Genetic structure of cod along the coast of Norway: Results from isozyme studies

A total of 521 cod in samples from eight coastal and fjord locations along the Norwegian coast, from the Russian border to mid-Norway, were assessed f or allele frequencies at six polymorphic tissue enzyme loci (LDH-3*. PGM-1* , MDH-3*, IDHP-1*, PGI-1* and PGI-2*). According to individual otolith depo sition patterns, the four northernmost samples contained both North-East Ar ctic ("arctic") cod and Norwegian coastal ("coastal") cod. At five of the l oci, cod appeared to be one genetically homogeneous unit throughout the sam pling area. Locus LDH-3*, however, showed substantial inter-sample genetic heterogeneity. At this locus, samples from northern Norway (north of the Lo foten Islands) had significantly lower frequencies of the LDH-3*100 allele than those from mid-Norway, whereas the frequencies within each of these gr oups were not significantly heterogeneous. Among cod typed as "arctic", how ever, there was a statistically significant surplus of LDH-3* heterozygotes on some locations and in some age groups, and genotypic and allelic freque ncies differed significantly between age groups and between specific sampli ng locations. These LDH-3* observations do not fit to the current cod manag ement model for northern Norwegian waters, which assumes that "arctic" cod is one panmictic population which individuals can be identified by otolith and are expected to show the same genetic characteristics throughout the di stribution range of this stock. Among possible explanations for this are: ( I) cod LDH-3* allele frequencies give, due to strong natural selection, an unreliable picture of the true population structure, (2) "arctic cod" is no t one homogeneous unit, i.e. two or more "arctic" populations were represen ted in the present materials, (3) classifying individual cod to "arctic" an d "coastal" by otolith is not a reliable procedure. Regardless of the answe rs to (2) and (3), the Hardy-Weinberg anomalies at LDH-3* documented in thi s and previous studies suggest that this locus is substantially affected by environmental selection and that its allele frequencies are not stable eno ugh to be used as population characteristics in cod. The differentiation pa tterns revealed by Various types of genetic markers are discussed under som e assumptions about the evolutionary frames for cod in the sampled areas.