Estimating the effect of wrasses (Labridae) and lumpfish (Cyclopterus lumpus) as control measures against salmon lice in Norwegian fish farms

Cleaner fish are commonly used as a biological control measure against salmon lice infestations in salmonid farming. However, there have been few attempts at quantifying the effect of cleaner fish on the louse abundance at commercial farms. Our study estimates a delousing effect for wrasses (Labridae) and lumpfish (Cyclopterus lumpus) by fitting a partially stage-structured lice development model to historical production data on all salmonid farms in Norway. We investigate different models, and evaluate them according to a statistical model selection criterion (BIC). The final model includes temperature dependence for the delousing effect of cleaner fish, as a quadratic function. The estimated optimal temperature for lice grazing was found to be 6.7 °C for lumpfish and 15.3 °C for wrasses. The final model also included separate delousing effects for adult female and other motile salmon lice. In general, the estimated delousing effect was larger for wrasses than for lumpfish. However, for temperatures below 8.3 °C, the estimated delousing effect of other motile lice was larger when using lumpfish compared to wrasses. The estimated delousing effect was larger for lower abundances of salmon lice. This implies that cleaner fish should be used at low louse abundances, and not as a delousing method during outbreaks. Our study is an important contribution to quantifying the temperature-dependent delousing effect of cleaner fish, which can be used to guide the farmers in their decision-making when planning cleaner fish strategies.