Effects of larger fish and size grading on growth and size variation in fingerling silver perch

This study presents two experiments addressing growth and size variation in fingerling silver perch (Bidyanus bidyanus. In the first experiment, fish close to mean population size were raised either in the presence or absence of five larger fish for 60 days. Mean specific growth rate (SGR) and incre ases in the coefficient of variation and skewness were lower in the presenc e of larger fish, indicating a negative effect of large fish on the growth of smaller ones. In the second experiment, fingerlings were graded into gro ups smaller and larger than the median size of the population and raised in size-sorted groups of 60 large or small fish and mixed groups of 30 fish o f each size category, for 60 days. There was no difference in mean SGR amon g groups, nor between the mixed group ad the weighted mean of the small and large groups. Biomass gain was higher in the mixed groups than in the weig hted small-and-large groups, probably due to a slightly lower survival in t he groups comprised of large fish. The fact that the effect of large finger lings on the growth of smaller ones was evident in the first, but not the s econd, experiment may be attributed to higher size disparity between large and small fingerlings in the first experiment.