A physiological basis for correlations among type of metabolism, behavior, and lifestyle is described for two early ontogenetic stages of coregonine fishes. Stage I extends from first feeding attempts until the end of metamorphosis, which usually occurs at a total length of 2–3 cm. Stage II extends through the juvenile period to a size of 8–10 cm. During the change from stage 1 to stage II, activities of several glycolytic and muscle enzymes increase severalfold. This increase in enzyme activities indicates a switch in major energy substrates from fatty and amino acids to glycogen. Prey selection data (prey ranked by size and mode of swimming), predator–prey reactive distance, and prey-handling time per unit energy return (profitability time) are given for coregonines in stages I and II. Results from studies of coregonine species from many different locations are incorporated into a deterministic model to estimate relationships between physical and biological environmental factors and fish growth. The growth rate peaks at a specific swimming speed, the optimum foraging speed. Simulation experiments demonstrate that optimum foraging speed increases exponentially as a function offish length and depends on density and type of prey. After further analysis of the relationship between mortality and growth rate, the bioenergetic data presented may facilitate the assessment of year-to-year variation in recruitment to the adult stock.