Environmental factors like temperature, water condition, light, meteorological . conditions, etc. are important factors controlling the reproduction of fish.
There is an optimal temperature range for induced breeding of culturable fishes. Critical temperature limits exist, above and below which fish will not reproduce. However, certain teleosts can be made to ripen below the critical temperature by using goandotropins. Warm temperature plays .a primary role in stimulating the maturation of gonads in many fishes. Temperature has a direct effect on gonads regulating their ability to respond to pituitary stimulation and effects on primary synthesis and release of gonadotropins. Major carps breed within a range of temperature varying from 24-31°C. Some scientists did not find any correlation between water temperature and percentage of spawning success in induced fish breeding. If an effective dose of pituitary, HCG or ovaprim is given to fish, they spawn successfully even if there is a substantial increase or decrease in water temperature.
Light is another important factor controlling the reproduction in fishes. Enhanced photoperiodic regimes result in early maturation and spawning of fishes like Fundulus, Oryzias, etc. Some fishes like Salmo, Salvelinus etc., attain delayed maturation and spawning. Cirrhinus reba attains early maturation when subjected to artificial day lengths longer than natural day even at low temperature. The requirement of light for activation of the reproductive cycle vary from species to species and from place to place, as the day length and temperature differ depending on the latitude of the place concerned.
Water currents and rain
Rheotaxtic response to water current is well established in fishes. Rain becomes a pre-requisite to spawning of fishes, even when they are subjected to induced breeding. Fresh rain water and flooded condition are the primary factors in triggering the spawning of carps. The sudden drop in the level of the electrolytes in the environment caused by the heavy monsoon rains induces hydration in the fish and stimulates the gonads resulting in its natural spawning. Successful spawning of fishes has been induced on cloudy and rainy days, especially after heavy showers.
Gonadotropins have been found to increase during spawning and decrease afterwards. Due to the presence of females, there is an increase in gonadotropin level in males. FSH and LH have been reported to influence gonadal maturity in carps. Ihere are other factors that influence the spawning of fishes. Availability of nest building site stimulate fish to spawn. Factors called the repressive factors like accumulation of metabolic eliminates (Ammonia, faecal pellets, etc.) inhibit spawnin.