Disease resistance and enzyme heterozygosity in rainbow trout

The relationship between heterozygosity at nine polymorphic enzyme loci and disease resistance was examined in 373 individually identified rainbow trout (Oncorhynchus mykiss) from 12 full-sib families and a pooled gamete cross. These fish were challenged with bacterial gill disease, a potentially lethal epizootic in freshwater fishes. The 213 surviving fish had significantly greater numbers of heterozygous loci per fish and were significantly larger than the 160 individuals that died. Survivors, on average, had higher heterozygosity at six out of nine loci than non-survivors. The differences were significant at three of these loci. These findings suggest that more heterozygous rainbow trout have superior disease resistance than less heterozygous fish.