Preamble Patricia Gallaugher, Director, Continuing Studies in

The impact of the salmon louse (Lepeophtheirussalmonis (Kroyer)) on populations of sea trout andsalmon in Ireland and other European countries isoutlined and recommendations are made for themanagement of marine salmon farms aimed atminimising such impacts. The paper describes thegrowth of the salmon farming industry in westernIreland during the 1980s and the resulting infestation ofsea trout post-smolts with the salmon louse. Data arepresented from 52 rivers around the Irish coast sampledannually over an eight year period and lice infestationparameters from rivers close to and distant from salmonfarming areas are presented. Correlations between licelevels on sea trout at river mouths and distance to thenearest salmon farm are described.The effect of whole bay fallowing in spring and theeffect of maintenance of good lice control measures onsalmon farms on sea trout recovery is presented. Newstrategies such as Single Bay Management, communalsmolt sites and the development of off-shore sites aredescribed. The results of sampling programmes at seatargeting wild salmon smolts in other Europeancountries is presented. In summary, the paper describesthe adverse consequences of the development of marinesalmon farming with particular reference to the salmonlouse on wild salmonid stocks in Ireland, Scotland andNorway and sets out the measures that can be taken toreduce or prevent these adverse impacts. Irish Sea Trout CatchesLittle accurate long-term data are available on Irish seatrout catches outside the Connemara region. During the1970s and 1980s there was evidence of a slow declinein sea trout stocks from the Burrishoole, north ofConnemara, attributed to afforestation and hillsideerosion due to overgrazing by sheep. Rod catch datafrom fifteen Connemara fisheries covering the period1974-1986 (Figure 11) shows no evidence of a seriousdecline. Rod catches began to show a decline over the1986-1988 period. By 1989 this had resulted in apopulation collapse in many mid-western andConnemara sea trout fisheries (Anon 1992). This isexemplified by the Connemara district rod catch data,which constitutes a large proportion of the mid-Westernregion, where the catch fell from an average of 9,570sea trout over the 1974-1988 period to 646 sea trout in1989 and 240 sea trout in 1990.Catch per unit effort (C.P.U.E.) data for twofisheries in this district, the OwenGowla and Invermore(Table 5) indicate that the catch collapse was not relatedto reduced effort but that an actual collapse in C.P.U.E.occurred. Data from the Burrishoole Fishery, where fulldownstream and upstream trapping takes place,confirmed that sea trout finnock sea survival fell froman average of 20% over the period 1971-1987 to 1.5%in 1989 (Poole et al. 1996).Sea trout rod catches in fisheries in other areasaround the Irish coast distant from salmon aquacultureareas continued to remain stable and no populationcollapse or breakdown of population structure occurred.Marine salmon farming began in Ireland in the early1980s and had reached 10,000 tonnes by 1991 (Figure11). In these early years the bulk of production was inthe Connemara area. The development of salmonfarming in Connemara in the late 1980s coincided witha dramatic reduction in sea trout rod catches inConnemara fisheries. Areas of Scientific Investigation in IrelandThe Sea Trout Action Group (STAG) was formed at theend of 1988 amid growing concern about decliningcatches of sea trout in the West of Ireland. The Group