Photoperiod

Photoperiod (day length) is the primary environmental cue controlling seasonal timing of reproduction, migration, and developmental transitions in temperate fish. Increasing daylength in spring triggers gonadal development and smoltification in salmonids; decreasing daylength in autumn triggers spawning migration in many cold-water species. The pineal gland detects photoperiod and converts it to melatonin signals. In aquaculture, artificial photoperiod manipulation is used to suppress or advance spawning and prevent premature maturation (e.g., using continuous light to suppress maturation in farmed salmon).

Norwegian salmon farms use constant light during the summer growing season to prevent premature sexual maturation, which would divert energy from growth to gonad development and reduce flesh quality.