THE ARTIFIULU PROPAGATION OB MARINE FISH
29
60 om x 30 cm x 30 cm moulded glaas incubators (capacity 40 1.) were
partially immersed in a wooden freshwater bath cooled by a coil linked
to a modified domestic refrigerator unit. Sea water ran into each
incubator at a slow controlled rate from a glass header tank, through
a 6-mm bore polythene tube delivering below the surface. The outlet
pipe was a right-angled length of glass tubing, the horizontal limb of
which carried a rubber seal where it p m d through a l-om hole drilled
in the end wall of the incubator, at 8 point 22 cm above the bottom.
The vertical limb, below water level, was tightly fitted into an inverted
polythene filter funnel with its mouth screened by 23 mesh per om
bolting silk to prevent the flushing out of eggs, larvae and larval food.
Exhaust sea water ran into a covered reservoir under the water bath,
from which it waa intermittently transferred back to the header through
an electrical centrifugal pump fitted with a plastic volute and operated
by a mercury float switch. The header contained washed fronds of
the green alga X n t e r o m q h intestinalis growing on pebbles and
colleoted from a local estuary. The alga received strong illumination
from a battery of tungsten filament lamps which were independently
switched; a rise in the pH index above 8-1 could be countered by
decreasing the illumination. The total capacity of the circuit waa about
200 1. The glass incubators were dimly illuminated with artifioial
light through slits in the water bath covers, and a slow circulation of
cooling water maintained in the beths by air pumps.
The shortcomings of this first closed sea-water circuit were many.
The irrigation rate was necessarily slow during larval feeding, to prevent
undue loss of larval food through the bolting silk of the outlet screen,
which, being of relatively small surface area, was always in danger of
blockage. A t a high ambient temperature and slow flow, temperature
gradients developed in the incubators ; these were a matter of conoern,
as their effect on larval survival was not thoroughly understood.
Enteromwphu is not adapted to live continuously submerged in high
salinity water, and required renewal from time to time. A slow
increase in salinity took place with evaporation, sometimes to a level
above that encountered in the sea. Mechanical failures were common.
Nevertheless, results were encouraging and confirmed Harald Damevig's discovery (1897) that plaice could be reared from the egg, through
and beyond metamorphosis, in small hatchery tanke.
A bigger system was built in 1959 (see Fig. 6). It had two mein
components ; a sunken reservoir 4.6 m x 3 m x 1.2 m deep, of 18 cm
reinforoed concrete containing 11 000 1. aea water, and an adjeoent
6.7 m x 4 m x 2-4 m high brick-built fish hatohery. E d w v h
W a s once again used for CO, and pH oontrol, being submerged on trap
29
60 om x 30 cm x 30 cm moulded glaas incubators (capacity 40 1.) were
partially immersed in a wooden freshwater bath cooled by a coil linked
to a modified domestic refrigerator unit. Sea water ran into each
incubator at a slow controlled rate from a glass header tank, through
a 6-mm bore polythene tube delivering below the surface. The outlet
pipe was a right-angled length of glass tubing, the horizontal limb of
which carried a rubber seal where it p m d through a l-om hole drilled
in the end wall of the incubator, at 8 point 22 cm above the bottom.
The vertical limb, below water level, was tightly fitted into an inverted
polythene filter funnel with its mouth screened by 23 mesh per om
bolting silk to prevent the flushing out of eggs, larvae and larval food.
Exhaust sea water ran into a covered reservoir under the water bath,
from which it waa intermittently transferred back to the header through
an electrical centrifugal pump fitted with a plastic volute and operated
by a mercury float switch. The header contained washed fronds of
the green alga X n t e r o m q h intestinalis growing on pebbles and
colleoted from a local estuary. The alga received strong illumination
from a battery of tungsten filament lamps which were independently
switched; a rise in the pH index above 8-1 could be countered by
decreasing the illumination. The total capacity of the circuit waa about
200 1. The glass incubators were dimly illuminated with artifioial
light through slits in the water bath covers, and a slow circulation of
cooling water maintained in the beths by air pumps.
The shortcomings of this first closed sea-water circuit were many.
The irrigation rate was necessarily slow during larval feeding, to prevent
undue loss of larval food through the bolting silk of the outlet screen,
which, being of relatively small surface area, was always in danger of
blockage. A t a high ambient temperature and slow flow, temperature
gradients developed in the incubators ; these were a matter of conoern,
as their effect on larval survival was not thoroughly understood.
Enteromwphu is not adapted to live continuously submerged in high
salinity water, and required renewal from time to time. A slow
increase in salinity took place with evaporation, sometimes to a level
above that encountered in the sea. Mechanical failures were common.
Nevertheless, results were encouraging and confirmed Harald Damevig's discovery (1897) that plaice could be reared from the egg, through
and beyond metamorphosis, in small hatchery tanke.
A bigger system was built in 1959 (see Fig. 6). It had two mein
components ; a sunken reservoir 4.6 m x 3 m x 1.2 m deep, of 18 cm
reinforoed concrete containing 11 000 1. aea water, and an adjeoent
6.7 m x 4 m x 2-4 m high brick-built fish hatohery. E d w v h
W a s once again used for CO, and pH oontrol, being submerged on trap