THE ARTIFICIAL PROPAGATION OF MARINE FISH
66
first-feeding ”, was maintained during the later stages of larval
development. About 66% of feeding larvae survived through metamorphosis, compared with 510/, in 1960, even though the survivor
density was trebled in 1961. A single environmental factor is unlikely
to account for this later improvement. Mean temperatures were lower,
and nearer the optimum, in the 1961 water-bath system ; tank deeign
had been improved by modifying the tank outlets to remove a cause
of larval mortality in earlier years ; tank hygiene was also better.
The effect of technical improvements can only be crit.ically assessed
by conducting rearing trials on egg stocks with similar viabilities. We
know little about fluctuations in the viability of pond-spawned eggs
from one year to another. The 1961 egg stock a t Port Erin was abnormal
in One respect ; shells were soft and imperfectly formed, giving toughness readings of 200 g or less, compared with higher values in the sea,
or from the Port Erin spawning pond in 1960. Despite this unpromising
start, survivals were the highest so far, in both treated and control
tanks.
Following the encouraging results in control tanks at Port Erin
during the 1961 season, the closed circulation work at Lowestoft was
abandoned, and the total research effort for 1962 concentrated at the
Port Erin hatchery. Four principal studies were undertaken :
1. The effect of bacterial control on the survival of plaice eggs
2. The optimum temperature regime for tank-reared stock.
3. Trials with layered plaice-rearing tanks.
4. The optimuw feeding rates for tank-reared stock.
A t the time of writing this paper, the analysis of experimental
results is still incomplete, but sufficient information is available to
warrant a preliminary account of the main findings.
3. The effect of bacterial control during egg incubation: 1962
The eggs of the plaice often become opaque and adhere one to
another during developmont in hatchery tanks. When the symptoms
are evere, heavy mortalities can be expected before hatching is
complete. In contrast, the shells of late plaice eggs caught at sea
Oppenheimer (1966)
demonstrated bacterial growth on cod egg shells by means of phaae
Photomicrographs, and found that the percenttlge hatch of cod, turbot
and Pacific sardine eggs in tanks could be increaaed by controlling
marine bacteria with a mixture of penicillin and streptomycin. Wahe
(l968) improved the spatfall in his oyster-rearing tanks by suppressing
and larvae.
show signs of opacity and adhesion.
‘.Y.B.-p
0
66
first-feeding ”, was maintained during the later stages of larval
development. About 66% of feeding larvae survived through metamorphosis, compared with 510/, in 1960, even though the survivor
density was trebled in 1961. A single environmental factor is unlikely
to account for this later improvement. Mean temperatures were lower,
and nearer the optimum, in the 1961 water-bath system ; tank deeign
had been improved by modifying the tank outlets to remove a cause
of larval mortality in earlier years ; tank hygiene was also better.
The effect of technical improvements can only be crit.ically assessed
by conducting rearing trials on egg stocks with similar viabilities. We
know little about fluctuations in the viability of pond-spawned eggs
from one year to another. The 1961 egg stock a t Port Erin was abnormal
in One respect ; shells were soft and imperfectly formed, giving toughness readings of 200 g or less, compared with higher values in the sea,
or from the Port Erin spawning pond in 1960. Despite this unpromising
start, survivals were the highest so far, in both treated and control
tanks.
Following the encouraging results in control tanks at Port Erin
during the 1961 season, the closed circulation work at Lowestoft was
abandoned, and the total research effort for 1962 concentrated at the
Port Erin hatchery. Four principal studies were undertaken :
1. The effect of bacterial control on the survival of plaice eggs
2. The optimum temperature regime for tank-reared stock.
3. Trials with layered plaice-rearing tanks.
4. The optimuw feeding rates for tank-reared stock.
A t the time of writing this paper, the analysis of experimental
results is still incomplete, but sufficient information is available to
warrant a preliminary account of the main findings.
3. The effect of bacterial control during egg incubation: 1962
The eggs of the plaice often become opaque and adhere one to
another during developmont in hatchery tanks. When the symptoms
are evere, heavy mortalities can be expected before hatching is
complete. In contrast, the shells of late plaice eggs caught at sea
Oppenheimer (1966)
demonstrated bacterial growth on cod egg shells by means of phaae
Photomicrographs, and found that the percenttlge hatch of cod, turbot
and Pacific sardine eggs in tanks could be increaaed by controlling
marine bacteria with a mixture of penicillin and streptomycin. Wahe
(l968) improved the spatfall in his oyster-rearing tanks by suppressing
and larvae.
show signs of opacity and adhesion.
‘.Y.B.-p
0