Haemoglobin polymorphism inGadus morhua: Genotypic differences in maturing age and within-season gonad maturation

Abstract

276 specimens of Atlantic cod (Gadus morhua L.) were caught during spawning in a restricted area of the Trondheimsfjord, Norway, in April and May 1979. Genotypes at the polymorphic haemoglobin locusHbl differed significantly with respect to mean age at maturation (in males) and mean gonadic development stage (in females). There was no indication of population mixing in the genotypic composition atHbl or at any of the 4 polymorphic tissue enzyme loci investigated (LDH-3, IDH-1, PGM, andPGI-1. The findings obtained were considered with regard to temperature-related differences in the functional properties of Hbl molecules, and genotypic differences in growth, age at maturation, and fishing mortality. At the present stage of investigation, the natural selection pattern seems directional and strong. However, theHbl allele frequencies observed in cod from the examined areas reveal no detectable changes over a period of two decades (∼ 4 generations). The current pattern of commercial exploitation causes, through size selection, a modification of the rate of erosion of the inferior allele, but additional factors must be in force, which play a role in its current abundance in an evolutionary perspective. The observedHbl genotypic differences in the exact within-season time for spawning might be one such factor. A potential sexual difference in genotypic fitness might be another, but this has yet to be confirmed. The apparent existence of considerable natural and artificial selection forces acting upon cod haemoglobin genotypes makesHbl allele frequencies unreliable for use in population structure analyses.