Abstract

Simple Summary Sea lice can generate wounds on fish and they are associated with the consumption of mucus, skin, and blood, inducing extensive epithelial erosion and inflammation. Knowledge related to the fin condition of fish with sea lice infections is scarce, limiting the range of parameters regarding their welfare status of fish. In the present study, the fin condition and two stress parameters were assessed inCaligus rogercresseyiinfected fish. An association between fin erosion, increasing fish stress and parasite abundances was observed, suggesting thatC. rogercresseyiinfection is a possible cause of fin degeneration in Atlantic salmon. Fin condition is a simple indicator of fish welfare, which anticipates detrimental effects on fish in aquaculture systems. This study evaluated the fin condition ofSalmo salarat different abundances of the parasiteCaligus rogercresseyi. Fish were exposed to infestation with copepodids and the cohort was allowed to develop to the adult stage. The relative fin index was measured. Significant differences between infested and control fish for both pectoral and anal fins were observed. Moreover, there were significant negative relationships between fin condition and parasite abundances for pectoral, anal, and pelvic fins, suggesting that infestations withC. rogercresseyicould be a possible cause for fin damage in Atlantic salmon. Moreover, this damage was associated with increased stress levels, suggesting that damage can be related to physiological changes on infested fish. According to these results, pectoral fin assessments have the potential to provide information on the welfare of fish withC. rogercresseyiinfestation. Determining the causes of poor fin development may improve fish welfare, even when infested by parasites.