| Summary: | Egg and sperm from Arctic and coastal cod (Gadus morhua L.) were collected during the spawning season in the Møre area. The parent fishes were classified to cod race according to type of otoliths, and the eggs were fertilized artificially as soon as possible. The fertilized eggs were sent by air to Statens Biologiske Stasjon Flødevigen and incubated in the laboratory. The eggs were divided. in three main groups depending of parent classification, A (coastal cod x coastal cod), B (coastal cod x Arctic cod), C (Arctic cod x Arctic cod) . Several polymorphic enzymes (phosphoglucose isomerase, phosphoglucomutase, isocitrate dehydrogenase, and lactate dehydrogenase were found both in the parent fish and in the newly hatched cod larvae. All phenotypes expected in the offspring were found. However, mendelian distribution could not be calculated due to mixing of larvae from more than two parents. During the first 30 days after hatching, offspring from the crossing between Arctic and coastal cod seemed to have a reduced growth rate compared to the other groups. Due to low enzyme activity in these larvae, the phenotypic classification in this group was difficult. A detailed genetic analysis of group B and C, before and after first feeding, show a significant change in the distribution of phenotypes and allel frequencies. The observed changes could be due to selection on phenotypes or may reflect different egg qualities for some of the parents used. The genetic analysis demonstrated, however, that changes occur during the first feeding stage, indicating that genetic selection can take place in this period of larval development.
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