Individual growth rate and age at sexual maturity in rainbow trout

Growth rate and possible connection between growth rate and age at sexual maturity were studied on individually tagged rainbow trout from different sib groups. Great variation in growth rate and age at first sexual maturity were found among different sib groups. Significant correlations were found between size (length) of the individual fish on group mean measured at different time during the sea phase. The mean lengths within groups of fish maturing during their third year of life were significantly higher than for fish still immature in the autumn preceding spawning as well as half a year before and even one year before spawning. Effect of sex on size of the fish was not clear, but in some groups the males were on average greater than the females. Fish maturing during their second year of life (mostly males) showed nearly the same mean size as immature fish before the spawning season. They grew considerably less during the spawning season (and showed somewhat increased mortality rate) but during next summer they matured again and showed greater relative growth rate than the other fish