THE ARTIFIUIAL PROPAQATION OF MARINE FISH
27
Stage 3.
Stage 4.
Stage 6.
Eyes still symmetrically placed. Notochord bent.
Eye migrating but not yet at edge of head.
Eye on or over the edge of the head.
These five stages are shown in Fig. 4.
B . Plaice-rearing in closed circulation at Lowestoft
1. Experimental system, 1961-61
Between 1961 and 1966 annual attempts to rear plaice beyond
stage 1 were usually abortive. Fertilized eggs, caught at sea in plankton nets during the spring spawning season in the southern North Sea,
were incubated in small, well-lighted glass jars (capacity 2-3 1. sea
water) standing in temperature-controlled water baths, at a stocking
rate of 100 gastrulae per jar. Incubators were usually aerated, also
intermittently refreshed from stocks of offshore water stored in glass
or polythene carboys. Even if egg mortalities were not unduly high
in these static conditions, lmval survivors seldom developed beyond
the early feeding stage, the food offered being the nauplii of Artemia
salina. Structural abnormalities frequently occurred, particularly a
condition involving oedema of the yolk-sac and distortion of the axial
musculature (Shelbourne, 19668). Premature hatching was also
common, embryos emerging from the eggs before their precociouslydeveloped eyes were completely pigmented. A disturbing fall in the
pH index of incubator sea water often preceded egg and larval mortalities in our early experiments, probably due to the excretion of
acid metabolites by livestock and bacteria.
By 1967, a closed circulation had been designed, giving a limited
degree of physico-chemical control. It incorporated a basic principle of
tropical aquarium technique-the use of photosynthesizing plants to
remove COa and other metabolites from the water-thus stabilizing
the pH and adding oxygen at the same time. pH can be regulated by
chemical means-the addition of lime, for instance (Cooper, 1932), or
sodium bicarbonate (Breder and Smith, 1932)-but
these methods
involve changes in the balance of salts. CO, ie also removed by vigorous
aeration (Downing, 1968) ; oxygen supersaturation, however, can be
detrimental to the survival of fish larvae (Henly, 1952; Bishai, 1960a).
Nitrogenous end-products of metabolism cannot at present be removed
from sea water by physico-chemical means, without affecting the survival of fish in circuit. It seemed that photosynthesizing green algae
gave the best chance of controlling metabolites, in a purely natural
manner, without jeopardizing the vitality of developing plaice stooke.
The design of the 1967 closed circulation is given in Fig. 5. Two
27
Stage 3.
Stage 4.
Stage 6.
Eyes still symmetrically placed. Notochord bent.
Eye migrating but not yet at edge of head.
Eye on or over the edge of the head.
These five stages are shown in Fig. 4.
B . Plaice-rearing in closed circulation at Lowestoft
1. Experimental system, 1961-61
Between 1961 and 1966 annual attempts to rear plaice beyond
stage 1 were usually abortive. Fertilized eggs, caught at sea in plankton nets during the spring spawning season in the southern North Sea,
were incubated in small, well-lighted glass jars (capacity 2-3 1. sea
water) standing in temperature-controlled water baths, at a stocking
rate of 100 gastrulae per jar. Incubators were usually aerated, also
intermittently refreshed from stocks of offshore water stored in glass
or polythene carboys. Even if egg mortalities were not unduly high
in these static conditions, lmval survivors seldom developed beyond
the early feeding stage, the food offered being the nauplii of Artemia
salina. Structural abnormalities frequently occurred, particularly a
condition involving oedema of the yolk-sac and distortion of the axial
musculature (Shelbourne, 19668). Premature hatching was also
common, embryos emerging from the eggs before their precociouslydeveloped eyes were completely pigmented. A disturbing fall in the
pH index of incubator sea water often preceded egg and larval mortalities in our early experiments, probably due to the excretion of
acid metabolites by livestock and bacteria.
By 1967, a closed circulation had been designed, giving a limited
degree of physico-chemical control. It incorporated a basic principle of
tropical aquarium technique-the use of photosynthesizing plants to
remove COa and other metabolites from the water-thus stabilizing
the pH and adding oxygen at the same time. pH can be regulated by
chemical means-the addition of lime, for instance (Cooper, 1932), or
sodium bicarbonate (Breder and Smith, 1932)-but
these methods
involve changes in the balance of salts. CO, ie also removed by vigorous
aeration (Downing, 1968) ; oxygen supersaturation, however, can be
detrimental to the survival of fish larvae (Henly, 1952; Bishai, 1960a).
Nitrogenous end-products of metabolism cannot at present be removed
from sea water by physico-chemical means, without affecting the survival of fish in circuit. It seemed that photosynthesizing green algae
gave the best chance of controlling metabolites, in a purely natural
manner, without jeopardizing the vitality of developing plaice stooke.
The design of the 1967 closed circulation is given in Fig. 5. Two