38
J. E. SHELBOURNE
condition^, that mme temperature may stimulate bacterial growth
in the rich organic environment of a closed circulation to the point
where shell contamination becomes critical , or where the speed of
ohemical change under bacterial influence outstrips the adaptability
of an embryo or larva. Larval activity is also influenced by temperature; high activity in the confined oonditions of a small rearing
tank is likely to have a bearing on subsequent mortality.
The temperature regime of offshore waters is much more stable
than that in shore aquaria, unless efficient hatchery control systems
are used. Different temperature regimes were adopted during the
period 1967-61 at Lowestoft; in retrospect, a practical schedule is to
incubate eggs at 6-7OC, rising to 7-8OC during early larval feeding,
followed by a gradual increase to between 10" and 12OC a t metamorphosis. These proposals do not conform to the recommendations
of Molander and Molander-Swedmark (1967), who considered 6°C to
be an unsuitable incubation temperature for Baltic plaice eggs.
The use of Enteromorpha intestinalis to control pH has already been
described. It was chosen, in the first place, to avoid the changes in
salt balance occurring with direct chemical control. During the plaice
spawning season, the pH of the sea usually stands between 8.1 and 8-3.
In static rearing tanks the pH index may quickly drop below 8.0
under the influence of metabolic CO, from eggs, larvae and bacteria.
Whitley (1906) thought the pH tolerance of plaice eggs to be slight,
particularly in conditions of increasing acidity rather than alkalinity ;
CO, can have a marked effect on their rate of oxygen uptake (Burfield,
1928). Since Enteromorpha was first introduced into our rearing trials,
pH levels within the favourable range (8.2-7.7) quoted by Bishai
(1960b) have been easily maintained, even with manual operation.
It would be a comparatively simple matter to achieve automatic
stabilization by linking a pH meter control unit to a variable light
system.
North Sea plaice spawn in high salinity water (35°/,0). Their eggs
and larvae are presumably adapted to develop at this high salt concentration, so from the outset, offshore sea water has been used in
our closed circulations. Increases in salinity due to evaporation can
be troublesome; adjustments were made from time to time with
distilled or tap water, cawing slight fluctuations that are unlikely to
occur in the stable environment of the sea. It remains to be seen
whether or not salinity stabilization is desirable, If so, it could be
achieved by coupling a sensitive conductivity cell to a solenoid valve
controlling the input of dhtined water into the system.
Tank illumination is necessary, einm plaice lamm are
Light.
J. E. SHELBOURNE
condition^, that mme temperature may stimulate bacterial growth
in the rich organic environment of a closed circulation to the point
where shell contamination becomes critical , or where the speed of
ohemical change under bacterial influence outstrips the adaptability
of an embryo or larva. Larval activity is also influenced by temperature; high activity in the confined oonditions of a small rearing
tank is likely to have a bearing on subsequent mortality.
The temperature regime of offshore waters is much more stable
than that in shore aquaria, unless efficient hatchery control systems
are used. Different temperature regimes were adopted during the
period 1967-61 at Lowestoft; in retrospect, a practical schedule is to
incubate eggs at 6-7OC, rising to 7-8OC during early larval feeding,
followed by a gradual increase to between 10" and 12OC a t metamorphosis. These proposals do not conform to the recommendations
of Molander and Molander-Swedmark (1967), who considered 6°C to
be an unsuitable incubation temperature for Baltic plaice eggs.
The use of Enteromorpha intestinalis to control pH has already been
described. It was chosen, in the first place, to avoid the changes in
salt balance occurring with direct chemical control. During the plaice
spawning season, the pH of the sea usually stands between 8.1 and 8-3.
In static rearing tanks the pH index may quickly drop below 8.0
under the influence of metabolic CO, from eggs, larvae and bacteria.
Whitley (1906) thought the pH tolerance of plaice eggs to be slight,
particularly in conditions of increasing acidity rather than alkalinity ;
CO, can have a marked effect on their rate of oxygen uptake (Burfield,
1928). Since Enteromorpha was first introduced into our rearing trials,
pH levels within the favourable range (8.2-7.7) quoted by Bishai
(1960b) have been easily maintained, even with manual operation.
It would be a comparatively simple matter to achieve automatic
stabilization by linking a pH meter control unit to a variable light
system.
North Sea plaice spawn in high salinity water (35°/,0). Their eggs
and larvae are presumably adapted to develop at this high salt concentration, so from the outset, offshore sea water has been used in
our closed circulations. Increases in salinity due to evaporation can
be troublesome; adjustments were made from time to time with
distilled or tap water, cawing slight fluctuations that are unlikely to
occur in the stable environment of the sea. It remains to be seen
whether or not salinity stabilization is desirable, If so, it could be
achieved by coupling a sensitive conductivity cell to a solenoid valve
controlling the input of dhtined water into the system.
Tank illumination is necessary, einm plaice lamm are
Light.