Transferrin Variation in North American Populations of the Atlantic Salmon, Salmo salar

A latitudinal cline was indicated by the frequency of the Tf4 transferrin allele in North American populations of the Atlantic salmon (Salmo salar) and an earlier suggestion of a major genetic discontinuity between the salmon populations of Newfoundland and those of the Canadian Maritimes is rejected.Male one-sea-winter fish had a statistically significant deficit of heterozygotes but transferrin phenotypes of one-sea-winter females and two-sea-winter or older fish of either sex agreed closely with the predictions of Hardy–Weinberg–Castle equilibria. Heterozygosity for Tf4 may be semilethal in male salmon programmed to return to spawn after one winter at sea.A comparison of anadromous and nonanadromous salmon populations from the same river system demonstrated that considerable genetic divergence can take place when postglacial crustal recovery produces barriers to upstream migration. The relative fitnesses of the transferrin phenotypes may be different for anadromous and nonanadromous life styles. In view of the demonstrated genetic divergence between anadromous and nonanadromous salmon stocks, the practice of "improving" salmon rivers by removing natural obstructions to upstream migration must be reconsidered in the case of river systems which have large stocks of nonanadromous salmon.