44
J. E. SHELBOURNE
In 1961, a bigger egg stock than in former years was collected by
plankton net at sea. Their condition was good at the time of transfer
to laboratory incubators, and a 99% hatch probably reflected improvements in collecting technique and hatchery conditions. With the aid
of the newly -installed reservoir cooling system, incubation temperatures
were more closely controlled at what is now thought to be optimal
(6-7OC) for the tank environment. The production of metamorphosed
plaice was more than double that of the previous year-2807 fhh
from 84 000 mixed eggs, giving a 3.3% survival and a final density of
336 young fish per m2. The trend between 1967 and 1961, in closed
circulation a t Lowestoft, was towards increased production of young
plaice, by the simple expedient of increasing egg stocks from year to
year. Mean survival rates remained low, however, at a figure seldom
exceeding 6%.
No special efforts were made at Lowestoft between 1957 and 1961
to count dead stock or to plot the course of the survival curve, from
the early egg stage up to the time when the emergent larvae started
feeding (early stage 2). Thereafter, the numbers of dead feeding larvae
removed from the circuit during cleaning operations were recorded,
usually on alternate days. As the plaice-rearing project progressed,
it became apparent that, when reasonable, though not strictly controlled, conditions prevailed, slight continuous egg loss could be
expected during the first 2 or 3 weeks of incubation, followed by a
significant increase at hatching. The greatest mortalities occurred in
the next phase (stage 1) corresponding to the " critical period " of
Fabre-Domergue and Bibtrix (1897). Mortalities then fell to a low rate
until the onset of metamorphosis, when a second protracted period
of larval loss could be expected. In the lightly-populated tank conditions at Lowestoft 1967-60, the death rate after metamorphosis
was generally low.
c. Plaice-rearing in open circuhtion at Port Erin, 18k3 of Man
On basic principles one might expect water conditions in a closed
circulation to have a limiting effect on larval survival and production,
if metabolites accumulate above the threshold tolerance of the
developing stock. The photosynthesizing Enter0morph.a fronds in the
1961 circuit at Lowestoft were unable to cope with a greatly increased
rate of metabolite produotion in the late stages of the experiment
(Riley and Thacker, 1963); this coincided with an unexpectedly high
poet-metamorphosis mortality.
Already in 1960 it WJJS evident that an accessible upp ply of eggs and
a continuous flow of good quality sea water were necessary for further
J. E. SHELBOURNE
In 1961, a bigger egg stock than in former years was collected by
plankton net at sea. Their condition was good at the time of transfer
to laboratory incubators, and a 99% hatch probably reflected improvements in collecting technique and hatchery conditions. With the aid
of the newly -installed reservoir cooling system, incubation temperatures
were more closely controlled at what is now thought to be optimal
(6-7OC) for the tank environment. The production of metamorphosed
plaice was more than double that of the previous year-2807 fhh
from 84 000 mixed eggs, giving a 3.3% survival and a final density of
336 young fish per m2. The trend between 1967 and 1961, in closed
circulation a t Lowestoft, was towards increased production of young
plaice, by the simple expedient of increasing egg stocks from year to
year. Mean survival rates remained low, however, at a figure seldom
exceeding 6%.
No special efforts were made at Lowestoft between 1957 and 1961
to count dead stock or to plot the course of the survival curve, from
the early egg stage up to the time when the emergent larvae started
feeding (early stage 2). Thereafter, the numbers of dead feeding larvae
removed from the circuit during cleaning operations were recorded,
usually on alternate days. As the plaice-rearing project progressed,
it became apparent that, when reasonable, though not strictly controlled, conditions prevailed, slight continuous egg loss could be
expected during the first 2 or 3 weeks of incubation, followed by a
significant increase at hatching. The greatest mortalities occurred in
the next phase (stage 1) corresponding to the " critical period " of
Fabre-Domergue and Bibtrix (1897). Mortalities then fell to a low rate
until the onset of metamorphosis, when a second protracted period
of larval loss could be expected. In the lightly-populated tank conditions at Lowestoft 1967-60, the death rate after metamorphosis
was generally low.
c. Plaice-rearing in open circuhtion at Port Erin, 18k3 of Man
On basic principles one might expect water conditions in a closed
circulation to have a limiting effect on larval survival and production,
if metabolites accumulate above the threshold tolerance of the
developing stock. The photosynthesizing Enter0morph.a fronds in the
1961 circuit at Lowestoft were unable to cope with a greatly increased
rate of metabolite produotion in the late stages of the experiment
(Riley and Thacker, 1963); this coincided with an unexpectedly high
poet-metamorphosis mortality.
Already in 1960 it WJJS evident that an accessible upp ply of eggs and
a continuous flow of good quality sea water were necessary for further