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 for 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 deposition patterns, the four northernmost samples contained both North-East Arctic ("arctic") cod and Norwegian coastal ("coastal") cod. At five of the loci, cod appeared to be one genetically homogeneous unit throughout the sampling area. Locus LDH-3*, however, showed substantial inter-sample genetic heterogeneity. At this locus, samples from northern Norway (north of the Lofoten Islands) had significantly lower frequencies of the LDH-3*100 allele than those from mid-Norway, whereas the frequencies within each of these groups were not significantly heterogeneous. Among cod typed as "arctic", however, there was a statistically significant surplus of LDH-3* heterozygotes on some locations and in some age groups, and genotypic and allelic frequencies differed significantly between age groups and between specific sampling locations. These LDH-3* observations do not fit to the current cod management 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 distribution 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 not one homogeneous unit, i.e. two or more "arctic" populations were represented in the present materials, (3) classifying individual cod to "arctic" and "coastal" by otolith is not a reliable procedure. Regardless of the answers to (2) and (3), the Hardy-Weinberg anomalies at LDH-3* documented in this and previous studies suggest that this locus is substantially affected by environmental selection and that its allele frequencies are not stable enough to be used as population characteristics in cod. The differentiation patterns revealed by Various types of genetic markers are discussed under some assumptions about the evolutionary frames for cod in the sampled areas.