Evidence for genetically distinct sympatric populations of anadromous and nonanadromous Atlantic salmon, Salmo salvar

Abstract

This study investigated the genetic relationship between sympatric anadromous and nonanadromous Atlantic salmon in the Gambo River system, eastern Newfoundland. Both forms were cultured in the laboratory and seasonal patterns of development were monitored and compared. Growth rates were not different during most of the first year of life, however smolting anadromous salmon (1+) grew more rapidly than nonanadromous salmon for several weeks prior to the time of seaward migration. Several other physiological parameters associated with the parr-smolt transformation were measured; no difference was noted in seasonal profiles of total body moisture, condition factor or plasma Na⁺ and Cl⁻ concentrations. Both groups exhibited increasing levels of integumentary silvering during the late winter and early spring although this pattern was more marked among the anadromous group. Gill Na⁺-K⁺ ATPase activity increased over the same period in both groups, however mean activities among the nonanadromous salmon were consistently lower. Similarly, anadromous salmon displayed more, and larger chloride cells in the gill epithelium as revealed by light microscopy. Salinity tolerance was well developed in both forms in April and June despite differences in chloride cell abundance and Na⁺-K⁺ ATPase activity. Sexual maturation did not occur among female postsmolts of either group nor among male anadromous postsmolts when cultured in freshwater and seawater. Most male nonanadromous salmon did mature as ‘postsmolts’. -- Mitochondrial DNA variation was also examined among wild salmon of both forms using restriction enzymes. The Atlantic salmon mitochondrial genome contains approximately 16,700 bp. No evidence was found for either length polymorphism or sequence heteroplasmy. Variable restriction fragment patterns were generated by five of eighteen enzymes; all variants could be accounted for by single base pair substitutions. Four distinct mitochondrial DNA genotypes were found. Pairwise sequence divergence estimates among genotypes range from 0.2 - 1.0 percent. Significant genotype frequency differences were observed among the two forms. -- Results of this study are consistent with the hypothesis that anadromous and nonanadromous salmon in the Gambo River system represent genetically distinct reproductive units. Reproductive isolation is supported by the observation that the two forms use different spawning sites and that spawning times differ to some extent. Until further research is done to verify these observations it would be prudent to manage the two forms in this system (and possibly others as well) as separate stocks.