The amount of genetic differentiation between stocks of Atlantic cod (Gadus morhua) was estimated from electrophoretically detectable protein loci expressed in skeletal muscle and liver. Variant alleles at 13 of these loci were detected among nine samples covering most of the species range: North America, Greenland, Iceland, Barents Sea, Norwegian coastal waters, the North Sea, and the Baltic Sea. A very low amount of genetic differentiation was observed among stocks. Only two loci (LDH-3 and PGI-1) showed large statistically significant heterogeneity of allele frequencies between samples. Standard genetic distances (Nei) between these samples based on 10 variable loci ranged from 0.00015 to 0.01072 with Baltic cod as the genetically most divergent sample. There was a highly significant correlation between genetic and geographic distance for the samples from the Atlantic suggesting that a substantial gene flow has occurred between these stocks with geographic distance being an inhibiting factor. The relatively larger genetic distance between the Baltic cod and other stocks may reflect isolation resulting from geographic and perhaps ecological barriers. However, the absolute amount of genetic differentiation in the Atlantic cod appears to be very low throughout its range.

Coexisting freshwater resident and anadromous (sea-run migratory) Brown Trout, Salmo trutta L., were compared genetically with landlocked populations (i.e. living above impassable waterfalls) in the same drainage system in western Norway. No genetic differentiation was found between resident and anadromous life-history types using the same locality and time for spawning. In contrast, significant genetic differences were found between Brown Trout (irrespective of lifehistory type) spawning in geographically separate localities, and particularly large differences were found between landlocked Brown Trout and those from localities accessible from the sea. These resuhs are consistent with other multiple-locus studies of salmonid fishes, showing larger genetic differentiation between localities than between coexisting life-history types that differ in morphology and ecology.

The systematica of coexisting morphotypes of Arctic charr, Salvelinus alpinus, has been a matter of dispute ever since the days of Linnaeus. Widespread allelic variation at an esterase locus has led some investigators to propose that the morphotypes reflect a complex of at least three sibling species. We tested this hypothesis by examining 42 electrophoretically detectable loci in natural and transplanted charr populations from 15 localities in S Norway. The absolute values of Nei's genetic distance between morphotypes and populations are small (typically in the order of 0.001), and morphotype changes may occur without accompanying changes in frequencies of esterase alleles. Differentiation among localities explains far more of the total gene diversity than differences between morphotypes in the four cases of naturally occurring sympatric morphotypes examined. The data are consistent with an intraspecific population structuring based on locality, and the multiple species hypothesis is rejected.

No abstract

Preservation of the genetic characteristics of a population is one of the primary objectives of many fish stocking programs. Using starch gel electrophoresis we have tested for temporal gene frequency stability at two polymorphic loci coding for α-glycerophosphate dehydrogenase and creatine phosphokinase. Three Swedish hatchery stocks of brown trout (Salmo trutta) and field samples from natural populations corresponding to two of these stocks were analyzed. Highly significant allele frequency changes at both loci indicated considerable lack of intra-stock genetic homogeneity. In the light of these findings we emphasize the importance of using large numbers of actual as well as effective parents to avoid inadvertent genetic changes and inbreeding. No stock should be founded or perpetuated using less than approximately 30 parents of the least numerous sex in any generation.Key words: inbreeding, genetic drift, gene bank, electrophoresis, stocking, trout