36
J. E. SHELBOURNE
Bight between January and April. It became normal practice to collect
offshore sea water during late January, allowing sufficient time for
thorough filtration and temperature adjustment before egg collection
started in early February.
A short sea survey with plankton net was usually enough to locate
an abundant egg population. Collections were made with a 2-m
Stramin net towed just below the surface at a speed not exceeding
1.6 knots. Expected catches can be of the order 8-10 000 plaice eggs/h
in various stages of development. Cod and dab eggs were also caught.
Catches were quickly but gently emptied into a large volume of Be8
water in a glass tank, and planktonic debris allowed to settle. Floating
eggs were then skimmed from the surface with a polythene coffee
strainer and transferred to large concrete-asbestos deck tanka twothirds filled with sea water, at a population density of 10000 eggs
per tank. As far as possible, egg oollections were timed to coincide
with short and infrequent spells of calm weather. Back in harbour,
eggs were once again skimmed and transferred to smaller glass containers for transport to laboratory incubators. By keeping an eye
on weather reports, and by gentle handling and good timing, it was
possible to complete collections with negligible egg mortality.
Egg etocking rate. Arbitrary stocking rates of 300-600 sorted midstage eggs (gastrulae) per glass incubator were used in 1967-69, and
4-6000 mixed eggs-in all stages of development-per large polythene
incubator in 1960. This was increased to roughly 10 000 mixed eggs
per incubator in 1961, without depressing survival rates. Gastrulae
were chosen aa experimental material in the early trials, following
Ibollefsen (1932) and Battle (1944), who found teleostean embryos to
be less vulnerable to mechanical damage after the completion of
epiboly.
Egg condition during incubation. The shells of plaice eggs in the sea
remain perfectly clear throughout the incubation phase. In contrast,
tank eggs usually show increasing shell cloudiness during development,
due to the growth of sessile bacteria and protozoan commensals.
Opacity varied from year to year in our Lowesteft experiments, which
lacked strict bacteriological control. In experimental aquaria, partioulmly with closed circulations, the bacterial hazard may be greatly
magnified; eggs quickly become contaminated and adhere to one
another in clumps. Methods of bacterial control in actual rearing
tanks, where direct irradiation with ultra-violet light can be dangerous,
are discussed in a later section.
Ratcs of waterjlow. The speed with which metabolites are removed
from, and oxygen transported to, a tank will depend largely on. the
J. E. SHELBOURNE
Bight between January and April. It became normal practice to collect
offshore sea water during late January, allowing sufficient time for
thorough filtration and temperature adjustment before egg collection
started in early February.
A short sea survey with plankton net was usually enough to locate
an abundant egg population. Collections were made with a 2-m
Stramin net towed just below the surface at a speed not exceeding
1.6 knots. Expected catches can be of the order 8-10 000 plaice eggs/h
in various stages of development. Cod and dab eggs were also caught.
Catches were quickly but gently emptied into a large volume of Be8
water in a glass tank, and planktonic debris allowed to settle. Floating
eggs were then skimmed from the surface with a polythene coffee
strainer and transferred to large concrete-asbestos deck tanka twothirds filled with sea water, at a population density of 10000 eggs
per tank. As far as possible, egg oollections were timed to coincide
with short and infrequent spells of calm weather. Back in harbour,
eggs were once again skimmed and transferred to smaller glass containers for transport to laboratory incubators. By keeping an eye
on weather reports, and by gentle handling and good timing, it was
possible to complete collections with negligible egg mortality.
Egg etocking rate. Arbitrary stocking rates of 300-600 sorted midstage eggs (gastrulae) per glass incubator were used in 1967-69, and
4-6000 mixed eggs-in all stages of development-per large polythene
incubator in 1960. This was increased to roughly 10 000 mixed eggs
per incubator in 1961, without depressing survival rates. Gastrulae
were chosen aa experimental material in the early trials, following
Ibollefsen (1932) and Battle (1944), who found teleostean embryos to
be less vulnerable to mechanical damage after the completion of
epiboly.
Egg condition during incubation. The shells of plaice eggs in the sea
remain perfectly clear throughout the incubation phase. In contrast,
tank eggs usually show increasing shell cloudiness during development,
due to the growth of sessile bacteria and protozoan commensals.
Opacity varied from year to year in our Lowesteft experiments, which
lacked strict bacteriological control. In experimental aquaria, partioulmly with closed circulations, the bacterial hazard may be greatly
magnified; eggs quickly become contaminated and adhere to one
another in clumps. Methods of bacterial control in actual rearing
tanks, where direct irradiation with ultra-violet light can be dangerous,
are discussed in a later section.
Ratcs of waterjlow. The speed with which metabolites are removed
from, and oxygen transported to, a tank will depend largely on. the