VARIATION AT ENZYME CODING LOCI AND CORRELATES OF FITNESS IN RAINBOW-TROUT - A COHORT ANALYSIS

An association between heterozygosity al six to nine enzyme coding loc i and correlates of fitness (survival, age at first maturation) was te sted in two cohorts of rainbow trout Oncorhynchus mykiss reared in the laboratory. A subsample of fish from the first cohort was reared toge ther for 3 years. There, fish sampled as embryos, juveniles and adults did not differ significantly in multilocus heterozygosity (MLH, numbe rs of heterozygous loci per fish). However, fish that died during a pe riod of high natural mortality had significantly greater MLH than thos e sampled before as hatched embryos and later as adults. The number of homozygotes and heterozygotes at three loci differed significantly am ong different aged fish but in different directions. Allele counts at two of these loci also differed among samples. In a second cohort, the heterozygosity of fish that died during the period of high natural mo rtality was compared with that of healthy fish collected at the same t ime; no significant differences in MLH or allele frequencies were dete cted. Taken together, the lower MLH of the natural mortalities as well as single locus effects observed in the first cohort might be explain ed by the differential survival of fish with particular genotypes lead ing to changes in heterozygosity of the entire cohort over time rather than heterozygosity per se. Age of first maturation and heterozygosit y (MLH and single locus) were not associated in males but were margina lly so in females. Time of ovulation within the spawning season was no t associated with the MLH of females. (C) 1996 The Fisheries Society o f the British Isles