A Two-Generation Experiment Comparing the Fitness and Life History Traits of Native, Ranched, Non-Native, Farmed And ‘Hybrid’ Atlantic Salmon Under Natural Conditions

Since Atlantic salmon from different rivers are genetically different, non-native introductions have the potential to change the genetic make-up and juvenile production (fitness) of the recipient wild populations. While there has been much theoretical discussion on the genetic and ecological impacts on native populations of the deliberate and inadvertent introductions of Atlantic salmon, there have been only a few empirical studies. The development of DNA profiling, involving minisatellites and later microsatellites, has enabled accurate parentage identification and opened the way to direct comparison of stocks from egg stage onwards under realistic natural conditions. An experiment, comprising three cohorts (1993, 1994 and 1998) of Atlantic salmon was undertaken in the Burrishoole system in western Ireland. This involved multiple families of the following eight groups: native wild (WILD - all cohorts); native ranched (RANCH - 1998 only); non-native from the adjacent Owenmore River (OWEN - 1998 only); farmed (FARM -all cohorts); F1 wild x farmed (male and female reciprocal groups) (F)Hy - 1993 and 1994 cohorts); F2 wild x farmed (F2Hy - 1998 cohort); Bl backcrosses to wild (BQW - 1998 cohort); and Bl backcross to farmed (BQF - 1998 cohort). The aim of the experiment was to look at genetic differences, without the confusion of behavioural differences. Survival, growth, migration and maturity characteristics were examined at each life stage and overall lifetime success was estimated. In total, 7033 parr and 1502 smolts from the experimental river together with 1385 returning adults were examined. In a situation where a river is not at its parr carrying capacity, farmed salmon have a lifetime success equivalent to 3% relative to wild fish. In a river at carrying capacity this increases to 6%. The ‘hybrids’ show intermediate fitness decreasing in the rank order of: BQ wild; Fi wild x farm; BQ farm; F2 hybrid (but marine stage not measured for this group); Fl farm x wild. The Owenmore group showed an overall success of 17 and 20% in the two scenarios. Only the ranched did not show a reduction in success relative to the wild group.