The Effect of Light on the Behaviour 
and Well Being of Marine Fish:
Who Shut Off the Lights?
by J. Charles Delbeek M.Sc.

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 significantly
related to the degree of illumination (Kikolsky 1963). Light exerts
a definite influence on a fish's metabolism, maturation, behaviour
and 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 metre
of water. Violet, on the other hand, becomes indistinguishable below
a depth of 100 m or more (Nikolsky 1963). Light exerts a definite
influence on a fish's metabolism, maturation, behaviour and
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 metre
of 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 are
generally either nocturnal or live at moderate depths (Chech and
Moyle 1982). In both situations, red light is virtually absent and a
red 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 have a number of adaptations to varying light levels. The rods
in the retina of the eye are specially adapted for sensing in weak
light. 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 due to the fact
that the upper part of the eye receives more dispersed light than the
lower 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 giveaway 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 also has profound effects upon the internal mechanisms of
fish. For example, light exerts a great influence on the maturation
of fish as well as their development and metabolism. For a number of
fishes an increase in the amount of illumination produces a
significant 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 it's
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).

The activity patterns of most fish are also related to light.
Vision-orientated predators are most active during the day, often
with peaks of feeding in the early morning and evening when
invertebrates become more available. Piscivorous fishes are also
most active at dawn and dusk; attracted by the feeding activities of
the smaller fish which form their prey. At night, the day-active
fish become quiescent and the nocturnal fish become active (Chech and
Moyle 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.

References
1. Chech, J.J. Jr. and P.B. Moyle. 1982. Fishes: An Introduction
to Ichthyology. Prentice-Hall Inc., Englewood Cliffs, N.J.

2. Nikolsky, G.V. 1963. The Ecology of Fishes. Academic Press,
London.