Sea Lice and Sea Trout: Are salmon farms causing increased parasitism on wild salmonids in Scotland

Over the past three decades, the development of salmon farming into a major industry in the Highlands and Islands of Scotland has been of huge benefit to employment opportunities in these rural communities, and to the Scottish economy as a whole. However, with the expansion of the industry has come rising concern about potential adverse environmental consequences for the marine ecosystems in which the industry is based. Possible impacts range from those on local scales (e.g. the export of carbon-rich fish wastes and chemicals used on fish farm sites) to wider regional-scale effects (e.g. a hypothesized link between dissolved nitrogen emissions from caged fish and increased risk of algal blooms). Over the past twenty years or so, considerable research effort has been invested in understanding local impacts, and developing appropriate predictive tools, and considerable progress has been made. More recently, scientists have been turning their attention to less tangible regional-scale effects. One such effect is a hypothesized, but as yet unproven, link between the sea lice populations that have been endemic on salmon farms, and infestations of lice on wild salmonids (salmon and sea trout). Declines in wild salmonid populations in Scottish rivers over the past few decades have been attributed by some to extreme sea lice parasitism caused by the expanding aquaculture industry, although the declines have been widely observed in salmonid populations throughout the North Atlantic region.