Diel and seasonal changes in the environment directly or indirectly control many biological functions. Chronobiology, the study of biological functions in relation to time of day, time of year, lunar phases, and other consistent cues, could have important applications in aquaculture. Physiological conditions that vary seasonally include feeding time, tolerance of fish to stress, the state of gonad development, lipid and water content of muscle tissue, and the dress-out weight of food fish. The quality of cultured fish may vary with the time of day when fish are fed, seined, graded, treated with therapeutic chemicals, injected with hormones, or physically disturbed. Efficiency of production and quality of aquacultural products might be improved if the activities of fish husbandry were timed to coincide with the biological rhythms of fish.