Parasite scars: the impact of salmon lice injury on sea trout populations.

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 (Salmo trutta) and infestation with salmon lice at a marine site in Norway. We further assessed the prevalence of such injuries on sea trout returning to spawning grounds from snorkelling observations in 16 rivers in Hardangerfjord, western Norway, from 2007 to 2021. The prevalence of injuries decreased with distance from the fjord outlet, from approximately 70% in middle and outer parts to approximately 10% in the inner parts. Additionally, a negative correlation was found between sea trout population size and the proportion of the population with dorsal fin damage. The results demonstrate that salmon lice inflict high levels of injury on significant portions of sea trout populations in areas with intensive fish farming, leading to poorer population status in affected populations.