The incidence of sea lice infestations on wild sea trout compared to farmed salmon

The high abundance of sea lice on wild salmonids, particularly sea trout post-smolts, is frequently said to be related to the presence of fish farms within the area. Within the Laxford Bay, Sutherland, the current management practice of a full year fallow, provides a unique opportunity to examine this. Wild salmonids were sampled monthly between March and October, over a four year period and the number of sea lice counted. Information on the fish farm was provided by the operators. From this study, there appears to be a weak relationship between lice abundance on the wild salmonids within Laxford Bay and the stage of production on the neighbouring fish farm. While this is as expected from other studies within the west coast of Scotland, the lack of conclusive statistically significant correlations between the lice abundance on the wild and farmed fish, indicates that other factors have a greater importance to lice abundance. In particular there is a seasonality apparent in the data, both in terms of lice abundance and the stages present.