Spillback effects of the parasitic salmon louse from fish farms to wild fish are a critical environmental concern for salmon-producing countries. This ectoparasite causes significant physiological damage to wild salmonids. However, strong empirical links between injuries and abundance lice has not been established in situ. This study empirically establishes a strong association between dorsal fin damage in sea trout (Salmo trutta) and infestation with mobile stages of salmon lice at a marine site. We further assessed the prevalence of such damages on returning sea trout to the spawning grounds through snorkelling observations in 16 rivers in Hardangerfjord, western Norway, from 2007-2021. The prevalence of injuries decreased with distance from the fjord outlet, from ~70% in middle and outer parts to ~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.

Data from sea trout with sea lice were collected from marine survey data on sea trout caught in a custom designed trap-net (Barlaup et al. 2013) in Herdlafjorden in the years 2011, 2013, 2014, and 2015 as a part of the national surveillance of salmon lice on wild salmonid fish (NALO). The traps were set up from the end of May to the end of start of July. In 2011, 2013 and 2014 fish were euthanized and placed in bags labelled with the date and frozen. The fish were later defrosted in the lab, weighed (wet mass, ± 1 g), and measured (total length, ± 1mm) and different stages of sea lice on each fish counted according to Hamre et al. (2013). In 2015, lice counts were instead conducted on live trout in the field. Each fish was placed in a bath of a mixture of ethanol and clove oil (1 part clove oil to 9 parts 70% ethanol) mixed in seawater at a ratio of 4.5 ml mixture per 10 liters of water. This was done to calm the fish so that lice could be counted (Figure 1). Once they had recovered from sedation, they were released back into the sea. Reporting dorsal fin damage is not part of the routine survey data collected in the survey, but the personnel at these locations were specifically asked to define whether or not the fish had dorsal fin damage or other damages. The dorsal fin was classified as either “no damage”, “partial missing” or “missing” (see figure 1 for examples).

Spawning count surveys are conducted annually in most of the rivers in Hardangerfjorden including 2007-2021. During spawning count surveys, personnel with snorkel using either a dry suit or a wet suit swim down the river in parallel (varying from 1- 4 people in a row) counting fish and classifying them according to size categories, species (trout or salmon) and origin (hatchery fish/escaped farmed fish/wild fish; Mahlum et al. 2019; Skoglund et al. 2021). Dorsal fin lesions were noted only on sea trout when fish are appropriately close and the water visibility allows it. This means that the fish observed for lesions are only a subsample of the fish in the river. The lesions are classified as a binomial outcome– damaged dorsal fin or not. Examples of a dorsal fin with a typical observable damaged dorsal fin is given in figure 1d. Dorsal fin damage classifications were only done by personnel with several years of experience snorkelling to maintain internal consistency within the dataset. To assess potential bias in observation we plotted the proportion of fish observed against the total number of fish observed for each river (Appendix A1), with the expectation that the proportion would correlate negatively with number of fish observed if there was an observation bias. No such pattern was found.