EFFECTS OF INBREEDING ON GROWTH AND REPRODUCTIVE TRAITS IN RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

Rate of inbreeding and inbreeding effects on body weight and reproduct ive traits were studied in three lines of rainbow trout from a Califor nian selection experiment, in which Line Y was selected for body weigh t at 364 days of age, Line E was selected for egg size and Line C was a random mated control. The coefficient of inbreeding, as estimated bo th from effective population size with the assumption of random mating and from observed pedigree increased from Gen. 0 to Gen, 5 by 7.8% an d 7.5% in Line C, by 5.7% and 8.3% in Line E, and by 5.2% and 6.5% in Line Y, respectively. The effects of inbreeding on the various traits were measured by the regression of individual performance on the coeff icient of inbreeding calculated from pedigree. The regressions of body weight on coefficient of inbreeding were positive but not significant in the age interval from 168 to 252 days of age, and negative but not significant from 280 to 336 days of age. After that, the regressions were significantly negative. Thus, inbreeding depression for body weig ht per 10% increase in inbreeding was 2.26% (females + males) at 364 d ays, and 3.92% (females) and 5.77% (males) at spawning as a percent of the mean. There was a tendency for inbreeding depression for body wei ght to increase with advancing age, A highly significant inbreeding de pression was found for spawning age of females and egg number. Per 10% increase of the inbreeding coefficient, spawning age of females was d elayed by 0.53% and egg number decreased by 6.10% of the mean, respect ively. In contrast, inbreeding did not significantly affect spawning a ge of males or egg size, The three kinds of inbreeding originating fro m male, female and embryo had quite different effects on fertility-hat chability. Inbreeding of male had no deleterious influence on this tra it, In contrast, inbreeding of female had a strong and significantly n egative effect and inbreeding of embryo itself had a noticeable though not statistically significant negative effect on fertility-hatchabili ty. Per 10% increase of the inbreeding coefficient of female and embry o, fertility-hatchability declined by 11.60% and 8.56% of the mean, re spectively. The following rank order of traits with regard to the magn itude of inbreeding depression seems to be in agreement with the rank order of the expected correlations of traits with fitness: (1) fertili ty-hatchability of females; (2) hatchability of embryo; (3) egg number ; (4) body weight of males and females at spawning; (5) body weight (f emales + males) at 308-364 days of age; and (6) spawning age of female s, body weight (females + males) at 280 days of age, and egg size.