THE ARTIFIUIAL PROPAGATION OF MARINE FISH
37
irrigation rate. Permissible flow in the circulations already described
was necessarily slow, for reasons of design. Up to and including 1959,
twenty-three meshes per om bolting silk was used to screen the outlet
pipe. Although retaining eggs and larvae, this mesh was not fine enough
to hold back small food organisms ; a fast flow quickly flushed away
daily rations. Accumulating debris on the screen caused backing up
of water in the tank, sometimes with ultimate overflowing. A finer
mesh becomes blocked more quickly at the same high rate of flow,
and any increase in screening area is not easily achieved without
contributing to surface contact hazards in rearing tanks.
For these reasons, the rate of water flow WBB kept to a minimum:
between 2 and 4 l./h through a small incubator and 40 l./h through
the larger polythene tctnks. I have found the practical irrigation rate,
under limitations of present tank design, to be about l/lOth of the
tank volume per hour.
Tank hygiene. Dead eggs, larvae and food debris were, as a general
rule, removed every day by pipette. Strong illumination during cleaning operations produces heightened larval activity, often accompanied
by a characteristic spasm in which the larval axis becomes momentarily contorted into an S-shape. Cleaning is best carried out by the
light of an electric torch ; larvae can avoid the beam if disturbed by it.
A common early larval abnormality in unhygienic surroundings
is the condition known as “water belly”, oedema of the abdomen.
Recent observations suggest a correlation between the occurrence of
‘* water belly ” and the presence of putrefying food in rearing tanks.
Although scavenging is not the general rule, some pelagic plaice larvae
do adopt this feeding habit. It is rewarding practice to remove decaying material as frequently as time permits, particularly in closed
circulations.
Coastal nursery grounds are usually found on sand. Tank bottoms
were therefore covered to a depth of 4 mm with washed sea sand during
the late pelagic stage of larval development, in an effort to simulate
natural conditions. This makes tank cleaning more difficult. Sand is
not essential to structural metamorphosis, and can be omitted in the
interests of tank hygiene.
Normally-developing plaice eggs
have been found in water of 10°C in the Dover Straits at the start
of the spawning season, and at P5’C farther north in March. They
may be assumed to tolerate temperatures within this range. Temperature toleration is one thing; optimum temperature for egg and
larval development in a particular set of circumstances is another.
Whereas 10°C may be perfectly suitable for egg development in natural
Temperature, pH and salinity.
37
irrigation rate. Permissible flow in the circulations already described
was necessarily slow, for reasons of design. Up to and including 1959,
twenty-three meshes per om bolting silk was used to screen the outlet
pipe. Although retaining eggs and larvae, this mesh was not fine enough
to hold back small food organisms ; a fast flow quickly flushed away
daily rations. Accumulating debris on the screen caused backing up
of water in the tank, sometimes with ultimate overflowing. A finer
mesh becomes blocked more quickly at the same high rate of flow,
and any increase in screening area is not easily achieved without
contributing to surface contact hazards in rearing tanks.
For these reasons, the rate of water flow WBB kept to a minimum:
between 2 and 4 l./h through a small incubator and 40 l./h through
the larger polythene tctnks. I have found the practical irrigation rate,
under limitations of present tank design, to be about l/lOth of the
tank volume per hour.
Tank hygiene. Dead eggs, larvae and food debris were, as a general
rule, removed every day by pipette. Strong illumination during cleaning operations produces heightened larval activity, often accompanied
by a characteristic spasm in which the larval axis becomes momentarily contorted into an S-shape. Cleaning is best carried out by the
light of an electric torch ; larvae can avoid the beam if disturbed by it.
A common early larval abnormality in unhygienic surroundings
is the condition known as “water belly”, oedema of the abdomen.
Recent observations suggest a correlation between the occurrence of
‘* water belly ” and the presence of putrefying food in rearing tanks.
Although scavenging is not the general rule, some pelagic plaice larvae
do adopt this feeding habit. It is rewarding practice to remove decaying material as frequently as time permits, particularly in closed
circulations.
Coastal nursery grounds are usually found on sand. Tank bottoms
were therefore covered to a depth of 4 mm with washed sea sand during
the late pelagic stage of larval development, in an effort to simulate
natural conditions. This makes tank cleaning more difficult. Sand is
not essential to structural metamorphosis, and can be omitted in the
interests of tank hygiene.
Normally-developing plaice eggs
have been found in water of 10°C in the Dover Straits at the start
of the spawning season, and at P5’C farther north in March. They
may be assumed to tolerate temperatures within this range. Temperature toleration is one thing; optimum temperature for egg and
larval development in a particular set of circumstances is another.
Whereas 10°C may be perfectly suitable for egg development in natural
Temperature, pH and salinity.