46
J. E. SHELBOURNE
Tank 3. 1000 late eggs (eyed) from pond-wintered stook
4. 1000 late eggs (eyed) from sea-wintered stock
6. 1000 early eggs (blastulae) from sea-wintered stock
6. 1000 mid-stage eggs (gastrulae) from sea-wintered stock
Representative egg samples from pond- and sea-wintered stock were
subjected to shell toughness tests by means of a spring balance modified
to record the applied load in grammes necessary to burst the egg shell ;
means and ranges are given in Table V. There was no temperature
control during the experiment ; tank temperatures rose from 9~4°C at
the start to around 1 7 O C at the end, with periods of rapid fluctuation
in between. The salinity of incoming sea water, taken from routine
hatchery records, remained between 33 and 34°/00 throughout the
season. The pH was similarly steady at 8.0-8.2.
As far as possible, dead eggs, larvae and food were removed every
other day. Four days after the first Iarva hatched in each tank, living
Arternia salina nauplii were offered as food; arbitrary daily rations
were subsequently fed in quantities thought sufficient to maintain a
residual food population. In fact, occasional food shortages did occur,
particularly after larval metamorphosis. Small increments of barnacle
nauplii were also offered as early food. At the first sign of approaching
metamorphosis, washed sand was spread over the bottom of each tank
to a depth of 4 mm. Towards the end of June, all survivors had metamorphosed and the overall mortality rate had fallen to a steady low
level. The experiment ended on June 29, when all survivors were
killed and preserved in 6% formaldehyde in sea water, for later stock
analysis. Table V summarizes the state of tank populations during
the course of the experiment.
The survival curves for tanks 2-6, each stocked with 1000 eggs, are
given in Fig. 11. Tank 1, stocked with 2000 eggs, has been omitted
for ease of presentation ; its curve was similar to that of tank 2. Tanks 3
and 4, both containing eyed eggs, showed a marked initial response to
handling during transfer from pond to incubator. The incubation
time per tank was variable, as eggs were in different stages of development at the start of the experiment. Hatching WM complete in tanks 3
and 4 (eyed eggs) within 7 days ; tanks 2 and 6 (gastrulae) after 9 days,
and tank 6 (blastulae) in 13 days. The incubation Gmperature
(86-10.8"C) was too high for good survival. Egg shells became rapidly
covered with an opaque film of sessile bacteria and losses were comparatively heavy.
A period of characteristically high larval mortality followed
hatohing in all t a r h during the so-called o r i t i d yolk-ertc phase. At
J. E. SHELBOURNE
Tank 3. 1000 late eggs (eyed) from pond-wintered stook
4. 1000 late eggs (eyed) from sea-wintered stock
6. 1000 early eggs (blastulae) from sea-wintered stock
6. 1000 mid-stage eggs (gastrulae) from sea-wintered stock
Representative egg samples from pond- and sea-wintered stock were
subjected to shell toughness tests by means of a spring balance modified
to record the applied load in grammes necessary to burst the egg shell ;
means and ranges are given in Table V. There was no temperature
control during the experiment ; tank temperatures rose from 9~4°C at
the start to around 1 7 O C at the end, with periods of rapid fluctuation
in between. The salinity of incoming sea water, taken from routine
hatchery records, remained between 33 and 34°/00 throughout the
season. The pH was similarly steady at 8.0-8.2.
As far as possible, dead eggs, larvae and food were removed every
other day. Four days after the first Iarva hatched in each tank, living
Arternia salina nauplii were offered as food; arbitrary daily rations
were subsequently fed in quantities thought sufficient to maintain a
residual food population. In fact, occasional food shortages did occur,
particularly after larval metamorphosis. Small increments of barnacle
nauplii were also offered as early food. At the first sign of approaching
metamorphosis, washed sand was spread over the bottom of each tank
to a depth of 4 mm. Towards the end of June, all survivors had metamorphosed and the overall mortality rate had fallen to a steady low
level. The experiment ended on June 29, when all survivors were
killed and preserved in 6% formaldehyde in sea water, for later stock
analysis. Table V summarizes the state of tank populations during
the course of the experiment.
The survival curves for tanks 2-6, each stocked with 1000 eggs, are
given in Fig. 11. Tank 1, stocked with 2000 eggs, has been omitted
for ease of presentation ; its curve was similar to that of tank 2. Tanks 3
and 4, both containing eyed eggs, showed a marked initial response to
handling during transfer from pond to incubator. The incubation
time per tank was variable, as eggs were in different stages of development at the start of the experiment. Hatching WM complete in tanks 3
and 4 (eyed eggs) within 7 days ; tanks 2 and 6 (gastrulae) after 9 days,
and tank 6 (blastulae) in 13 days. The incubation Gmperature
(86-10.8"C) was too high for good survival. Egg shells became rapidly
covered with an opaque film of sessile bacteria and losses were comparatively heavy.
A period of characteristically high larval mortality followed
hatohing in all t a r h during the so-called o r i t i d yolk-ertc phase. At