Farm Atlantic salmon escape and invade rivers throughout the North Atlantic annually, which has generated growing concern about their impacts on native salmon populations. A large-scale experiment was therefore undertaken in order to quantify the lifetime success and interactions of farm salmon invading a Norwegian river. Sexually mature farm and native salmon were genetically screened, radio tagged and released into the River Imsa where no other salmon had been allowed to ascend. The farm ¢shes were competitively and reproductively inferior, achieving less than one-third the breeding success of the native ¢shes. Moreover, this inferiority was sex biased, being more pronounced in farm males than females, resulting in the principal route of gene £ow involving native males mating with farm females. There were also indications of selection against farm genotypes during early survival but not thereafter. However, evidence of resource competition and competitive displacement existed as the productivity of the native population was depressed by more than 30%. Ultimately, the lifetime reproductive success (adult to adult) of the farm ¢shes was 16% that of the native salmon. Our results indicate that such annual invasions have the potential for impacting on population productivity, disrupting local adaptations and reducing the genetic diversity of wild salmon populations.

Genetic diversity within and between two wild and one farmed population of Atlantic salmon was estimated by 12 variable enzyme loci, three single locus‐ and one multilocus DNA minisatellite probe. The farmed salmon were fifth‐generation fish from one of the principal commercial strains in Norway and the wild salmon were from the rivers Numedalslågen and Tana, Norway. All three classes of markers detected significant differentiation between the populations, with the farmed population being genetically most divergent. The farmed fish showed less genetic variability than the wild populations, as all techniques revealed a lower (14%−45%) number of variable bands/alleles in this group. However, only multilocus DNA fingerprinting detected a significantly lower level of heterozygosity within the farmed population. Estimates of average heterozygosity within populations were almost identical for single locus and multilocus minisatellites (0.558 and 0.548, respectively), which were more than three times higher than for polymorphic allozymes (0.162). Estimates of interpopulation genetic differentiation, however, gave highly concordant results between the allozymes and minisatellites (Fstrange: 0.106‐0.135). Our results show that fifth generation farmed Atlantic salmon differ significantly from wild salmon in loci others than those chosen for selection in the breeding program. The study also suggests that the choice of genetic markers for studies of Atlantic salmon depends on the aim of the study (within‐versus between‐population variability).

The influence of sperm competition and individual mating behaviour in an externally fertilizing species of fish, the Atlantic salmon (Salmo salar), is estimated from video observations of multiple-male spawnings and subsequent paternity analyses. One male dominated the paternity during polygamous spawnings, fathering more than 80% of the progeny in a single nest. Behavioural analyses of the spawnings showed that the first-mating male had sperm precedence in 6 out of 10 cases. In three of the other spawnings, sperm limitation likely influenced individual success, as the first-mating male had participated in a large number of spawnings. In the final, nearly simultaneous spawning, male size was more important than the 0.6-s difference in spawning times. Thus, male fertilization success can be influenced by a variety of factors, including sperm precedence, male size, and spawning history.

Atlantic salmon (Salmo salar) juveniles were electrofished along a 300-m stretch of a river to test for possible associations between genetic similarity and geographical distance between individuals. Multilocus DNA fingerprinting showed that genetically similar juveniles (1-4 years old) were found closer together in the river than less related individuals. However, the association between genetic similarity and geographical distance, although significant, was not strong. This may indicate that factors other than genetic relatedness influence the positioning in the river. A sex bias in the relationship between genetic similarity and geographical distance was caused by a difference between sexually mature and immature males. The study shows that sampling of salmon juveniles should be spread over a wide stretch of the river in order to avoid sampling relatives. Moreover, by including several year classes, the overall degree of genetic similarity is effectively reduced compared with sampling individuals of similar age.1999 The Fisheries Society of the British Isles