The relationship between allozyme polymorphism and morphometric variation in pink salmon from Southern Sakhalin was examined in order to determine the effects of individual genes. Dramatic differences were found between individuals of some genotypes. For instance, the difference in average male body length between the two prevailing PGDH* genotypes reached 36sigma, which corresponds to 5% contribution to the total variance of the trait. It would take only several generations for selection in favor of extreme phenotypes to rapidly shift the allele frequencies at loci of such a strong effect, which should be taken into account both in fishery management and in practical hatchery. Long-term selection can result in irreversible genetic changes in the population, which may be hazardous in the context of causing irreparable harm to the biological resources of the Earth.