Fish behaviour and light

LIGHT AND BAHAVIOUR
Light, both direct and indirect, is of great importance in the lives of fishes. In the majority of fishes, the visual organs play a substantial role in orientation during swimming, towards prey, predators, other individuals of the same species in a school, or immobile objects.

The behaviour of fish, particularly their diurnal activity and many other aspects of their life, are significantlyrelated to the degree of illumination (Kikolsky 1963). Light exerts a definite influence on a fish's metabolism, maturation, behaviour and colouration.

LIGHT WAVELENGTHS AND FISH COLOURATION
Light conditions in water differ from those on land not only in their intensity but also in the depth of penetration of the various wavelengths. The longer wavelengths (i.e. red, orange) are absorbed first, with over 25% of red light being absorbed in the first metreof water. Violet, on the other hand, becomes indistinguishable below a depth of 100 m or more (Nikolsky 1963).

Since a high percentage of red light is filtered out in the first few metres of water, bright red fishes are common. Fishes that are solid red in colour aregenerally either nocturnal or live at moderate depths (Chech andMoyle 1982).

In both situations, red light is virtually absent and ared fish tends to fade into the background rather easily. However,many shallow water fish also have red spots or lines. It is thought that such colouration is important in recognition and/or breeding; red colours would be highly visible over short distances but is difficult to see over large lateral distances (water absorbs the red)(Chech and Moyle 1982).

The visual spectrum of fish depends on the nature of their habitat; fish which live in predominantly shallow waters are more sensitive to the longer wavelengths of light (red) while, as depth increases, the visual spectrum narrows considerably, starting with the longer wavelengths (Nikolsky 1963). The majority of fish can distinguish colours quite well although the maximum distance they can see is apparently not greater than 15 metres (Nikolsky 1963).

FISH ADAPTATION TO LIGHT
Fish have a number of adaptations to varying light levels. The rodsin the retina of the eye are specially adapted for sensing in weaklight. During periods of bright illumination (i.e. daylight), the rods become buried in between pigment cells lining the retina. In contrast, the cones which are adapted to sensing brighter light, move to the surface in response to higher illumination (Nikolsky 1963). The lower part of the retina, in the majority of fish, possesses more cones, and fewer rods, than the upper surface. This is due to the fact that the upper part of the eye receives more dispersed light than thelower half of the eye (Nikolsky 1963).

A rather obvious adaptation to low light levels in a relative increase in the size of eye. This affords a much greater area for light detection. A fish with large eyes is always a give away that it should be kept in a tank with either low light levels or caves and overhangs (i.e. Squirrel fish[Holocentridae] and Cardinal Fish [Apogonidae]).

LIGHT EFFECTS ON FISH BREEDING
Light also has profound effects upon the internal mechanisms of fish. For example, light exerts a great influence on the maturationof fish as well as their development and metabolism. For a number of fishes an increase in the amount of illumination produces asignificant acceleration in the development of the egg. This often takes the form of an increase in development rate in the light and a decrease in the rate in the dark (Nikolsky 1963).

In many fish species, the maturation of the gonads depends to a large extent on the duration of light (the so called photoperiod) and its intensity. In tropical seas, where the photoperiod is not as variable as in temperate areas, the role of light is not as important and many tropical species breed year round (Nikolsky 1963).

If fish do not receive the correct amount and intensity of light, they can be severely crippled and may not develop properly. In many fish species the normal course of metabolism is disturbed if they are reared in light conditions which are abnormal for them (Nikolsky 1963).

LIGHT EFFECTS ON PREDATION
The activity patterns of most fish are also related to light.Vision-orientated predators are most active during the day, oftenwith peaks of feeding in the early morning and evening when invertebrates become more available. Piscivorous fishes are alsomost active at dawn and dusk; attracted by the feeding activities ofthe smaller fish which form their prey. At night, the day-activefish become quiescent and the nocturnal fish become active (Chech andMoyle 1982).

In this article I have pointed out some of the basic reactions of fish to light; both its duration and intensity. It can be seen that light plays a very important role in the lives of fish and this must be taken into account when keeping them in enclosed systems.