Population genetic differentiation of sea lice (Lepeophtheirus salmonis) parasitic on Atlantic and Pacific salmonids: analyses of microsatellite DNA variation among wild and farmed hosts

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Abstract
The copepod Lepeophtheirus salmonis is ectoparasitic on Atlantic and Pacific wild salmonids. It is a major pest to Atlantic salmon (Salmo salar) aquaculture and may be implicated in recent declines of certain European wild salmonid stocks. Variation at six microsatellite loci was assessed among L. salmonis from wild and farmed salmonids in Scotland, wild sea-run brown trout (Salmo trutta) in Norway, and farmed Atlantic salmon in eastern Canada. An outgroup North Pacific sample was obtained from farmed Atlantic salmon in British Columbia. No significant differentiation was found between L. salmonis from the host species or among samples from throughout the North Atlantic. This is consistent with long-distance oceanic migration of wild hosts and larval interchange between farmed and wild host stocks being sufficient to prevent genetic divergence of L. salmonis throughout the North Atlantic. These results have important management implications for both wild stock conservation and aquaculture in that genetically, L. salmonis in the North Atlantic comprises a single population: there is no evidence of isolation of populations on farmed hosts from those on wild fish. Comparison between North Pacific and North Atlantic L. salmonis populations showed significant but low differentiation (FST= 0.06).

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