Population genetic studies in cod (Gadus morhua L.) by means of the haemoglobin polymorphism; observations in a Norwegian coastal population
1984
A total of 1205 immature and mature cod (Gadus morhua L.) of both sexes, caught during1976-1981 in the Trondheimsfjord, Norway, were investigated in a population-genetic study bymeans of the polymorphic haemoglobin locus HbI (SICK 1961). Genetic heterogeneity at this locuswas observed both between and within samples, even when taken on the spawning ground. In1977, 602 specimens from the spawning group were genotyped for HbI, tagged with Lea hydrostatictag, and released. The spatio-temporal distribution of genotypes in recaptures during four yearsreinforced the impression of genetic imbalance at HbI for cod in this area. Much of the HbIheterogeneity in adults was shown to be correlated with fish size, and an analysis of the lengthdistribution among immatures revealed significant differences in mean lengths between the threecommon HbI genotypes.The goodness of fit, to our data, of two different population models is evaluated; in model 1, theobserved HbI heterogeneity and genotypic size differences are explained as effects of physicalpopulation mixing while in model 2, these observations are interpreted as effects of genotypicgrowth differences and size-selecting fishing gear.The observations in the present study seemed most consistent under the latter model, althoughthe former could not be formally excluded. The present findings are related to previous studiesconcerning the selection aspect of the cod HbI polymorphism, and the necessity of experimentaltesting of some field observations is pointed out. However, it is concluded that there at present isreason to doubt the reliability of HbI characteristics when used in cod population structureanalyses.
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