Gunnar Ståhl

Genetic relationships among landlocked, resident, and anadromous Brown Trout, Salmo trutta L.

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 life-history 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 results 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.

Differences in the amount and distribution of genetic variation between natural populations and hatchery stocks of Atlantic salmon

Abstract Tissue samples from a total of 1643 Atlantic salmon collected from naturally reproducing populations and hatchery stocks representing nine major river systems draining into the Baltic Sea were electrophoretically analyzed for 37 enzyme loci. Pronounced spatial genetic heterogeneity was generally observed among samples within as well as between different river systems. Samples representing hatchery stocks exhibit a significantly lower amount of genetic variability than natural populations. This is displayed as both a reduced variation within hatchery stocks and a lower amount of genetic divergence between them.

EVOLUTION OF ATLANTIC AND PACIFIC COD: LOSS OF GENETIC VARIATION AND GENE EXPRESSION IN PACIFIC COD

An allozyme investigation of 41 protein‐coding loci in two morphologically similar fishes, Atlantic and Pacific cod, indicates that Pacific cod experienced a severe population bottleneck that led to the loss of gene diversity and gene expression. Pacific cod possesses a significantly lesser amount of gene diversity (H = 0.032) than Atlantic cod (H = 0.125) and lacks gene expression for Me‐3. Allele‐frequency distributions differ between species as predicted by neutral theory: Atlantic cod has a U‐shaped distribution, which is expected for populations in drift‐mutation equilibrium, whereas Pacific cod has a J‐shaped distribution with an excess of low‐frequency alleles. This excess may be explained by the appearance of new alleles through mutation which have not yet reached intermediate frequencies through drift. The population bottleneck in Pacific cod was most likely associated with founder populations that dispersed into the Pacific Ocean after the Bering Strait opened. Under the molecular‐clock hypothesis a Nei genetic distance of 0.415 (based on 41 loci) suggests that Pacific cod dispersed into the Pacific Ocean soon after the Bering Strait opened in the mid‐Pliocene, 3.0 to 3.5 million years ago.