BackgroundThe 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).MethodsWe 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.ResultsThe 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.ConclusionsThe 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 materialThe online version of this article (10.1186/s13071-018-3189-6) contains supplementary material, which is available to authorized users.

The rivers that drain into the Hardangerfjord were historically known to have numerous populations of both sea trout (Salmo trutta) and Atlantic salmon (Salmo salar). After a decline in catches during the last decades many of the rivers have been closed for fishing. In this study we use snorkelling observations from rivers and catch statistics from 2004 to 2011 to describe the current situation and analyse the patterns of density of wild salmon, sea trout and escaped farmed salmon in the Hardangerfjord rivers. We hypothesize that some of the variance in density of salmon and sea trout can be explained by the location of the river in the fjord, with fish from rivers with a longer fjord exposure having a lower density. A median number of 3.5 salmon )ha(1 and 14.9 sea trout )ha (1 were observed in the rivers. Farmed salmon were observed in all rivers and constituted on average 23.3% of the total number of observed salmon. For salmon, there was an inverse log-linear relationship between density in the river and migration distance to coast, with a higher density in rivers with a shorter migration distance. For sea trout there was no evident relationship with location within the fjord. We suggest that the spatial patterns observed for salmon and sea trout can be related to the species-specific differences in habitat use within the fjord system.

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