Rearing strategies alter patterns of size-selective mortality and heritable size variation in steelhead trout (Oncorhynchus mykiss)

Abstract

Steelhead trout (Oncorhynchus mykiss) reared under two different regimes: high food ration for 1 year (S1; typical strategy) or low ration for 2 years (S2) were subjected to a seawater challenge during the corresponding spring outmigration period. The S1 smolts were smaller and suffered greater seawater challenge mortality (23.9% compared with 0.7% for the S2 smolts) that was significantly and negatively related to body size. Heritability of body size was similar for the two treatments during the parr stage (fork length: S1 = 0.181, S2 = 0.245; mass: S1 = 0.372; S2 = 0.447), but higher for the S1 treatment during the smolt stage for length (S1 = 0.212, S2 = 0.002) and body mass (S1 = 0.145, S2 = 0.015). Strong family effects for both traits and significant family by environment interactions for parr mass and smolt length indicated significant phenotypic plasticity. A genetic response to size-selective mortality caused by insufficient growth opportunity in the S1 treatment is plausible and may affect fitness in the natural environment through effects on correlated traits.