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.