Spillback effects of the parasitic salmon louse from fish farms to wild fish are a critical environmental concern for countries with wild salmon. The ectoparasitic louse causes significant physical and physiological damage to wild salmonids that probably have long-term impacts on individuals and population dynamics. However, empirical links between injuries and abundance of lice have not been established in the wild. This study establishes a strong association between dorsal fin injuries in wild sea trout (