THE ARTIFICIAL PROPAOATION OF MARINE FISH
39
visual feeders (Shelbourne, 1953). Their tolerance of light intensity
is thought to be quite wide, but sudden changes, such as occur during
tank inspections, are undesirable. Strong sunlight is thought to
decrease the survival of cod larvae in tanks (Dannevig, 1932 ; Dannevig
and Sivertsen, 1933).
Between 1967 and 1961, rearing tanks at Lowestoft were blacked
out except for a central slit in the top cover, through which dim light
was admitted. Later observations suggest that higher light intensities
of 400-600 lux at the water surface (as recorded with a photometer
corrected by filter to the wavelength response of the human eye),
assist early larvae to capture food, and that decreased light, later on,
may help survival by depressing activity in the restricted space of a
rearing tank. Illumination was continuous in 1967-68 ; in 1959-60
tanks were illuminated only during the day. Some doubt has recently
been cast on the suitability of light from a fluorescent source for
biological experiments (Perlmutter, 1962 ; Scott and Bennet-Clark,
1963). The quantitative effect of light of different wave-lengths on
the survival of plaice larvae is not known. Good results have been
achieved, however, using standard domestic fluorescent fittings.
The value of a black tank wall lies in the way it contrasts an illuminated food organism, making capture easier for early larval feeders.
Translucent polythene tanks do not give this contrast, and are unsuitable for plaice-rearing experiments.
Tank design. A pelagic plaice larva is adapted to an active life
in the open sea, out of contact with surfaces until metamorphosis.
Surface contact is an unavoidable tank hazard which may unduly
strain the adaptive resources of a larva, and it is therefore important
to keep a tank interior as simple as possible, with no unnecessary
inclusions. Two closely apposed surfaces can act as a lethal trap for
roaming larvae. Crevices are a particular menace-larvae swim into
them and seem unable to back out.
The very delicate skin of an early feeder is an important barrier to
the loss of water and the entry of chlorides under the osmotic gradient.
Disturbance of skin secretions by surface contact, or other structural
damage, will impair the efficiency of this barrier, and may present a
salt control problem beyond the capabilities of a larva. Tank outlet
screens have been a continual source of trouble and mortality shoe
this rearing project started ; Schach (1939) encountered similar problem
with herring larvae. Our tank design has given some memure of sucoem,
but could bear considerable improvement.
Aeration. Eggs were not directly aerated during incubation, nor
subjected to the agitation recommended by Fabre-Domergue and
39
visual feeders (Shelbourne, 1953). Their tolerance of light intensity
is thought to be quite wide, but sudden changes, such as occur during
tank inspections, are undesirable. Strong sunlight is thought to
decrease the survival of cod larvae in tanks (Dannevig, 1932 ; Dannevig
and Sivertsen, 1933).
Between 1967 and 1961, rearing tanks at Lowestoft were blacked
out except for a central slit in the top cover, through which dim light
was admitted. Later observations suggest that higher light intensities
of 400-600 lux at the water surface (as recorded with a photometer
corrected by filter to the wavelength response of the human eye),
assist early larvae to capture food, and that decreased light, later on,
may help survival by depressing activity in the restricted space of a
rearing tank. Illumination was continuous in 1967-68 ; in 1959-60
tanks were illuminated only during the day. Some doubt has recently
been cast on the suitability of light from a fluorescent source for
biological experiments (Perlmutter, 1962 ; Scott and Bennet-Clark,
1963). The quantitative effect of light of different wave-lengths on
the survival of plaice larvae is not known. Good results have been
achieved, however, using standard domestic fluorescent fittings.
The value of a black tank wall lies in the way it contrasts an illuminated food organism, making capture easier for early larval feeders.
Translucent polythene tanks do not give this contrast, and are unsuitable for plaice-rearing experiments.
Tank design. A pelagic plaice larva is adapted to an active life
in the open sea, out of contact with surfaces until metamorphosis.
Surface contact is an unavoidable tank hazard which may unduly
strain the adaptive resources of a larva, and it is therefore important
to keep a tank interior as simple as possible, with no unnecessary
inclusions. Two closely apposed surfaces can act as a lethal trap for
roaming larvae. Crevices are a particular menace-larvae swim into
them and seem unable to back out.
The very delicate skin of an early feeder is an important barrier to
the loss of water and the entry of chlorides under the osmotic gradient.
Disturbance of skin secretions by surface contact, or other structural
damage, will impair the efficiency of this barrier, and may present a
salt control problem beyond the capabilities of a larva. Tank outlet
screens have been a continual source of trouble and mortality shoe
this rearing project started ; Schach (1939) encountered similar problem
with herring larvae. Our tank design has given some memure of sucoem,
but could bear considerable improvement.
Aeration. Eggs were not directly aerated during incubation, nor
subjected to the agitation recommended by Fabre-Domergue and