Physiology of Finfish and Shellfish (2+1)

3.1.22. Seasonal regulation of reproduction/ environmental influence on the reproductive cycle

The general, cues that trigger gamete growth and development appear to be environmental in nature. These environmental cues are often linked to the annual cycles of daylength and temperature variations. For eg, Cyprinus carpio and many other cyprinids, gonadal maturation process is thought to be triggered by a combination of increasing daylength and the commencement of the rise in water temperature. The spawning season comes to an end during the summer. It is thought that the cessation of spawning activity may in part be due to inhibitory effects of high summer water temperatures on gonadal differentiation and maturation. Spawning does not however, resume as temperature begins to fall with the approach of winter. This indicates that temperature is not the only environmental factor exerting a controlling influence over reproduction. Thus, it is thought that both photoperiod and temperature are major environmental cues responsible for the mediation of the reproductive cycle in several species of fish that inhibit temperate latitudes.

In the tropics, there may be only very minor seasonal changes in photoperiod and water temperature. Consequently, these 2 environmental factors are unlikely to act as major cues influencing the reproductive cycle, the timing and duration of the spawning season. In some tropical species, there does not appear to be any distinct spawning season and members of the population can be found to be engaged in reproductive activities at almost any time of the year. In other species, spawning activities do seem to vary on a seasonal basis with factors such as variations in rainfall possibly acting as the environmental cues triggering changes in gonadal development and maturation.