GENETIC DIFFERENTIATION OF POPULATIONS OF THE COPEPOD SEA LOUSE LEPEOPHTHEIRUS-SALMONIS (KROYER) ECTOPARASITIC ON WILD AND FARMED SALMONIDSAROUND THE COASTS OF SCOTLAND - EVIDENCE FROM RAPD MARKERS

Sea trout are the sea-going migratory form of the freshwater brown tro ut (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 cope pod sea louse, Lepeophtheirus salmonis (Kroyer), has featured prominen tly in the controversy, especially with regard to the role of inshore commercial salmon farms asa possible source of infestation of wild sal monids by sea lice. This study focused on the population genetics of L . salmonis around the coasts of Scotland: We sampled fish from wild an d cultured stocks and included salmon (Salmo salar L.), rainbow trout (Oncorhynchus mykiss mykiss Walbaum) and sea trout as host species. An alyses of allozyme variation of sea lice were confined to data for two polymorphic loci (Fum, Got-2) and conformed to our initial expectatio n - 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, wou ld enhance gene flow and preclude genetic differentiation of L, salmon is populations as a result of random drift alone. DNA polymorphism was quantified by means of PCR and RAPD analysis. Six primers were screen ed for 16 samples (from wild and farmed salmon, wild sea trout and far med rainbow trout) - including the east, north and west coasts of Scot land - and the data analyzed by AMOVA (Analysis of Molecular Variance) . In contrast to the allozyme results, the RAPD analysis showed striki ng patterns of genetic differentiation around the coasts of Scotland. The overall pattern was one of genetic homogeneity of L, salmonis popu lations sampled from wild salmon and sea trout. All of the: L, salmoni s samples taken from farmed salmon and rainbow trout did, however, sho w highly significant levels of genetic differentiation,, both between wild and farmed salmonids and among the various farms themselves. Evid ence of high levels of small-scale spatial or temporal heterogeneity o f RAPD marker band frequencies was shown for the one farm from which r epeat samples (July and November, 1995) were analysed. Samples of sea lice taken from west coast wild sea trout subjected to RAPD analysis a lso revealed the occurrence of putative ''farm markers'' in some indiv idual parasites, indicating that they had possibly originated from sal mon farms. (C) 1997 Elsevier Science B.V.