A news item released to the Journal a few weeks ago
reported that EXXON was starting to make loud underwater
sounds in waters off the coast of Santa Barbara to explore
for oil. These sounds (known to us as shock waves) will be
240DB every few seconds round the clock for 45 days. The
shock waves bounce back from the ocean floor as echoes to
receivers on board ship to be interpreted by geologists as
areas of potential oil deposits.
No matter that this is
happening right about the time of the gray whale migration
southward. The National Marine Fisheries Service in
Washinton DC stated that the authorization issued by this
agency would provide adequate protection for the marine mammals.
We wonder whether the whales understand this, and will they
make a detour around Santa Barbara
This brings to mind some experiences we had at the Marine
Products Laboratory, University of Maryland, Crisfield, MD
about 25 years ago with similar underwater sounds.
A few miles to our north the jet bombers of the US Navy were
wreaking havoc on the uninhabited and now uninhabitable
Bloodsworth Island. The Navy was bombing the living bejabbers
out of the island. It rationalized that it was playing a vital
role in training combat pilots.
In addition to the bombing, the Navy frogmen were administering
their own brand of havoc in the surrounding waters. They were
detonating several hundred pounds of high explosives to study
the effects of underwater demolition on whatever they were
demolishing. The detonations killed enough yellowfin trout to
glut the fish markets for days.
Of interest to us at the laboratory was the fact that the frogmen
observed that organisms like oysters and clams were gaped open
and remained open for several minutes, and were still alive, after
each detonation. We saw something here.
Sometimes it's an ill wind that blows no good. The intent and
philosophy of the laboratory since its inception in 1950 was to
assist the Maryland seafood industry in studying and adapting
any technological advance for its progress.
At that time, the decline in the numbers of experienced oyster
shuckers became a hardship to the industry. The average age of
a shucker was 55 years. The yearly attrition by retirement or
disability was leaving a void in the work force. Younger people
were simply not attracted to this type of employment. The
decline also reflected a decrease in oyster production as the
result of the then, as now, prevalent MSX parasite infecting
the oyster beds of the Chesapeake Bay. The industry, therefore,
needed some kind of a boost to survive. It needed outside help,
such as ours, to do this.
With the newly found observation that shock wave energy caused
the shell of an oyster to gape, we went to work investigating
how this phenomenon could be adapted for the industry.
The force from an underwater explosion has the form of an intense
high pressure shock wave. As the wave travels through the water
at sonic velocity, the intensity of its initial peak pressure
decreases sharply and rapidly, followed immediately by a rarefaction
or negative pressure phase. It is this phase which causes extensive
damage to fishes with an air bladder.
Detonations can be produced by several means. As employed by the
petroleum industry for offshore oil exploration these include:
(1) discharging an electric spark in water (known as electrohydraulics),
(2) releasing a small volume of high pressure air into the water,
and (3) igniting a mixture of combustible gases in a closed end tube
under water.
The laboratory entered a joint research program with the General
Electric Company in Schenectady,NY to explore the potential of the
electrohydraulic effect to open oysters. Special equipment to
conduct these tests was designed and fabricated at GE.
The equipment consisted of a stainless steel tank-conveyor system
for carrying oysters on a continuous belt through fresh running
water in the tank under a submerged transducer housing the electrodes.
Uder the test condtions employed 87% of the oysters were gaped, with
49 oysters per minute receiving treatment as the conveyor traveled
under the electrodes. Very little effort was required by even an
inexperienced shucker to insert a knife into a gaped oyster to sever
its muscle. The ungaped 0ysters bled liquor when squeezed and were
thus much easier to shuck after treatment. The shucked meats were
organoleptically of excellent quality.
The research was then directed toward a second method of producing
a detonation from a non-explosive source. This is done by releasing
a small volume of air under high pressure directly into the water.
A device known as the PAR AIR GUN was used for this purpose. The air
gun uses compressed air (2400 psi) in a firing chamber and releases
it explosively through four ports into the surrounding water.These
tests were run in a 4 ft diam steel tank. Oysters were placed on
trays of expanded metal and immersed two feet below the surface of
the water. The air gun was placed eight inches above the oysters.
Essentially similar results were obtained by this treatment.
The study demonstrated that oysters could be gaped by shock wave
energy. Because of its simplicity and much lower cost the air gun
would serve the purpose adequately.
Insofar as oyster production on the Chesapeake Bay is approaching
the null point, there appears to be no need to so mechanize the
industry. Experienced shuckers are still available.
Reference:
Gaping Oysters by Shock Wave Energy
M. W. Paparella and M. Allen
Chesapeake Science, Vol 11 No 2, June 1970
Natural Resources Institute
University of Maryland
December 3, 1995 Michael W. Paparella All Rights Reserved
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