Photoperiod

Photoperiod is the daily duration and timing of light an animal receives, and in aquaculture it is deliberately manipulated because day length, sensed through the retinal-pineal melatonin axis, is one of the two master environmental cues (with temperature) controlling reproduction in most temperate fish. Long-day species such as salmon and cod become reproductively ready under lengthening days, while short-day species such as European seabass respond to shortening days; by holding broodstock under artificial light regimes, hatcheries induce out-of-season maturation and spawning, decoupling seed production from the natural calendar and allowing year-round fry supply. In salmon farming, constant light during winter blocks the smoltification trigger, while a winter-shortening signal is later used to initiate parr-smolt transformation, giving farms precise control over seawater transfer timing. Photoperiod also influences feeding activity, growth, and feed efficiency in juveniles, and extended light is used commercially in cages and tanks to enhance appetite — though light management must respect species-specific needs, since inappropriate regimes can cause maturation in grow-out fish (grilse, early-maturing tilapia), diverting energy from growth to gonad development. Light intensity (lux), spectrum, and dawn-dusk ramping matter alongside duration, particularly for larvae.