice numbers over time. RESULTS: Ballan wrasse and lumpfish were both effective at delousing sea lice in Scottish summer and winter water temperatures with lice numbers significantly reduced after 4 days, showing that both species are effective delousers all year round. There was no significant difference in delousing between small (40 g) and large (80 g) lumpfish. Ballan wrasse were effective at delousing pigmented and cryptic lice but lumpfish were less effective at delousing cryptic lice, especially male lice. CONCLUSION: This study demonstrates the potential for year-round efficacy of cleaner fish in salmon aquaculture, although it is highly dependent on the health and welfare of cleaner fish in commercial conditions. It also tested for the first time delousing of cryptic lice under experimental conditions, and results indicate that they should be considered in sea lice management strategies involving lumpfish. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.