Egg size–related traits during the first year of growth and smolting in hatchery and wild juveniles of steelhead trout Oncorhynchus mykiss

Within-clutch variation in the size or the amount of yolk in the egg has been widely studied because of its importance for both maternal and offspring fitness. In particular, egg size has been used as a proxy for survival in first-feeding salmonids alevins. We tested the hypothesis that egg size has implications for the phenotypes (development, physiology, and behavior) of juvenile steelhead trout ( Oncorhynchus mykiss ) up to 1 year post fertilization in both hatchery and wild fish. Egg diameter was measured for 38 families; the smallest and largest eggs were selected and reared individually to study embryonic development, and then collectively as “small” and “large” egg groups. The development and growth of individual embryos were followed up to first feeding. We showed that hatchery fish have smaller eggs than wild fish, offspring from small eggs grow better during their first year than those from larger eggs, offspring from large eggs hatch later and at a more advanced stage than those from small eggs, and the degree of smolting and saltwater preference differed between hatchery and wild offspring. By looking at not only mean egg size but also individual mediated–egg size phenotypic traits, we show the significant contribution of egg size in promoting phenotypic variation within a population and its potential for among-population variation.