Does Early Growth Cause a Phenotypically Plastic Response in Egg Production of Atlantic Salmon?

Abstract

1. To evaluate how differences in juvenile and subadult growth influence egg size and fecundity, we investigated wild and hatchery reared Atlantic Salmon smolts originating from the River Imsa population. The smolts were individually tagged and released into a Norwegian river. Specific growth rate of hatchery reared juveniles in fresh water averaged 1.38 times higher than that of wild conspecifics. Specific growth rate at sea of the subadults, on the other hand, was 1.14 times higher for wild than hatchery reared fish. 2. Mean egg weight increased with adult body weight. On average, eggs of wild fish were 1.12 times heavier than those of hatchery reared fish of corresponding sea ages. 3. Somatic fish weight (total fish weight less egg weight) explained 27% of the variance in egg weight in wild, 40% in hatchery reared salmon and 30% for the combined sample. Moreover, juvenile growth rate in fresh water as a second independent variable, significantly improved the explanatory power (r 2 = 0.43 for wild, 0.46 for hatchery salmon and 0.48 for the combined sample). Neither growth rate at sea nor length increment as juveniles in fresh water (smolt length) added significantly to the explanatory power. 4. Fecundity increased with somatic weight in both wild (r 2 = 0.81) and hatchery reared fish (r 2 = 0.74), with hatchery fish having about 30% more eggs for a given body size than wild females. By adding specific growth rate in fresh water as a second independent variable, 76% of the variance in fecundity of hatchery fish was explained. In wild fish, the addition of egg weight as the second independent variable increased the coefficient of determination of the regression model to 0.88. Use of length increment at sea and smolt length as independent variables did not improve the explanatory power. 5. Total ovary weight increased with somatic weight (r 2 = 0.83) and the pattern was similar for wild and hatchery reared fish. Neither growth rate in fresh water nor at sea increased the explanatory power of the regression model.