Genetic differentiation of populations of the copepod sea louse Lepeophtheirus salmonis (Krøyer) ectoparasitic on wild and farmed salmonids around the coasts of Scotland: Evidence from RAPD markers

Sea trout are the sea-going migratory form of the freshwater brown trout (Salmo trutta L.) and since 1989 there have been marked declines in their stocks on the west coasts both of Scotland and Ireland. Various factors have been attributed as possible causal agents in these stock declines, including fresh water acidification, overfishing, climatic fluctuations, habitat degradation and sea lice parasitic burdens. The putative impact of infestations of sea trout by the ectoparasitic copepod sea louse, Lepeophtheirus salmonis (Krøyer), has featured prominently in the controversy, especially with regard to the role of inshore commercial salmon farms as a possible source of infestation of wild salmonids by sea lice. This study focused on the population genetics of L. salmonis around the coasts of Scotland: We sampled fish from wild and cultured stocks and included salmon (Salmo salar L.), rainbow trout (Oncorhynchus mykiss Walbaum) and sea trout as host species. Analyses of allozyme variation of sea lice were confined to data for two polymorphic loci (Fum, Got-2) and conformed to our initial expectation — that the inclusion of a planktonic larval phase in the life cycle of the copepod, in addition to the high mobility of the host fish, would enhance gene flow and preclude genetic differentiation of L. salmonis populations as a result of random drift alone. DNA polymorphism was quantified by means of PCR and RAPD analysis. Six primers were screened for 16 samples (from wild and farmed salmon, wild sea trout and farmed rainbow trout) — including the east, north and west coasts of Scotland — and the data analyzed by AMOVA (Analysis of Molecular Variance). In contrast to the allozyme results, the RAPD analysis showed striking patterns of genetic differentiation around the coasts of Scotland. The overall pattern was one of genetic homogeneity of L. salmonis populations sampled from wild salmon and sea trout. All of the L. salmonis samples taken from farmed salmon and rainbow trout did, however, show highly significant levels of genetic differentiation, both between wild and farmed salmonids and among the various farms themselves. Evidence of high levels of small-scale spatial or temporal heterogeneity of RAPD marker band frequencies was shown for the one farm from which repeat samples (July and November, 1995) were analysed. Samples of sea lice taken from west coast wild sea trout subjected to RAPD analysis also revealed the occurrence of putative “farm markers” in some individual parasites, indicating that they had possibly originated from salmon farms.