Aquaculture of tench
361
reproduction is mostly carried out in bigger ponds where the fry spend the
first summer growing to l—summer fish
The number of spawning fish to be stocked depends on pond productivity and
how the pond is used. There must be enough offspring to fully utilize the
natural food resources, but, too many spawners produce surplus fry. The
fingerlings remain small and are difficult to fish in autumn. Twenty to
forty 3 to 4—year old females with a weight of 300 to 400 9, and at least
as
many males, per ha may be stocked into special spawning ponds (0.5—l.0 ha
of water area) (Schaeperclaus, l967). Koch (l960) recommends 20 to l00 pairs.
In normal
carp fingerling and fattening ponds, the number of stocked pairs
varies between l and 6 per ha. The spawning fish are chosen on the basis
of positive mass—selection and should be free of Ergasilus .
Fingerlings must be fished in the dark, and the water level must be lowered
very slowly when draining the pond. The draining channels must be clean
at all times. Fingerlings can be collected in traps behind the outlet, if
there is a sufficient water supply in the discharge water channel. One—
summer
tench may be wintered separately or together with carp in special
ponds. It may be better to wait until spring to empty the pond in which
the fish grow, provided that it is deep enough and receives a small,
constant water influx.
6. Propagation in modern hatcheries
_
6.l
Induced breeding
Tench adapt easily to carp breeding techniques and can be bred in the same
technical facilities. Acetone—dried pituitary gland extract of common carp
can
be used for the hormonal treatment of tench. The traditional dose is
9 to l0 mg/fish but two intraperitoneal injections (2 and lO—l6 mg,
respectively), given 8 h apart, produce better results. There is no
difference made between the males and females.
The optimal water temperature for spawner ripening is 24—25°C. Ovulation
begins l2 to l4 h after the second hormonal treatment and continues for
a
couple of hours.
Formerly it was usual to strip the brood fish several times {every hour),
and to treat the eggs with carbamide and salt for fertilization
and With
tannin to prevent stickiness before incubation. This method gives good
results and about 90% of the eggs are fertilized. However,
it is time—
consuming and the brood fish are easily injured by handling.
As a result of research in recent years, stripping and artifical
insemination have proven to be superfluous because tench spawn voluntarily
in tanks if not disturbed. As a rule, spawning lasts
4 to 6 hours and the
discharged eggs stick to the wall of the tank. Since they do not
sticky, they are knocked off after a while by the movement
of the spawning
fish and sink to the bottom where they are slowly carried toward the tank
outlet by the water current. If'the outlet is surrounded by a narrow—meshed
9Vld, the eggs gradually accumulate there and can be removed eas1ly by a
narrow—meshed net or a rubber hose. The fertilization rate in spawning tanks
is 70—80%.
361
reproduction is mostly carried out in bigger ponds where the fry spend the
first summer growing to l—summer fish
The number of spawning fish to be stocked depends on pond productivity and
how the pond is used. There must be enough offspring to fully utilize the
natural food resources, but, too many spawners produce surplus fry. The
fingerlings remain small and are difficult to fish in autumn. Twenty to
forty 3 to 4—year old females with a weight of 300 to 400 9, and at least
as
many males, per ha may be stocked into special spawning ponds (0.5—l.0 ha
of water area) (Schaeperclaus, l967). Koch (l960) recommends 20 to l00 pairs.
In normal
carp fingerling and fattening ponds, the number of stocked pairs
varies between l and 6 per ha. The spawning fish are chosen on the basis
of positive mass—selection and should be free of Ergasilus .
Fingerlings must be fished in the dark, and the water level must be lowered
very slowly when draining the pond. The draining channels must be clean
at all times. Fingerlings can be collected in traps behind the outlet, if
there is a sufficient water supply in the discharge water channel. One—
summer
tench may be wintered separately or together with carp in special
ponds. It may be better to wait until spring to empty the pond in which
the fish grow, provided that it is deep enough and receives a small,
constant water influx.
6. Propagation in modern hatcheries
_
6.l
Induced breeding
Tench adapt easily to carp breeding techniques and can be bred in the same
technical facilities. Acetone—dried pituitary gland extract of common carp
can
be used for the hormonal treatment of tench. The traditional dose is
9 to l0 mg/fish but two intraperitoneal injections (2 and lO—l6 mg,
respectively), given 8 h apart, produce better results. There is no
difference made between the males and females.
The optimal water temperature for spawner ripening is 24—25°C. Ovulation
begins l2 to l4 h after the second hormonal treatment and continues for
a
couple of hours.
Formerly it was usual to strip the brood fish several times {every hour),
and to treat the eggs with carbamide and salt for fertilization
and With
tannin to prevent stickiness before incubation. This method gives good
results and about 90% of the eggs are fertilized. However,
it is time—
consuming and the brood fish are easily injured by handling.
As a result of research in recent years, stripping and artifical
insemination have proven to be superfluous because tench spawn voluntarily
in tanks if not disturbed. As a rule, spawning lasts
4 to 6 hours and the
discharged eggs stick to the wall of the tank. Since they do not
sticky, they are knocked off after a while by the movement
of the spawning
fish and sink to the bottom where they are slowly carried toward the tank
outlet by the water current. If'the outlet is surrounded by a narrow—meshed
9Vld, the eggs gradually accumulate there and can be removed eas1ly by a
narrow—meshed net or a rubber hose. The fertilization rate in spawning tanks
is 70—80%.