62
.
J. E. SRELBOURNB
supply, for long periods of time. The incubators were protected
ctgainst high room temperature, and the air-lift pump aerated the
water in addition to transferring it from reservoir to header. These
triple-tank units can be used to test any long-term chemical effeot
on larval survival, without running the risk of seoondary effeote
inherent in a static sea-water system.
Procedure. Three such units, holding a total of nine incubators,
were employed in 1961. On 22 March, each incubator was stocked
with 1000 plaice gastrulae sorted from egg catches brought into the
hatchery from the spawxiing pond. Without exception, egg shells
were severely contaminated, and covered with both sedeptary and
motile commensals. When hatching was complete, the fluorescent
tubes above the rearing tanks were lit for 12 h each day, from 0900
to 2100 hr.
The feeding technique differed from that of former years in one
important respect: during the early feeding phase, the hinged flaps
above the tanks were opened to receive maximum illumination, and
this practice was continued until all survivors were well established.
In addition, a special effort was made to keep all tanks free from
faeces, bacterial films and decaying food. Salinity conditions were
varied in experimental units 1 and 3 ; unit 2 ww the untreated control.
Its three incubators were continuously irrigated with normal hatchery
sea water. The experiment ended on 23 June, when all survivors were
killed and preserved in 6% formaldehyde in sea water.
Rtmdts. A summary of stock data per tank is contained in Table VI,
together with mgah size measurements and light intensities. The
present account is concerned mainly with survivals in the three control
tanks of unit 2, which finalized at 36%, 33% and 31% of original eggs,;
giving a mean density of 1790 young fish/ma. This is about three times:
the previous highest survival rate and survivor density at Port Erin
in 1960.
Results for both years are compared in Fig. 14, survival
expressed as percentage of original egg stock. The readings are
of the three best tanks in 1960 and the three control unit incu
in 1961. Both curves show the usual trends. Firstly, an
egg lose during incubation, as a result of initial handling
bacterial effects. Secondly, a phwe of greatly increased
the so-called
feeding. Thirdly, 2 months of slow larval loas between th
feeding and metamorphosis, interrupted by slightly incre
talities for a short time at the late pelagic stage.
critical ” period between hatching and
The curve for 1961 is displaoed to the right, b e a w e (a) the on
.
J. E. SRELBOURNB
supply, for long periods of time. The incubators were protected
ctgainst high room temperature, and the air-lift pump aerated the
water in addition to transferring it from reservoir to header. These
triple-tank units can be used to test any long-term chemical effeot
on larval survival, without running the risk of seoondary effeote
inherent in a static sea-water system.
Procedure. Three such units, holding a total of nine incubators,
were employed in 1961. On 22 March, each incubator was stocked
with 1000 plaice gastrulae sorted from egg catches brought into the
hatchery from the spawxiing pond. Without exception, egg shells
were severely contaminated, and covered with both sedeptary and
motile commensals. When hatching was complete, the fluorescent
tubes above the rearing tanks were lit for 12 h each day, from 0900
to 2100 hr.
The feeding technique differed from that of former years in one
important respect: during the early feeding phase, the hinged flaps
above the tanks were opened to receive maximum illumination, and
this practice was continued until all survivors were well established.
In addition, a special effort was made to keep all tanks free from
faeces, bacterial films and decaying food. Salinity conditions were
varied in experimental units 1 and 3 ; unit 2 ww the untreated control.
Its three incubators were continuously irrigated with normal hatchery
sea water. The experiment ended on 23 June, when all survivors were
killed and preserved in 6% formaldehyde in sea water.
Rtmdts. A summary of stock data per tank is contained in Table VI,
together with mgah size measurements and light intensities. The
present account is concerned mainly with survivals in the three control
tanks of unit 2, which finalized at 36%, 33% and 31% of original eggs,;
giving a mean density of 1790 young fish/ma. This is about three times:
the previous highest survival rate and survivor density at Port Erin
in 1960.
Results for both years are compared in Fig. 14, survival
expressed as percentage of original egg stock. The readings are
of the three best tanks in 1960 and the three control unit incu
in 1961. Both curves show the usual trends. Firstly, an
egg lose during incubation, as a result of initial handling
bacterial effects. Secondly, a phwe of greatly increased
the so-called
feeding. Thirdly, 2 months of slow larval loas between th
feeding and metamorphosis, interrupted by slightly incre
talities for a short time at the late pelagic stage.
critical ” period between hatching and
The curve for 1961 is displaoed to the right, b e a w e (a) the on