| Recent
work conducted at the University of Louisiana Lafayette
has shed new light on a species that has a long fishery
history in the state. The river shrimp is well known in
the Mississippi, Atchafalaya and Red River systems. It was
a common item on New Orleans menus early in the 1900s, but
now supports only a limited bait fishery in some areas of
Louisiana.
River
shrimp are still common in many rivers, and undoubtedly
are a major component of riverine ecosystems.
Macrobrachium
ohione are the best-known of the freshwater shrimp
in North America. They grow to around four inches long,
in contrast to the much larger Malaysian prawn, M. rosenbergii.
The Malaysian prawn is the species that is being successfully
pond cultured in the southern U.S. (and other places in
the world, particularly India) and is sometimes available
in our seafood markets. Both of these freshwater shrimp
are delicious and are quite different from saltwater shrimp.
The meat is white and not as dense as that of marine shrimp
and the flavor tends to be mild.
In 1937,
Gordon Gunter, the greatest of early Louisiana naturalists,
wrote: “In Louisiana many fishermen along the Mississippi
derive part of their income during the summer months from
the catch of M. ohionis (ohione). As sales are
often made from door to door no adequate statistics of the
catch are available. It is believed to amount to several
thousand pounds a year. Commercially, the shrimp are taken
in box traps made of wood strips, baited with fish and meat
scraps, and sunk in shallow water not far offshore. These
traps have an inverted V-shaped inset running lengthwise
along the bottom of the box with an open groove at the angle
of the V. Pressed cottonseed cake is sometimes used as bait,
but fishermen state that decayed meat is better and is more
frequently used. The cottonseed cake is often sprinkled
on the shrimp before they are taken
to market, in order to make the fastidious buyer believe
the less objectionable bait had been used.
“Shrimp
attack fish kept alive in live-boxes in the river, feeding
on whatever part of the body to which they happen to attach.
Catfish are especially vulnerable and are often virtually
skinned alive. Liveboxes are therefore covered with screenwire
to protect the fish. Shrimp are sometimes captured by lifting
willow bushes out of the water and catching the animals
in a dip net as they drop off.”
UL professor
Ray Bauer has been studying Atchafalaya populations of river
shrimp with Lafayette naturalist Jim Delahoussaye. These
scientists have documented the downriver migration of reproducing
female shrimp, followed by massive migrations of juveniles
out of the estuaries and back up the river.
Additionally,
they have shown that brackish water is essential for the
early development of the “freshwater” shrimp
larvae. Juvenile shrimp were found to move upstream in late
summer and early fall. At night, in the quiet water along
the bank, a band of juvenile shrimp several yards wide swim
steadily upstream. These researchers calculated that an
embryonic shrimp hatched at the Atchafalaya Delta would
take about 100 days to reach Butte La Rose, a distance of
91 river miles. During this time the shrimp grow to about
2 inches.
This
cycle of repeated migrations between rivers and estuaries
is a type of diadromous life cycle that has been called
amphidromy (where the migration is for purposes other than
breeding, which in M. ohione occurs in the river).
While
the populations of river shrimp in Louisiana are still robust,
the species is becoming rare in the northern portions of
its range, the Missouri and Ohio Rivers. Dams, water control
structures and changes in water quality may be factors in
the loss of upstream populations.
Bauer
and Delahoussaye are looking for folks who can help sample
river shrimp along their Atchafalaya migration route. A
couple of sweeps of a dipnet on a few summer nights would
provide needed information on growth and body composition
changes. Folks who have easy river access – particularly
camps along the Atchafalaya – are encouraged to contact
the researcher:
Raymond
T. Bauer, Professor of Biology
Dept. of Biology, University of Louisiana,
Lafayette, LA 70504-2451
email: rtbauer@louisiana.edu
telephone: 337-482-6435
Research Website: www.louisiana.edu/~rtb6933
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