Aquaculture of tench
363
on
a
few square meters of the pond surface. Since mown grass is an
appropriate substrate for the deposit of chironomid and tendipedid eggs,
a
few days later the pupae of these insects become the favourite food of
tench fry longer than l.5 cm. This procedure is of particular importance
if not enough natural food can be supplied by the inoculation of cladocera
(Moina). Parallel with a continuous natural food supply, high—value feeds
shou d also be given every day, e.g. starter feed for common carp fry.
Once or twice & week, nutritional and health status as well as fry growth
should be checked. After 2 to 3 weeks, the fry may stop growing and turn
a
dark colour. A veterinary inspection is then necessary, as tench fry are
susceptible to ectoparasitic infestation of Trichodina sp. In case of a
high incidence, it is advisable to treat the pond With copper—oxychloride,
calculating 4 g of the chemical per l m3 of water for a 24—hour treatment.
To prevent infestation during the 4th week of nursing, it is suggested to
ensure
a
continuous exchange of pond water by through—flow and to fit the
inlet device with a fine—meshed mosquito net. In a pond of 200 2, a through—
flow rate of 30 to 40 liters/min is sufficient.
During a nursing period of 5 to 6 weeks, tench fry reach a length of 3 to
4 cm and are then advanced. Tench is known to be a tough fish which stands
crowding and harvesting. However, this is not true of small tench. Prior
to harvesting, the biocenosis of the nursery pond has to be examined. If
Daphnia magna and water bugs like Notenecta,
Nepa have
pro Tfërated in the pond, it is recommended to treat the water with one
of the trichlorphon compounds at a l.0 to l.5—ppm concentration. This
treatment eradicates Daphnia, which otherwise disturbs harvesting
and
counting operations, and rëdUCes the number of water bugs con5iderably.
The bites and stings of these bugs are very detrimental to tench crowded
in the_net or transport tanks, and a water—bug invasion may even lead to
a
total fish-kill.
Harvesting the fry after nursing requires special care. It is recommended
to reduce the water level to half of the normal operational level and to
seine with a fine—meshed tulle net at least twice to harvest 60—70% of the
stock. The remaining fish can be gathered in a pool
in front
of the outlet
or
caught in a trap behind it when the pond is drained. It is very practical
to keep the captured fish in a holding net for a few hours. Thus, With ample
through—flow, mud is removed from the gills and the digestive tract is
emptied.
The nursery technology described promises a survival rate of 50—60% on the
average in large—scale production conditions (table 2).
Pond
Number of
Stocked
Harvested
Length
Survival
area
ponds
fry per
advanced fry
rate
m2
n
pond
n
mean
n
cm
%
200
l
100 000
90 300
2.5—3.0
90.3
200
3
80 000
52 300
65.4
100
2
50 000
27 500
2.0-3.0
55-0
200
2
50 000
l8 500
2.0—5.0
37.0
Table 2 Survival of tench fry stocke?
at various densities in experimental
nursing ponds of l00 — 200 m
363
on
a
few square meters of the pond surface. Since mown grass is an
appropriate substrate for the deposit of chironomid and tendipedid eggs,
a
few days later the pupae of these insects become the favourite food of
tench fry longer than l.5 cm. This procedure is of particular importance
if not enough natural food can be supplied by the inoculation of cladocera
(Moina). Parallel with a continuous natural food supply, high—value feeds
shou d also be given every day, e.g. starter feed for common carp fry.
Once or twice & week, nutritional and health status as well as fry growth
should be checked. After 2 to 3 weeks, the fry may stop growing and turn
a
dark colour. A veterinary inspection is then necessary, as tench fry are
susceptible to ectoparasitic infestation of Trichodina sp. In case of a
high incidence, it is advisable to treat the pond With copper—oxychloride,
calculating 4 g of the chemical per l m3 of water for a 24—hour treatment.
To prevent infestation during the 4th week of nursing, it is suggested to
ensure
a
continuous exchange of pond water by through—flow and to fit the
inlet device with a fine—meshed mosquito net. In a pond of 200 2, a through—
flow rate of 30 to 40 liters/min is sufficient.
During a nursing period of 5 to 6 weeks, tench fry reach a length of 3 to
4 cm and are then advanced. Tench is known to be a tough fish which stands
crowding and harvesting. However, this is not true of small tench. Prior
to harvesting, the biocenosis of the nursery pond has to be examined. If
Daphnia magna and water bugs like Notenecta,
Nepa have
pro Tfërated in the pond, it is recommended to treat the water with one
of the trichlorphon compounds at a l.0 to l.5—ppm concentration. This
treatment eradicates Daphnia, which otherwise disturbs harvesting
and
counting operations, and rëdUCes the number of water bugs con5iderably.
The bites and stings of these bugs are very detrimental to tench crowded
in the_net or transport tanks, and a water—bug invasion may even lead to
a
total fish-kill.
Harvesting the fry after nursing requires special care. It is recommended
to reduce the water level to half of the normal operational level and to
seine with a fine—meshed tulle net at least twice to harvest 60—70% of the
stock. The remaining fish can be gathered in a pool
in front
of the outlet
or
caught in a trap behind it when the pond is drained. It is very practical
to keep the captured fish in a holding net for a few hours. Thus, With ample
through—flow, mud is removed from the gills and the digestive tract is
emptied.
The nursery technology described promises a survival rate of 50—60% on the
average in large—scale production conditions (table 2).
Pond
Number of
Stocked
Harvested
Length
Survival
area
ponds
fry per
advanced fry
rate
m2
n
pond
n
mean
n
cm
%
200
l
100 000
90 300
2.5—3.0
90.3
200
3
80 000
52 300
65.4
100
2
50 000
27 500
2.0-3.0
55-0
200
2
50 000
l8 500
2.0—5.0
37.0
Table 2 Survival of tench fry stocke?
at various densities in experimental
nursing ponds of l00 — 200 m