Towards direct evidence of the effects of salmon lice (Lepeophtheirus salmonis Krøyer) on sea trout (Salmo trutta L.) in their natural habitat: proof of concept for a new combination of methods

Abstract

Studies addressing the impact of salmon lice (Lepeophtheirus salmonis Krøyer) on sea trout in their natural habitat are scarce and mostly limited to prophylaxis-based experiments. The main drawbacks with this approach are that lice infestations on control fish are not known and the anti-parasitic treatment is of unknown efficacy and may have unwanted side-effects. We tested an innovative approach where prophylaxis is replaced with artificial infestation of the fish. Twenty-nine sea trout post-smolts were caught in a farming-free area in southern Norway and half the fish were artificially infested with lice. Survival and behavior of individual fish was investigated using acoustic telemetry. Furthermore, salinity values were extracted from an hydrodynamical model simulation and connected to individual 3D positions. Results from this pilot study show consistent trends in behavioral differences between artificially infested and control fish. All fish that died or prematurely returned to freshwater were artificially infested fish, although results were not statistically significant. Besides, power analysis confirmed the limitations of this small pilot study for delivering statistically significant inferences. We found also indications of artificially infested fish remaining in shallower waters and within shorter distance to low salinity habitats, but only differences in modelled salinity values were statistically significant. Methodological progress and limitations with this original approach are discussed, and we recommend further studies using this combination of methods and the lessons learnt from this pilot study to provide better quantitative evidence on the effect of salmon lice on sea trout in the wild.

The hydrodynamic foundation for salmon lice dispersion modeling along the Norwegian coast

“…The salmon louse Lepeophtheirus salmonis is a natural parasite in Norwegian waters that feeds on the skin, fat, and mucus of salmonid fish (Pike & Wadsworth 1999). With the industrialization of aquaculture, the numbers of potential hosts have increased, leading to unnaturally high lice infestations on both farmed and wild fish (Finstad et al 2000;Bjorn et al 2001;Heuch & Mo 2001;Heuch et al 2009Heuch et al , 2011Serra-Llinares et al 2018). Salmon lice carry eggs in two eggstrings at the rear of the body.…”

Impacts of salmon lice on mortality, marine migration distance and premature return in sea trout

“…Moreover, other secondary effects associated with the treatment are not expected. Serra-Llinares et al (2018) performed a pilot study to test this new method using wild trout post-smolts (n = 29) caught in bag nets shortly after they entered the sea in Sandnesfjord, southern Norway. The authors concluded that, despite limitations in the statistical power rendered by the study, results indicated consistent trends in behavioural differences between control and artificially infested fish, suggesting that this shift in method (i.e.…”