Crayfish - Asset or Liability?

Fish and Lake Management
Crayfish fishing
Pond Boss moderator Dave Davidson examines a crawfish trap. Pondmeister Johnny Tanner, from Carrollton, Georgia, has a small pond where he raises crawfish to.feed his bass in a larger lake. Inset: Oronectes, 'papershell crayfish' from Missouri.

Any bass angler who fishes Midwestern lakes and impoundments can testify to the importance or frequency of finding remains of crayfish in stomachs of largemouth bass (LMB). When food habits of LMB were studied at Murphy Flowage, Wisconsin many years ago, the most prominent item was crayfish. A conclusion of the study was that LMB were not an effective predator on the high-density population of slow growing bluegill (BLG). Biologists failed to recognize that panfish outweighed the total weight of LMB by about 10:1. Apparently when LMB have relatively little competition for food they chose crayfish over fish. What would you choose in your favorite cafeteria offering free food, lobster or fish?

A sidelight of the Murphy Flowage studies was a major effort to improve BLG growth rate by removing as many fish as possible with large seines. Large seines have relatively large mesh and were effective at harvesting the largest fish of the adult population. It was a surprise to discover that subsequent to the removal, BLG growth rate declined. Harvesting adult bluegill resulted in reduced competition for food, an explosion of bluegill reproduction and a dramatic increase in BLG numbers.

More recent studies have shown that in order to have a balanced population of BLG with good growth and fish of quality (6-inch) and larger sizes, at least 90% annual mortality of age-I and age-II BLG (1.5 to 3.5 inches long) is needed: i.e. 10,000 age-I reduced to 1000 age-II which are reduced to 100 age-III. BLG of this size and age are prey for LMB 12 to 15 inches long. In well balanced communities of BLG and LMB a favorable total weight ratio is 3 or 4 to 1 at any point in time.

Two other unsuccessful efforts to improve growth rate and size structure of prey are informative. In one case, in Wisconsin, adult northern pike were introduced into a population of stunted yellow perch. The result was a decline in the number of quality-size perch. A second case in Nebraska was an effort to reduce numbers and improve size distribution of bullheads by stocking an effective predator, adult flathead catfish. In both cases the introductions did not result as intended because both predators were selective for the largest of the prey available. The principle learned is the importance of the relationship between size of predator and size of prey.

Back to the question of crayfish. The rusty crayfish (Orconectes rusticus) is an unwelcome visitor in some northern lakes. It has been introduced by anglers using crayfish as bait... not the first time the bait bucket has been a source of an introduction. The impact in many of these lakes has been loss of valuable habitat and native aquatic plants.

I had a similar experience with a different species of Orconectes in a one-acre pond in Missouri. The pond was about 8 feet deep and with clear water. There was a ring of vegetation growing to a depth of 7 feet. In mid-summer the pond looked like a donut with a small opening in the deepest part. Fishing was difficult or impossible except in early spring.

In the fall I had the opportunity to introduce about 10 gallons of crayfish, a byproduct of some other pond research. From my perspective the result was a success. The following year water transparency declined to about 2 feet and vegetation was limited to water less than 2 feet deep. It was easy to fish the pond from shore all summer long. The species of crayfish in this genus do not burrow but have a strong thigmotaxis- a desire to be touching something. In native habitat that could be a log or rock that they could get next to or under. In clay-bottom ponds they work to form a shallow depression where they rest during the day. This tactic may help to avoid predation.

In a small shallow pond without fish and wall-to-wall vegetation, I introduced a small number of Orconectes. The next summer the pond looked muddy; the water transparency was reduced to a few inches and a plant hook came back empty. The aquatic plants were gone. The impact of the crayfish was excessive.

In an established pond with an excess of rooted vegetation into deep water the introduction of Orconectes might yield positive results by improving conditions for fishing, increasing capacity for production of LMB, possibly promoting better wind induced circulation, an improved oxygen profile and a reduction or avoidance of excessively high pH due to decreased aquatic plant mass. The problem is how to find a source of Orconectes?

Fish farmers often produce crayfish in minnow ponds. There, crayfish can be a liability because they injure minnows when ponds are seined or harvested. On fish farms, crayfish are an unwanted byproduct. It is unlikely to find crayfish in catfish ponds because they are a preferred food item.

An easy source of crayfish is the red swamp crayfish (Procambarus clarkii) sold for human food. Production is a major industry in rice fields. Although Orconectes prefers to feed on vegetation, the red swamp crayfish can be carnivorous and eat insect larvae, tadpoles, snails and small fish. Such food habits would not be an asset in most fish ponds. Red swamp crayfish have been introduced into many areas. An undesirable attribute of the genus and a relative, Cambarus, is the habit of burrowing. Burrowing activity can damage water control structures, dams and levees. P. clarkii is an intermediate host for parasitic helminthes of vertebrates. Because of adverse affects, efforts for eradication have been attempted in some areas.

What might a pond owner do to introduce something with limited ability for natural dispersion that would feed primarily on plant material or detritus and increase the production of food for fish? A critter that has appeal to me is a different decapod, Palaemonetes, a freshwater prawn also called glass shrimp. They are relatively small, up to 2 inches long, and free swimming. I introduced some into a sediment trap pond above a 35-acre lake in Missouri. They moved downstream and have persisted in the lake for many years.

What about simply buying red swamp crayfish and using them as supplemental food for LMB? Sure, it has been done, countless times. As a pond manager, your job is to learn the facts and make the best decisions you can make. One pondmeister, in central Oklahoma, has bought different kinds of living supplemental food for his bass for years. During the crayfish season, he might buy 2,000 pounds to stock into his 10 acre lake. His mission is to supplement the natural food chain. For him, it works. His bass are so overcrowded that crayfish don't stand a chance to establish themselves. There is relatively little crayfish habitat and so many bass that the mudbugs don't stand much of a chance at survival. Advantage, bass.

But, look at another real world case. In 1984, lake manager Billy Cooper, from Houston, Texas, helped renovate a well known lake which sits less than fifteen miles from the Texas coast. The lake was drained, bulldozers did their magic, rearranging heavy clay soils in the flat country. The lake refilled and restocked with forage fish to set the table to grow giant largemouth bass.

Within months, the lake looked like chocolate milk. What happened?

This 30 acre lake sits in the middle of rice country along the Texas gulf coast. After the bulldozers left, the landowner seeded part of the lake bottom with rye grass and let it grow to six inches before flooding the lake in January... about the time crayfish are feeding and growing as fast as they can.

The source of the muddy water was literally hundreds of pounds of growing, thriving crayfish, fighting for food and for space and stirring bottom clay soils. These creatures were outcompeting the fish, overeating the foodchain and making the lake muddy.

After a concerted effort at trapping, crayfish numbers were reduced to the point that fish could begin to thrive and the water cleared well enough the lake could perform as wished. In this case, crayfish cost the landowner some time and were a problem.

Crayfish - asset or liability? The best answer is, it all depends. If they are introduced under appropriate circumstances, it would be important to prevent overharvest of LMB and maintain numbers and sizes to take advantage of an increased food supply. If you get a good population established you may decide to trap out a mess to make a poor-man shrimp cocktail or a New Orleans gumbo. Bon Apetit.