POPULATION-STRUCTURE AND IMPACT OF SUPPORTIVE BREEDING INFERRED FROM MITOCHONDRIAL AND MICROSATELLITE DNA ANALYSES IN LAND-LOCKED ATLANTIC SALMON SALMO-SALAR L

Four tributaries of Lake St-Jean (Quebec, Canada) are used for spawnin g and juvenile habitat by land-locked Atlantic salmon. Spawning runs h ave drastically declined since the mid-1980s, and consequently, a supp ortive-breeding programme was undertaken in 1990. In this study, we an alysed seven microsatellite loci and mtDNA, and empirically estimated effective population sizes to test the hypotheses that (i) fish spawni ng in different tributaries form genetically distinct populations and (ii) the supportive breeding programme causes genetic perturbations on wild populations. Allele frequency distribution, molecular variance a nd genetic distance estimates all supported the hypothesis of genetic differentiation among salmon from different tributaries. Gene flow amo ng some populations was much more restricted than previously reported for anadromous populations despite the small geographical scale (40 km ) involved. Both mtDNA and microsatellites revealed a more pronounced differentiation between populations from two tributaries of a single r iver compared with their differentiation with a population from a neig hbouring river. The comparison of wild and F-1-hatchery fish (produced from breeders originating from the same river) indicated significant changes in allele frequencies and losses of low-frequency alleles but no reduction in heterozygosity. Estimates of variance and inbreeding p opulation size indicated that susceptibility to genetic drift and inbr eeding in one population increased by twofold after only one generatio n of supplementation.