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.