11. HYDROSTATIC PRESSURE
455
while under high pressures. Semenov (1954, cited by Belyi, 1963) reported
that sturgeon eggs and larvae developed better under increased pressures
of unspecified magnitude than they did at 1 atm.
Bishai (1961b) found that herring, Clupea harengus, larvae survived
and behaved normally at 2 atm pressure during exposures to pressure
lasting 12 days. Indeed, larvae under pressure survived somewhat better
than controls maintained at 1 atm. Similar results were obtained with
other larvae subjected to the following regime: 1 day at 2 atm, 1 day
at 3 atm, and 3 days at 3 4 atm. Young plaice, Pkuronectes plutessa, were
similarly unaffected by pressures of 2 atm maintained for up to 8 days.
All experiments involved rapid changes in pressure (5-30 min) for
both compression and decompression. The swim bladders had not yet
developed in the herring larvae used; the plaice does not have a swim
bladder.
Similar experiments carried out on larvae and young of salmonid
fishes (Salmo salar; Salmo truttn, both sea and brown trout types)
showed a distinct dependence of pressure tolerance on age (Bishai,
1961b). Alevins can tolerate pressures up to 5 atm until the disappearance of the yolk sac. Older alevins, to at least 56 days old and having
partly and fully developed swim bladders, survived exposures to 25
atm pressure for up to 8 days, although many of them experienced
periods of acute disequilibrium following decompression at fairly slow
rates. Some of these fish died, apparently from asphyxia resulting from
gill blockage by gas bubbles. Young fish 98-178 days old all died within
24 hr when subjected to pressure increases of more than 0.2 atm. Fish
older than 272 days once again survived high pressures, in these cases
up to at least 4 atm. Bishai interpreted these results as reflections of
the gradual development of greater ability to control swim bladder gas
volumes in older fish. No suggestion was offered as to why the 98-178day old fish die following even very small pressure increases.
A more extensive survey of low pressure tolerances of a variety of
freshwater fishes all having well-developed swim bladders, but some
being physostomous, others physoclistous, has also been made b y Bishai
(1961a, 1963). In these experiments pressures of only 2 atm were used,
with rapid compressions (1-2 min) and variable rates of decompression
(1-35 min). Durations at high pressure were 1-5 days, with observations
of experimental fishes often continued for weeks after decompression.
All species studied ( 5 physoclists and 5 physostomes) survived the
experiments undamaged. Some mortality occurred, but it was attributed
to gas disease rather than pressure effects. The physoclists had more
difficulty in adjusting, and they took longer to adjust to both pressure
increase and fall than did the physostomes.
455
while under high pressures. Semenov (1954, cited by Belyi, 1963) reported
that sturgeon eggs and larvae developed better under increased pressures
of unspecified magnitude than they did at 1 atm.
Bishai (1961b) found that herring, Clupea harengus, larvae survived
and behaved normally at 2 atm pressure during exposures to pressure
lasting 12 days. Indeed, larvae under pressure survived somewhat better
than controls maintained at 1 atm. Similar results were obtained with
other larvae subjected to the following regime: 1 day at 2 atm, 1 day
at 3 atm, and 3 days at 3 4 atm. Young plaice, Pkuronectes plutessa, were
similarly unaffected by pressures of 2 atm maintained for up to 8 days.
All experiments involved rapid changes in pressure (5-30 min) for
both compression and decompression. The swim bladders had not yet
developed in the herring larvae used; the plaice does not have a swim
bladder.
Similar experiments carried out on larvae and young of salmonid
fishes (Salmo salar; Salmo truttn, both sea and brown trout types)
showed a distinct dependence of pressure tolerance on age (Bishai,
1961b). Alevins can tolerate pressures up to 5 atm until the disappearance of the yolk sac. Older alevins, to at least 56 days old and having
partly and fully developed swim bladders, survived exposures to 25
atm pressure for up to 8 days, although many of them experienced
periods of acute disequilibrium following decompression at fairly slow
rates. Some of these fish died, apparently from asphyxia resulting from
gill blockage by gas bubbles. Young fish 98-178 days old all died within
24 hr when subjected to pressure increases of more than 0.2 atm. Fish
older than 272 days once again survived high pressures, in these cases
up to at least 4 atm. Bishai interpreted these results as reflections of
the gradual development of greater ability to control swim bladder gas
volumes in older fish. No suggestion was offered as to why the 98-178day old fish die following even very small pressure increases.
A more extensive survey of low pressure tolerances of a variety of
freshwater fishes all having well-developed swim bladders, but some
being physostomous, others physoclistous, has also been made b y Bishai
(1961a, 1963). In these experiments pressures of only 2 atm were used,
with rapid compressions (1-2 min) and variable rates of decompression
(1-35 min). Durations at high pressure were 1-5 days, with observations
of experimental fishes often continued for weeks after decompression.
All species studied ( 5 physoclists and 5 physostomes) survived the
experiments undamaged. Some mortality occurred, but it was attributed
to gas disease rather than pressure effects. The physoclists had more
difficulty in adjusting, and they took longer to adjust to both pressure
increase and fall than did the physostomes.