Climate, photoperiod and temperature directly influence reproduction as does food availability which is an indirect effect. The effects of photoperiodism are more varied than those of temperature but long-term thermal variations also influence reproduction. Some parameters of the aquatic medium, such as pH, O sub(2), water salinity, social environment, are also important, affecting gametogenesis and fertility. Man represents the main environmental influence having a direct (domestication, manipulation) or indirect effect by profoundly changing the medium (pollution). The various environmental factors, interacting with endogenous parameters, depend on the physiology and health of the animal at a given time of the year or the day and on the degree of domestication.