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northern crayfish: an update
INTRODUCTION Crayfish wild harvest and
aquaculture may have the potential to develop into a significant industry in Minnesota
and Wisconsin. It is premature to speculate how large an industry will develop.
As with new commercialization of a natural resource, there will probably be
conflicts among user groups and between users and management agencies. Openness
and direct communication are important if crayfish wild harvest and aquaculture
opportunities are to be realized in an efficient and environmentally sound manner.
Well directed research is also needed to answer the many questions that will arise
as the industry grows. Some of those questions are listed below.
- Can
crayfish be commercially cultured in Minnesota?
- Can soft crayfish be commercially
produced with our northern crayfish species?
- Can Midwestern markets be
developed for hard-shell, soft-shell, and bait crayfish, and for peeled and deveined
tail meat?
- What effects will intensive harvests have on wild crayfish
and other fish/wildlife populations?
- What effects are exotic rusty crayfish
having on native fish, crayfish, and aquatic plants?
- What user conflicts
will arise, and how can they be dealt with?
HARD-SHELL
CRAYFISH Between 2,000 and 3,000 metric tons of crayfish are consumed
annually at a festival in Sweden. Because of a crayfish plague in Europe, the
availability of crayfish from those waters has declined drastically. The rusty
crayfish (Orconectes rusticus) may be the best substitute crayfish seen
in Sweden in 50 years. Swedish importers have placed a large order for rusty crayfish
more than 3 1/2 inches long. The rusty crayfish is not native to either Minnesota
or Wisconsin, but it is now found in both states. It was probably brought in as
bait. Because the rusty crayfish is much more widespread in Wisconsin, most of
the wild harvest was from those waters.
Three of the five other crayfish species in Minnesota may also
have market potential. Although smaller and not initially as highly
regarded by the Swedish buyers as the rusty crayfish, O. virilis,
O. immunis, and O. propinquus may eventually find a niche
in European or U.S. markets. These native species and any rusty
crayfish that are smaller than Sweden's 3 1/2 inch minimum import
size, may also be suitable for the domestic tail meat market.
Tail meat from cooked hard-shell crayfish is peeled, deveined, and
sold fresh or frozen. Development of an efficient tail meat picking machine could
increase the potential for economically marketing Minnesota crayfish in the U.S.
Crayfish culture is the largest aquaculture industry in the U.S. in surface
acres of water under production. Louisiana's 140,000 acres produce an average
of 500 to 600 pounds each. Including wild harvest, Louisiana produces about 120
million pounds (80 percent of the world's supply of crayfish) each year from November
to June. The Louisiana species are the red swamp crayfish (Procambarus clarkii)
and white river crayfish (P. acutus acutus). Aquaculture of crayfish
in northern states is in its infancy but a small industry is developing. Native
crayfish have become so abundant in some Minnesota wild rice paddies that they
are considered a nuisance. Paddies might, therefore, be a suitable environment
for commercial crayfish culture. It has also been suggested that small wild-caught
crayfish could be grown to marketable size in culture ponds. The current U.S.
market for crayfish is primarily in Louisiana, but demand is beginning to grow
elsewhere. Very few crayfish are marketed for food in the Midwest. Nevertheless
crayfish production in Minnesota and other Midwestern states during the summer
months could help ensure a stable 12-month supply and establish new U.S. markets.
SOFT-SHELL CRAYFISH Crayfish periodically
molt or shed their hard exoskeleton in order to grow. During the molting process
the crayfish expands and forms a flexible new inner shell while dissolving and
cracking the old shell. After the crayfish emerges from the old shell, the new,
larger shell begins to harden. The new exoskeleton remains soft for only about
three hours after molting unless the crayfish is chilled or frozen. Up to 92 percent
of a soft crayfish is edible compared to only 10 to 20 percent of a hard crayfish.
Soft crayfish are considered a delicacy and are also popular as fishing bait.
Prices to the producer for soft crayfish sold as food range from $6 to $9 per
pound. Soft crayfish sold as bait may bring $12 to $30 per pound at certain times
of the year.
There was no way to commercially produce soft crayfish, especially
the larger ones used as food, until recently, when Sea Grant researchers
at Louisiana State University developed a recirculating soft crayfish
shedding system. Soft crayfish production increased from 10 tons
in 1987 to 50 tons in 1988. The potential food market for soft crayfish
is estimated at 1,500 tons per year.
It is not
known if wild-caught or cultured northern crayfish will respond to the intensive
shedding system developed for Louisiana's red swamp crayfish. Research has already
begun to address this question. Developing a shedding system for northern
crayfish and a reliable market for soft crayfish would provide a use for crayfish
smaller than Sweden's 3 1/2 inch minimum import size and would increase the likelihood
of developing a significant crayfish industry in Minnesota and Wisconsin.
TRAPPING CRAYFISH Crayfish are generally
harvested by trapping although seines and lift nets can also be effective at times.
Traps are typically made from coated wire mesh or plastic mesh. Hardware cloth
can also be used. A 3/4 inch mesh allows under-sized crayfish to escape. Of the
numerous trap designs available, two of the easiest to construct are the traditional
minnow-style trap and the Louisiana-style pillow trap. A recent study by the
University of Wisconsin-Stout found the minnow-style trap was more effective than
the pillow trap. They also developed a new trap, currently being refined, with
the entrance on top. Plans for this trap, which out-performed all the others,
may be available soon. The minnow-style trap is simple to constrict and is
widely used by crayfish trappers. It is a mesh cylinder about 28 inches long and
10 inches in diameter. Inward pointing funnels are fashioned from mesh and attached
to each end of the trap. Entrance holes are two inches in diameter. A door is
constructed in the middle of the trap to remove crayfish and add bait. When completed,
the trap is shaped like a pillow about 20 inches by 28 inches by 8 inches.
Traps can be baited with fish heads or racks, canned pet food, beef scraps, liver,
or commercial pelleted baits. Carp was a more effective bait than either sucker
or smelt in a UW-Stout study; suckers were next most effective. Smelt is least
effective. Crayfish typically feed at night, but large adults will feed in daylight
during the warmer months. Where there are large crayfish populations, they may
consume the bait and leave the trap within hours. Check your traps more frequently
if this seems to be happening. Mark your traps with your name and address.
Attach a small styrofoam float to each trap with a strong nylon cord or rope.
The rope should be slightly longer than the water is deep, but not so long that
it obstructs boaters or anglers. Try to avoid placing traps in areas with high
boat traffic. Crayfish can be found in almost any aquatic setting: lakes,
ponds, rivers, and streams. They can be found on any type of bottom, from muddy
to rocky, and may or may not be associated with aquatic vegetation. Experimenting
in each body of water will reveal the best trapping locations. New Minnesota
Department of Natural Resourses (DNR) Commissioner's Orders regulating crayfish
harvest, transportation, and use as bait went into effect in Spring 1989.
Contact your local DNR office for a copy of the regulations.
IDENTIFICATION
Identifying crayfish can be very difficult. Positive identification requires
looking at a number of characteristics and having enough experience to interpret
them. Here are some general, easily observed characteristics that can be used
to help you identify mature adults of the four species most often encountered
in Minnesota. They are general identification guidelines and should not be
used when positive identification is needed. Rusty crayfish can generally
be identified by their more robust claws than either O. immunis or O.
virilis, and by the dark rusty spots on each side of their carapace (pinchers).
The spots are placed as though you picked up the crayfish with paint on your forefinger
and thumb (Figure 4A). The spots may be
obscured by the dark coloration of rusty crayfish from some waters. Compared to
the rusty crayfish, O. virilis can often be distinguished because its claw
is more blue and has distinct white wart-like bumps. The rusty claw, by comparison,
is greyish-green to reddish-brown and is smoother. O. propinquus has
a claw very similar to the rusty crayfish, but O. propinquus lacks the
dark spots on each side of the carapace. Instead, it has a dark brown to black
patch on the top of the abdomen ("tail section"). This gives the impression that
a light colored stripe runs along each side of the abdomen (Figure 4B).
Click here to see
table of distinguishing claw traits
For more information regarding crayfish identification and life
history, refer to The Crayfishes and Shrimps of Wisconsin
by Hobbs and Jass, published in 1988 by the Milwaukee Public Museum,
800 W. Wells St., Milwaukee, WI 53233, telephone 414.278.2702.
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