Abstract: Light compares a complex of external and ecological factors, including colour spectrum, intensity and photoperiod. Light characteristics are very specific in an aquatic environment and light is extremely variable in nature. `Receptivity' of fish to light profoundly changes according to the species and the developmental status. Specific photoreceptor cells are present in both eye and pineal. If it is easy to change the light in experimentation and to observe the effects on fish growth, it is much more difficult in nature to make such determinations.
In larvae, many studies have been dedicated to the influence of intensity and photoperiod on growth: generally, species need a minimal threshold intensity to be able to develop normally and grow. This is probably related to the aptitude أهلية to localize, catch and ingest prey. Light is also indispensable for body pigmentation, an important phenomenon involved in early development and growth. Too intense light can be stressful or even lethal.
A few species are able to develop and grow at very low intensities or, sometimes, in the absence of light. Generally, long daylength improves larval rearing quality. The synergistic effect of `food availability-daylength' appears to be determining at this stage.
In older fish, there is very little information about the influence of light `quality' but more about intensity and much more about photoperiod. Light intensity effects are not so clear and depend on the species and the experimental procedures: it is probably not an important factor for growth stimulation. Daylength appears much more important. Many species, including both marine species and salmonids, react to photoperiod treatments and long daylength stimulates growth.
The most studied species is the Atlantic salmon, which is very sensitive, both during the freshwater stage, with the parr–smolt transformation very dependent on the photoperiod, and also in sea water. In this last condition, lighting also influences early maturation. An important point is to be certain that light affects fish growth through a better food conversion efficiency and not just through stimulated food intake. Also included in this review is a discussion about the endolymph–otolith system , which is very sensitive to daylight and seasonal cycles and a review of the present knowledge on the involvement of light influence on hormone levels (melatonin, somatotropin, thyroid hormones and other hormones).