The copepod parasite (Lepeophtheirus salmonis (Kroyer), Caligus elongatus Nordmann) interactions between wild and farmed Atlantic salmon (Salmo salar L.) and wild sea trout (Salmo trutta L.): a mini review

Ectoparasitic copepods are major pathogens of farm and wild salmonids throughout the North Atlantic. Since the early 1990s, there has been controversy regarding the extent to which infective larvae of Lepeophtheirus salmonis originate from aquaculture sites and impact wild salmon (Salmo salar) and sea trout (Salmo trutta). Because of the impracticality of tracking individual planktonic larvae from hatching to final host colonization, reliance has been placed on non-genetic and genetic experimental approaches. Microsatellite analyses show that L. salmonis comprises a single panmictic population throughout the Atlantic; gene flow between parasites on wild and farmed hosts is sufficiently high to prevent population genetic differentiation by random drift. However, because of this lack of significant differentiation, no estimates of the levels of gene flow between farm and wild are possible. The possible evolution of resistance to chemotherapeutants by caligids is of especial concern to the aquaculture industry. Decreased efficacy has been reported for numerous compounds and identification of a point mutation of a sodium channel gene might be indicative of knockdown resistance to pyrethroids. An additional concern is that the more host generalist C. elongatus might become an especially severe pathogen to both salmonid and gadid host populations as the emerging cod (Gadus morhua) aquaculture industry develops.