Salmon louse infection reduces condition and induces mortality in wild Atlantic salmon post-smolt

The marine ectoparasitic copepod Lepeophtheirus salmonis, commonly known as the salmon louse, poses a significant risk to Atlantic salmon (Salmo salar L.) in farm-intensive areas in the North Atlantic. Previous studies have documented the direct effects of the lice on hatchery-reared salmon in laboratory studies and wild salmon as part of ecosystem-level modeling approaches. However, the effects of L. salmonis on wild post-smolt salmon in a controlled environment remain poorly understood. In this study, we investigated the impact of the parasite on individual fish in two separate laboratory infection studies with wild post-smolt salmon caught during smolt migration from two rivers in western Norway (n = 47, average body weight 24.3 ± 6.0 g). We found negative correlations between L. salmonis infection intensity and host-specific growth rate (β1: -1.15% d−1, p < 0.001) and condition factor (β1: -0.08, p < 0.001). We observed significant positive correlations between infection intensity and salmon heart size (β1: 4.022e-04% body weight, p = 0.002) and liver size (β1: 0.03% body weight, p < 0.001), suggesting a trend toward increased organ size. However, the poor model fit and minimal β1 estimates limit the ability to draw conclusions about wild host fitness. Mortality (n = 11) was observed between 18 and 35 days post-infection at 9°C water temperature. This time frame coincided with the female preadult lice stages. Obtained lice-induced mortality estimates were determined in the absence of additional natural environmental stressors such as predators, which are likely to preferentially target fish weakened by infection. Notably also, the use of UV-treated water excluded opportunistic pathogens that may infect fish through lice-associated wounds. This study confirms the established link between reduced condition and higher mortality due to L. salmonis as observed in laboratory experiments using hatchery-reared fish also extends to wild post-smolt salmon entering the fjords, underscoring the considerable risk the parasite poses to these populations.