Sea lice (Lepeophtheirus salmonis) transmission between farmed and wild salmon in Muchalat Inlet, British Columbia

Objective: To evaluate the association between the abundance of sea lice recorded on salmon farms and the infestation levels on wild out-migrating Pacific salmon (Oncorhynchus spp.).

Materials and methods: This study included 10 years of sea lice count and management data (2007-2016; aggregated at the sampled month level) from all Atlantic salmon farms (n = 5) and wild Pacific salmon monitoring sites (n = 16) along Muchalat Inlet, British Columbia, Canada. The potential effects of farm-origin sea lice pressure from farms (X) on the prevalence of infestation on Chum salmon (O. keta), the dominant species in the study region, (Y) during their outmigration period (March-June) were assessed using mixed-effects logistic and linear regression models. Fixed effect of months and random effect of study years were included in the final models. To define X, sea lice output from each farm was weighted using a Gaussian Kernel density function based on seaway distance.

Results: Our analyses indicated a significant positive association between the sea lice abundance on farms and the probability that wild fish would be infested. However, increased abundance of lice on farms was not significantly associated with the prevalence of infestation observed on Chum salmon.

Conclusion: Our results suggest that Atlantic salmon farms may be an important source for the introduction of sea lice to wild Pacific salmon populations, but that the absence of a dose-response relationship indicates that any estimate of farm impact requires more careful evaluation of causal inference than can typically be found in the extant scientific literature.