Photoperiodism in Fish

The control of reproduction involves a complex network of factors in the brain-pituitary-gonads axis (BPG) of fish. The hormones and corresponding receptors involved, display daily and annual variations allowing reproduction events to occur at the right time. The two main external synchronizers in the system are photoperiod and temperature. They act via the retina and the pineal gland, which send neuronal photoperiodic information to the brain centers controlling reproduction. The pineal also provides daily and seasonal hormonal information via melatonin, which acts at all levels of the network. Melatonin is one main coordinator of the BPG rhythmic activity.