Selective exploitation of early running fish may induce genetic and phenotypic changes in Atlantic salmon

Genetic evidence for the selective exploitation by anglers of early running fish was examined in four Iberian Atlantic salmon populations (R. As¢n, Pas, Nansa, Deva), using protein and mtDNA markers. The populations studied had been exploited exclusively by rod and line over a nearly fixed fishing season (March to July) since 1949. Genetic variation at six protein loci was small and was accounted for by the MEP-2* and MDH-3,4* polymorphisms, which generally remained stable over time and were in Castle-Hardy-Weinberg equilibrium during the fishing season. However, early running multi-seawinter fish (MSW) generally showed higher frequencies of the common MEP-2* (100) allele than did late running, one sea winter (1SW) grilse, which were significantly smaller and tended to escape the fishery. Spawners differed from angled fish in their mtDNA frequencies and consistently showed a lower sea age and a smaller body size. Spawners also smolted at an older age and displayed lower frequencies of the MEP-2* (100) allele in three of the four populations studied. These results suggest that anglers in these rivers are selectively exploiting a distinct component of the population, and inadvertently causing a differential mortality of genetic types that is likely to be detrimental. Key words: Atlantic salmon; isozymes; mtDNA; selective exploitation; conservation; population structuring.