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Background  The causal relation between parasitic sea lice on fish farms and sea lice on wild fish is a controversial subject. A specific scientific debate has been whether the statistical association between infestation pressure ( IP ) from fish farms and the number of parasites observed on wild sea trout emerges purely because of a confounding and direct effect of temperature ( T ).    Methods  We studied the associations between louse infestation on wild sea trout, fish farm activity and temperature in an area that practices coordinated fallowing in Nordhordland, Norway. The data were sampled between 2009 and 2016. We used negative binomial models and mediation analysis to determine to what degree the effect of  T  is mediated through the  IP  from fish farms.    Results  The number of attached lice on sea trout increased with the  T  when the  IP  from fish farms was high but not when the  IP  was low. In addition, nearly all of the effect of rising  T  was indirect and mediated through the  IP . Attached lice remained low when neighbouring farms were in the first year of the production cycle but rose substantially during the second year. In contrast to attached lice, mobile lice were generally seen in higher numbers at lower water temperatures. Temperature had an indirect positive effect on mobile louse counts by increasing the  IP  which, in turn, raised the sea trout louse counts. Mobile louse counts rose steadily during the year when neighbouring farms were in the first year of the production cycle and stayed high throughout the second year.    Conclusions  The estimates of the  IP  effect on louse counts along with the clear biennial pattern emerging due to the production cycle of fish farms clearly indicate that fish farms play an important role in the epidemiology of sea lice on wild sea trout. Furthermore, the mediation analysis demonstrates that a large proportion of the effect of  T  on louse counts is mediated through  IP .    Electronic supplementary material  The online version of this article (10.1186/s13071-018-3189-6) contains supplementary material, which is available to authorized users.

Parasitic sea louse infestations on wild sea trout: separating the roles of fish farms and temperature