The cleaning efficacy of lumpfish (Cyclopterus lumpus L.) in Faroese salmon (Salmo salar L.) farming pens in relation to lumpfish size and seasonality

Sea lice infestations causes major economic losses in the Atlantic salmon aquaculture industry, and cleaner fish, e.g. the lumpfish, Cyclopterus lumpus L., are therefore increasingly deployed as a biological control method. However, large variations in the cleaning efficacy have been observed, and in the Faroe Islands, the most prominent variation is seasonal. Over a period of approximately two years 5511 lumpfish stomachs were analysed. The stomach contents, where present, were identified and grouped as, a) sea lice, b) lumpfish feed, c) salmon feed, d) organisms associated with biofouling, e) zooplankton organisms and/or f) other. The presence of zooplankton organisms had a significant, and negative, influence on the cleaning efficacy, while the presence of organisms associated with biofouling had a more moderate, but positive, influence on the prevalence of sea lice in the lumpfish diet. Our findings indicate that biofouling, and the subsequent availability of alternative prey organisms, does not reduce the cleaning efficacy of lumpfish, while zooplankton does, i.e. reducing it by a factor of approximately five. The lumpfish size only seemed to play a minor role in the variation observed in the cleaning efficacy, while it had a significant influence on the proportion of empty stomachs, i.e. the smaller lumpfish (< 50 g) had the highest occurrence of empty stomachs.This is convenient knowledge for implementation in the sea lice strategies of farming sites using lumpfish as cleaner fish in general, but especially in farming areas with large seasonal variations in the zooplankton abundance such as in the Faroes. The high occurrence of empty stomachs in small lumpfish highlights the necessity to adapt husbandry in the first period post lumpfish deployment, especially when the opportunity for naturally occurring food is sparse. Furthermore, our findings of biofouling having a positive influence on the grazing efficacy of lumpfish indicate that net cleaning might have a negative influence on lumpfish grazing efficacy.