| Summary: | A total of 1205 immature and mature cod (Gadus morhua L.) of both sexes, caught during 1976-1981 in the Trondheimsfjord, Norway, were investigated in a population-genetic study by means of the polymorphic haemoglobin locus HbI (SICK 1961). Genetic heterogeneity at this locus was observed both between and within samples, even when taken on the spawning ground. In 1977, 602 specimens from the spawning group were genotyped for HbI, tagged with Lea hydrostatic tag, and released. The spatio-temporal distribution of genotypes in recaptures during four years reinforced the impression of genetic imbalance at HbI for cod in this area. Much of the HbI heterogeneity in adults was shown to be correlated with fish size, and an analysis of the length distribution among immatures revealed significant differences in mean lengths between the three common HbI genotypes. The goodness of fit, to our data, of two different population models is evaluated; in model 1, the observed HbI heterogeneity and genotypic size differences are explained as effects of physical population mixing while in model 2, these observations are interpreted as effects of genotypic growth differences and size-selecting fishing gear. The observations in the present study seemed most consistent under the latter model, although the former could not be formally excluded. The present findings are related to previous studies concerning the selection aspect of the cod HbI polymorphism, and the necessity of experimental testing of some field observations is pointed out. However, it is concluded that there at present is reason to doubt the reliability of HbI characteristics when used in cod population structure analyses.
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