Juvenile lake trout (Salvelinus namaycush) were held at different stocking densities (48 and 96 kg m⁻³) in rearing-units of different design (rectangular plug-flow, circular and cylindrical cross-flow) and the effects on behavior, growth and metabolism were examined. Ambient light levels and current velocities were measured in each of three tank sectors (upstream, middle and downstream) to determine their relative contributions to fish behavior and distribution. Rearing-unit design affected orientation to current, contact with surfaces, agonistic responses, turn rates and avoidance of or contact with other fish. Stocking density primarily affected contact with tank surfaces, avoidance and contact with other fish. Fish in circular tanks distributed themselves uniformly, whereas those in plug-flow tanks crowded upstream and those in cross-flow tanks congregated downstream. Stocking density affected fish distribution most in cross-flow and least in circular tanks. Ambient light, current velocity and total in situ aggression modified fish distribution at low stocking density, but at high density the effects of current velocity and total aggression were undetectable. Growth and food conversion were best in plug-flow, followed by cross-flow and then circular tanks. Net ammonia excretion and oxygen consumption were highest in circular tanks and lowest in cross-flow (NH₃) or cross-flow and plug-flow (O₂) tanks. Ammonia production was correlated with stocking density, but oxygen consumption was unrelated to density. Results demonstrate the interrelatedness of abiotic and biotic factors in fish culture and the ability to control fish behavior, growth and metabolism through the alteration of several physical and biological variables in the rearing unit.