60
J. E. SHELBOURNE
larval feeding began. These temperatures were maintained through
the oritical early feeding phase, which was of longest duration at the
lowest temperature (water bath 1). Thereafter, the lower temperatures
were slowly raised to 9"C, followed by an increase in all baths, at an
equal rate, to a maximum of 11°C at metamorphosis. Stock survivals
are displayed in Fig. 17. At all temperatures, a higher percentage
hatoh occurred in tanks treated with anti-biotics, although differences
between treated and untreated tanks were not as marked as in the
experiments of Oppenhcimer (1956). The effect will no doubt vary
with the degree of egg contamination at the time tanks are stocked.
This in turn will be related to the abundance of contaminants in the
spawning pond.
Slight differences in survival between treated and untreated egg
stocks were magnified after hatching and during larval feeding, at all
temperatures, indicating that the benefits of bacterial suppression
during incubation are felt long after hatching is complete. Embryos
are not necessarily killed, but are undoubtedly weakened by shell
contaminants; in each case, the treated stock produced the more
competent feeders. Arguments based on laboratory experience,
supporting the idea of a corresponding '' critical " phase in the sea,
must inevitably be weakened by the results of this experiment. The
artificial hazards of the tank environment clearly influence the
amplitude of non-feeding mortalities. A " critical period " will still
occur in the sea, however, if suitable planktonic food is marce during
the early larval stages.
The temperature rbgime in water bath 4 was above the optimum.
The other three rbgimes gave comparable results, with water baths 2
and 3 having a very narrow margin of advantage. Coupling these
data with previous experience, it would seem, at the moment, as if the
optimum temperature for plaice egg incubation, using Irish Sea stock,
lies within the range 6-7"C, and that a slight shift to 8°C facilitates
I' first-feeding ". A slow temperature rise from 8' to lO-ll"C during
later development is likely to minimize mortalities among feeding
and metamorphosing larvae.
The survival in water bath 2 (treated tank) was the highest ever
-66%
of original egg stock passed safely through metamorphosis,
giving a survivor density of 3660 young fish/mz of tank bottom. At
these densities (about 3/ine), post-metamorphosis competition is
fierce, and continuing mortalities can be expected under such conditions of high population pressure. This oontrasts with the flatness
of the post-metamorphosis survival curve at lower survival rates and
low survivor densities in earlier experiments.
J. E. SHELBOURNE
larval feeding began. These temperatures were maintained through
the oritical early feeding phase, which was of longest duration at the
lowest temperature (water bath 1). Thereafter, the lower temperatures
were slowly raised to 9"C, followed by an increase in all baths, at an
equal rate, to a maximum of 11°C at metamorphosis. Stock survivals
are displayed in Fig. 17. At all temperatures, a higher percentage
hatoh occurred in tanks treated with anti-biotics, although differences
between treated and untreated tanks were not as marked as in the
experiments of Oppenhcimer (1956). The effect will no doubt vary
with the degree of egg contamination at the time tanks are stocked.
This in turn will be related to the abundance of contaminants in the
spawning pond.
Slight differences in survival between treated and untreated egg
stocks were magnified after hatching and during larval feeding, at all
temperatures, indicating that the benefits of bacterial suppression
during incubation are felt long after hatching is complete. Embryos
are not necessarily killed, but are undoubtedly weakened by shell
contaminants; in each case, the treated stock produced the more
competent feeders. Arguments based on laboratory experience,
supporting the idea of a corresponding '' critical " phase in the sea,
must inevitably be weakened by the results of this experiment. The
artificial hazards of the tank environment clearly influence the
amplitude of non-feeding mortalities. A " critical period " will still
occur in the sea, however, if suitable planktonic food is marce during
the early larval stages.
The temperature rbgime in water bath 4 was above the optimum.
The other three rbgimes gave comparable results, with water baths 2
and 3 having a very narrow margin of advantage. Coupling these
data with previous experience, it would seem, at the moment, as if the
optimum temperature for plaice egg incubation, using Irish Sea stock,
lies within the range 6-7"C, and that a slight shift to 8°C facilitates
I' first-feeding ". A slow temperature rise from 8' to lO-ll"C during
later development is likely to minimize mortalities among feeding
and metamorphosing larvae.
The survival in water bath 2 (treated tank) was the highest ever
-66%
of original egg stock passed safely through metamorphosis,
giving a survivor density of 3660 young fish/mz of tank bottom. At
these densities (about 3/ine), post-metamorphosis competition is
fierce, and continuing mortalities can be expected under such conditions of high population pressure. This oontrasts with the flatness
of the post-metamorphosis survival curve at lower survival rates and
low survivor densities in earlier experiments.