Salmon Louse Infestation Impairs the Long-Term Survival of Sea-Run Brown Trout

Anadromous salmonids, including sea-run brown trout, are exposed to ectoparasitic salmon lice during their sea migrations. The development of intensive aquaculture in coastal areas has promoted louse epidemics by substantially increasing the number of hosts available to the parasite. We employed a mark-recapture study involving large-scale traps to capture and PIT-tag 676 wild sea-trout during their early marine migration in spring 2020 and 2021. Each trout was examined for lice, tagged with passive integrated transponders (PIT), and monitored for subsequent survival using a PIT antenna system installed at the river Yndesdalsvassdraget. Using a Cormack-Jolly-Seber capture recapture model of individual re-detections the subsequent years, we found a significant negative correlation between lice per gram of fish weight and the survival probability. Increasing lice load from 0 to 1 louse per gram fish reduced the survival probability by approximately 73% in 2020 and 58% in 2021. This is among the first field studies to demonstrate a statistically significant association between individual survival of brown trout and their parasite loads in the wild. Our findings demonstrate the critical need for robust marine spatial planning and lice management in coastal fisheries. Effective control of lice loads is essential to mitigate their deleterious effects on brown trout, ensuring sustainable fish populations and maintaining ecological balance in regions affected by aquaculture.