Abstract
Securing the welfare and maximising the lice removal efficacy of ~60 million cleaner fish used each year on salmonid farms is essential to develop a productive and ethical industry with low salmon lice Lepeophtheirus salmonis levels. We tested whether ballan wrasse Labrus bergylta welfare and lice consumption differed depending upon whether they were acclimatised in cages with either no Atlantic salmon Salmo salar, salmon without lice, or lice-infested salmon, prior to deployment in sea cages. After 1 wk, commercial densities of lice-infested salmon were stocked with the acclimatised wrasse in cages and kept for a further 3 wk. Each week, the number of lice on salmon was counted and a subset of wrasse was sampled for gut contents and physical welfare. Lice occurred less frequently in wrasse guts (6% of all dietary items) compared to caprellid amphipods (76%). Ballan wrasse consumed more lice if they were pre-exposed to lice-infested salmon (mean ± SE = 0.79 ± 0.4 lice) compared to wrasse not pre-exposed to salmon (0.15 ± 0.1 lice, p = 0.003) or wrasse pre-exposed to salmon without lice (0.01 ± 0.09 lice, p = 0.03). This did not affect overall mobile lice loads on salmon, which doubled over the study period regardless of acclimatisation strategy. Decline in condition factor (K) and the increase in specific physical damage over time were not affected by acclimatisation strategy. If welfare can be secured, acclimatisation could be tested on cleaner fish at commercial scales as a strategy to improve biological control agents.
Highlights
Biological control is considered a sustainable solution to the rapid evolution of pesticide and medicinal resistance in pests (Bale et al 2008)
Ballan wrasse acclimatised with liceinfested salmon are more likely to consume lice than individuals with no prior experience of salmon or lice
This sheds light on a potential strategy to improve cleaner fish efficacy on salmon farms, higher levels of lice found in their stomachs did not translate to lower lice loads on salmon or improved welfare of ballan wrasse during the experiment
Summary
Biological control is considered a sustainable solution to the rapid evolution of pesticide and medicinal resistance in pests (Bale et al 2008). For this reason, Norwegian salmonid farms stock ~60 million cleaner fish per year (rock cook wrasse Centrolabrus exoletus, goldsinny wrasse Ctenolabrus rupestris, ballan wrasse Labrus bergylta, corkwing wrasse Symphodus melops and lumpfish Cyclopterus lumpus) to control salmon lice Lepeophtheirus salmonis (hereafter lice) infestations (Overton et al 2020). As an alternative to chemical and mechanical control methods, 62.5% of Norwegian salmon farms stocked cleaner fish in 2019 (469 of 750 active sites: www.lusedata.no). A recent nation-scale analysis of 488 farms in Norway that completed a full production cycle between 2016 and 2018 concluded that while some sites achieved lice reduction results with cleaner fish, suboptimal use was widespread (Barrett et al 2020)
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