Abstract:

Arguably the most striking characteristic of fish is their seasonality, with the timing of developmental and maturational events dominated by and, in parallel, synchronised with seasonal changes in climate, daylength and food supplies. This coordination and the associated internal processes of control ensure that young fish are produced when environmental conditions are most suitable for their survival. Thus, the first-feeding fry of mid and higher latitude species are produced when temperatures and daylengths are increasing in the spring, whereas the corresponding stages of tropical and sub-tropical fish appear to be timed to the increased productivity which follows seasonal rainfall or movements in oceanic currents. Since gonadal development and subsequent gamete and embryo formation take a long time to complete, it is clear that gonadal recrudescence must have been initiated many months earlier to ensure that the production of first-feeding fry is coincident with seasonal improvements in climate and nutrient availability. Fish, along with most other seasonally breeding animals, rely on cues from the external environment to achieve synchronisation of maturational events with changing season.