Environmental effects on feed utilization.

Both external and internal factors affect the response of fish to variations in dietary quantity and quality. An attempt is made to review major, recent studies on the series of intermediate steps (intake, digestion, metabolism, excretion and retention) involved in the global response of fish to environmental changes. Among these external factors, greater attention is however devoted to those that are the most important natural effectors within the aquatic environment: temperature, ambient oxygen and salinity. The changes brought about by a change in temperature at different levels of nutrient utilization have been studied to a great extent in the recent past. As temperature affects in the first instance, the voluntary food intake, a discussion on current nutrient requirement data should preferably be dealt with in absolute terms. While critical levels of oxygen below which growth is hindered are sufficiently defined for many species, precise data on the direct effects of oxygen deficiency on nutrient utilization are still fragmentory. With regard to salinity, a distinction between stenohaline and euryhaline species and a knowledge of the physiological mechanisms corresponding to their life cycles are required before attempting comparative analyses. Within euryhaline species, best performances are noted at salinities isotonic to the internal medium. Despite accumulating evidence on the effects of cyclical phenomena, the chronobiological approach to fish culture remains practically unexplored.