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Salmon farming has multiplied from a side business of coastal farmers to one of the world's major aquaculture species. This has dramatically altered the disease dynamics between farmed and wild salmonids. As salmon fish farming has increased, new restrictions have been enforced to combat emerging density-dependent impacts of pathogen spillover. In most northern and arctic regions, the effects of pathogens from fish farms on wild salmonids have been minimal for two key reasons: (i) relative low density of fish farms in the north and (ii) cold water temperatures. However, both factors are set to change dramatically. On one side, there is an increasing interest in utilizing northern areas for fish farming due to limited capacity for expansion in mid-latitude regions. On the other side, climate change is rapidly changing these northern ecosystems. High-latitude regions inhabit some of the largest remaining wild Atlantic salmon populations in the world along with sea trout and Arctic charr. Wild salmonids in the north have most likely seldom been exposed to high infection pressure, and we question how these populations will cope with changes that are coming. We identify 12 research questions emerging from these imminent changes and discuss methodologies for addressing them. We conclude that policies related to fish farming must consider uncertainties with respect to pathogen dynamics in the north until these research questions are fully addressed. [ABSTRACT FROM AUTHOR]

Bjørn, P-A., Finstad, B., Kristoffersen, R., Rikardsen, A. H., and McKinley, R. S. 2007. Differences in risks and consequences of salmon louse, Lepeophtheirus salmonis (Krøyer), infestation on sympatric populations of Atlantic salmon, brown trout, and Arctic charr within northern fjords. – ICES Journal of Marine Science, 64: 386–393. Differences in salmon louse (Lepeophtheirus salmonis) infestation on sympatric populations of fjord-migrating, Atlantic salmon post-smolts (Salmo salar), brown trout (Salmo trutta) (sea trout), and Arctic charr (Salvelinus alpinus) were studied in three fjords with fish-farming activity in northern Norway during the period June–August 2000. Atlantic salmon post-smolts were only captured in the fjords during late June and early July, and probably left them subsequently. No fish were infested with salmon lice. In contrast, brown trout and Arctic charr had similar infection patterns during their sampling periods, with very low prevalence and mean infection intensity during June (0–21% and 0–6 lice per fish, respectively), slightly increasing in July (8–70% and 6–12 lice per fish, respectively), and peaking in August (80–88% and 19–27 lice per fish, respectively). The chalimus stages dominated during June and July, with a few pre-adult and adult stages observed in July, and all stages were found frequently during August. The observations indicate that Atlantic salmon may have a mismatch between the time of louse infestation and their post-smolt fjord migration in northern fjords. In contrast, brown trout and Arctic charr feed within the fjords throughout summer and have a higher risk of harmful infestation in years with suitable environmental conditions for salmon louse development, especially in fish‐farming areas. Arctic charr usually spend the shortest time at sea of the three species, and the salmon lice may not have time to develop to the adult stage on this species. [ABSTRACT FROM PUBLISHER]

The abundance of salmon lice was examined in two stocks of sympatric anadromous Arctic char and sea trout in sub-Arctic regions in northern Norway in June, July, and August 1992 and 1993. One stock feeds in a coastal area exposed to moderate salmon farming activity (exposed area), while the other feed in a region without salmon farms (unexposed area). The salmon lice infestation on both species differed significantly between the exposed and unexposed area as well as between years and also between weeks within the same year. We did not detect, however, any clear significant differences in salmon lice abundance between sympatric populations of Arctic char and sea trout, or between different size groups of the species. The 1992 and 1993 infestation pattern in the exposed area showed an epidemic tendency in both Arctic char and sea trout, characterised by a sudden increase in both prevalence and abundance of lice larvae in July 1992 (23.6±25.7 lice/fish) and August 1993 (19.9±20.8 lice/fish). We therefore suggest that salmon lice epidemics, previously only observed on sea trout, may also occur in populations of Arctic char, and that fish farming contributes to the elevated lice level in wild fish. The fish in the unexposed area were also infested, although at significantly lower levels than fish from the exposed area. The infestation peaked in August 1992 at 13.0±18.1 lice/fish and August 1993 at 3.9±4.5 lice/fish, suggested that lice originating on ascending wild Atlantic salmon, or lice larvae drifting from farming areas, may infest Arctic char and sea trout also in unexposed localities in Subarctic areas. [Copyright &y& Elsevier]