Characteristics of lumpfish (Cyclopterus lumpus) with high cleaning efficacy in commercial Atlantic salmon (Salmo salar) production

Cleaner fish such as lumpfish (Cyclopterus lumpus) are widely used as a biological method for delousing in Atlantic salmon (Salmo salar) aquaculture. Previous studies have mainly focused on the number of salmon lice (Lepeophtheirus salmonis) per salmon or on the proportion of individuals grazing on sea lice (L. salmonis and Caligus elongatus) and reported large variations in cleaning efficacy. Understanding what parameters cause these differences in cleaning efficacy is crucial to optimize the use of lumpfish as a biological delouser. In addition, it is uncertain how many sea lice are commonly ingested per lumpfish, even though this constitutes decisive information in the use of lumpfish for delousing. This study focused on the impact of lumpfish weight, diet and sex on cleaning efficacy in commercial Atlantic salmon net pens. A total of 2104 lumpfish were sampled from 8 Norwegian salmon farms over 2 years. Weight and sex was determined for each individual. Stomach content was visually identified and categorized as sea lice, lumpfish feed, salmon feed, biofouling organisms and zooplankton. Proportions of lumpfish with sea lice in their stomachs varied from 0 to 47% (average of 8.7% for the whole study) and the number of ingested sea lice varied from 0 to 120 (average of 0.6) between Atlantic salmon farms and sampling dates. Our results suggested that estimating the average number of sea lice ingested per lumpfish is necessary to determine cleaning efficacy. Weight was found to be a determining factor as lumpfish >300 g were very unlikely to consume sea lice. Individuals with the highest cleaning efficacy were small (< 100 g) and fed on additional prey items, mainly biofouling organisms. Lumpfish sex did not influence cleaning efficacy. We developed a simple model based on logistic regression analysis to predict lumpfish predict cleaning efficacy. The model predicted that maintaining lumpfish in net pens at a relatively small size (< 300 g) may raise grazing up to >1 sea louse per lumpfish.