BAROTRAUMA IN FISH
Betty Staugler, Florida Sea Grant Marine Agent - UF/IFAS Extension, Charlotte County
Anglers who have been offshore fishing have most likely seen
the bulging eyes, protruding stomach and distended
intestines of a fish brought up from deep water on hook and
line. You might wonder if there is hope for such fish. As with
human divers, fish that experience rapid pressure change can
suffer a host of physiological problems, including air
embolisms and strokes. Many also have an internal ballasting
mechanism, a swim bladder that during a rapid ascent can
swell with enough force to push a fish’s stomach out through
its mouth or distend its eyes.
Distended stomach of a red grouper experiencing
barotrauma—Photo FSG
Understanding how rapid ascent affects the swim bladder
requires some understanding of the swim bladder itself and
the role it plays during normal functioning.
Swim bladders are gas-filled sacs located in the upper body
cavity below the spine that develop as an outgrowth or
pouch of the gut. Swim bladders can be ‘open’, if the link
with the gut, referred to as the pneumatic duct, remains in
adult fish, or they may be ‘closed’ (at least 2/3rd of all modern
fish) where the link is only present during the larval stages.
Many ancient fish as well as modern lung-fish use their swim
bladders to collect and store air swallowed at the surface.
Some modern fish also use their swim bladders to make
sounds, drums and groupers do this. The primary role of
swim bladders however, for those species that have them, is
to allow a fish to conserve energy and remain neutrally
buoyant throughout the water column, even when
stationary.
The gas that fills the bladder is oxygen, carried by
hemoglobin in the blood of the fish. This happens as a result
EAS-012914-001
of the gas gland excreting lactic acid and producing carbon
dioxide. High concentrations of carbon dioxide in the blood
cause the blood surrounding the swim bladder to become
slightly acidic; in turn resulting in hemoglobin releasing
oxygen into the swim bladder and causing it to expand (Root
Effect). There are a number of other processes happening
here, but we won’t get into those.
Swim bladder volume follows a principle known as Boyle’s
law which says pressure at the surface is 1 atmosphere (atm)
and increases by 1 atm, or 14.7 pounds per square inch, for
every 33 feet of descent. So a fish swimming at the surface is
subject to 1 atm of pressure. At 33 feet the pressure
increases to 2 atm, resulting in compression of the swim
bladder to half its original surface volume. This causes the
fish to become heavier than the water around it allowing for
descent. To return to the surface, the swim bladder must be
inflated to its original volume because returning to the
surface decreases the pressure in the swim bladder by half.
Some fish, those with open swim bladders, can deflate their
swim bladders quickly by releasing gas through the
pneumatic duct as bubbles through the mouth or gills. A few
fish with closed swim bladders, Amberjacks for example,
have a specialized purge valve in the gill cover area that
allows them to burp or fizz gas on ascent. However, most
fish with closed swim bladders, groupers and snappers for
instance, cannot deflate their swim bladders quickly and as a
result, when brought to the surface from deep water are
unable to decompress quickly enough to compensate for the
fast pressure changes. This often results in swim bladder
rupture.
When the swim bladder ruptures, swim bladder gasses are
released into the fish’s body cavity causing internal trauma.
This trauma, referred to as barotrauma, is visible in the form
of bulging eyes and protruding stomachs. Survival of a fish
experiencing barotrauma is dependent upon a number of
factors, including severity of damage, both from barotrauma
as well as hook trauma, stress due to temperature changes,
and landing and handling time.
Species specific differences in swim bladder size and
thickness also play a role in fish survival. Red grouper, for
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BAROTRAUMA
January 29, 2014
instance, have thin
swim bladders which
are able to hold
greater amounts of
swim bladder gasses
than the small thick
swim bladder of a red
snapper. This size and
thickness difference
between species
results in red groupers
experiencing much
Modified Cabella lip grip—Photo FSG larger swim bladder
tears during rapid
ascent.
DESCENDING GEAR EXAMPLES
New research findings
from the West Coast
and elsewhere
indicate that the
survival of fish
experiencing
barotrauma can be
significantly increased
Seaqualizer—Photo FSG
using a variety of
methods that quickly
return the fish to
depth while
minimizing injury.
Research shows high
survival rates for
rockfish in depths up
to 300 feet. A number
of ingenious anglers
have developed a
variety of descending
Inverted Dungeness crab net—
gear devices that can
Photo FSG
be used to accomplish
this with minimum injury to the fish. Some of these devices
are now available for purchase, most directly from the
manufacturer.
Several descending devices utilize modified lip grips that are
weighted. Another method involves using a weight and a
large, inverted, barbless hook. Other methods involve using
a container, such as an upside-down milk crate or an
inverted, collapsible net, weighted with lead and attached to
a rope.
Another practice known as venting releases gases from the
body cavity, thus eliminating the pressure on the internal
organs. Venting also will allow the fish to overcome
buoyancy
problems and
swim down to
habitat depth,
enhancing its
immediate
survival.
Descending gear
on the other
hand rapidly
descends a fish
Warsaw grouper being vented—Photo FSG
back to depth
where swim bladder gasses will recompress on their own.
When venting or descending gear is used, if damage is not
excessive, the organs will return in place on their own, once
the gases are expelled or recompressed.
When using a venting tool, insert into the fish at a 45-degree
angle approximately 1 to 2 inches from the base of the
pectoral fin, just deep enough to release the gases.
Your ability to judge which fish requires assistance will
improve with practice and experience. After reeling in a fish,
closely observe its condition. If the fish is bloated and floats
(is unable to control its buoyancy) or if the fish’s stomach is
distended out of the mouth, the fish is experiencing
barotrauma. If the fish appears normal, not bloated, and is
able to swim down to habitat depth on its own, no
intervention is necessary.
For more information on venting and descending gear use,
including some great videos visit www.catchandrelease.org
Sources:
Burns, K. 2009. J and Circle Hook Mortality and Barotrauma
and Consequences for Red Snapper Survival. SEDAR 24RD47.
Moyle, B. and J. Cech, Jr. 2000. Fishes: An Introduction to
Ichthyology, 4th Ed. Prentice-Hall, Inc. New Jersey. 612pp.
Parkyn, D. 2013. Email communication.
Pribyl, A. et.al, 2012. Recovery potential of black rockfish,
Sebastes melanops Girard, recompressed following barotrauma. Journal of Fish Disease, 35, 275-286.
Theberge, S. 2013. Telephone conversation.
BETTY STAUGLER
Florida Sea Grant Agent
[email protected]
UF/IFAS EXTENSION, CHARLOTTE COUNTY
25550 Harbor View Road, Suite 3 - Port Charlotte, Florida 33980
941.764.4340 - 941.764.4343 (fax) - http://charlotte.ifas.ufl.edu