Body shape cylindrical. Cephalotorax conspicuously granular (roughened) in adults, provided of numerous small tuberculi and also having strong cervical, cephalic, branchiostegal, and marginal spines. Rostrum long with margins straight, convergent, having marginal spines near its tip, ending in a triangular acumen. Chelae narrow and long, notch in proximal portion of dactyl, leaving gap and delimited by tubercle. Large tubercle opposite end of gap on fixed finger; large scarlet tubercles on the palm and fingers. Carapace not separated at the middle (dorsally) by a space, the areola. Colour in adults dark red, some in shades of brown. A wedge-shaped black stripe is present on the abdomen. Chelae with bright red tubercles. Juveniles uniform grey, sometimes overlain by dark wavy lines. 

 

Red swamp crawfish
Procambarus clarkii above and P. zonangulus below	Stocking crawfish
Crawfish pond near the end of the harvesting season	Harvesting crawfish

Although captive fisheries for introduced Procambarus clarkii exist in several other countries (such as China, Spain, and Portugal), there is no place where crayfish are more highly regarded socially and have had as much impact to the economy of a region than in the southern United States. Crawfish are cultivated and consumed for food in several southern states but Louisiana dominates the crawfish industry of North America in both aquaculture and wild capture fisheries, where the industry contributes well in excess of USD 150 million to the state's economy annually. Commercial sales of crawfish from natural waters began in Louisiana in the late 1800s, and with the development of improved transportation and cold storage, crawfish markets shifted from local consumption in rural areas to higher-volume markets in cities such as Baton Rouge and New Orleans, and beyond. Annual supplies of wild harvests were extremely variable from year to year and the season was often short-lived. Therefore, entrepreneurs began experimenting with the farming of crawfish by the mid 20th century as a source of more dependable supplies. Pond culture of crawfish soon became integrated with other farming operations, and today, pond-reared crawfish constitutes the majority of the annual harvest. Over the last decade, farm-reared crawfish have accounted for well over 75 percent of the total harvest. Approximately 48 000 ha are devoted to the culture of crawfish in Louisiana and the state accounts for about 90-95 percent of the total production in the USA.

At the time that this fact sheet was prepared, no information or documentation was available that indicated that China is intentionally culturing P. clarkii on a commercial scale, despite the large amount reported to FAO in its aquaculture statistics. It is possible that there are some culture ponds where P. clarkii are raised but we have no details on their number or size. It is believed that the crawfish exports from China are a result of both captured stocks (from rivers, streams, canals, etc.) and incidental catches from the seining of finfish ponds.

Procambarus clarkii was originally distributed from northern Mexico to Florida, and north to southern Illinois and Ohio. It has been widely introduced in the USA (Arizona, California, Georgia, Hawaii, Idaho, Indiana, Maryland, Nevada, New Mexico, North Carolina, Ohio, Oregon, South Carolina, Utah, Oklahoma), south and central America (Belize, Brazil, Costa Rica, Dominican Republic), Europe (Portugal, Spain, France, Cyprus), and other more dispersed areas (Japan, Kenya, China, Taiwan Province of China and Uganda).

P. clarkii is found in lentic and lotic freshwater habitats: sluggish streams and lentic habitats, swamps, ditches, sloughs and ponds, etc., especially in vegetation, leaf litter, etc. It avoids streams and ditches with a strong flow, where it is replaced by other species. It exhibits territorial behaviour and is aggressive with its own species. It burrows during periods of drought or cold. It is benthic and omnivorous, feeding on insects, larvae, detritus, etc., with a preference for animal matter. Procambarid crawfish in natural habitats have been classified as obligate carnivores, which means that while they can subsist on living and detrital plant matter they require some form of animal matter in the diet for optimal growth and health.

The life cycle of this burrowing crawfish is well suited to the annual sequence of spring flooding and summer dry period common to large river systems and floodplains in the southern USA where it is most abundant.

Mature animals mate in open water, and although spawning can take place in open water, the burrow provides protection while the eggs and offspring are attached to the abdomen. Burrowing activity for reproduction can occur at any time of the year but it is most prevalent in late spring/early summer in the South. Egg development in mature females usually begins prior to burrowing and maturity is completed in the burrow. At maturity, the eggs are expelled through the oviducts, fertilized externally with sperm that has been stored in the seminal receptacle, and are then attached to the swimmerets on the tail (abdomen). Although crawfish can survive in a high humid environment within the burrow, standing water is necessary for spawning. The number of eggs laid varies with size and condition of the female and will usually range from 200 to 500 eggs.

Hatchlings remain attached to the female's swimmerets through two moults, after which they become free and can forage on their own. It is a fast-growing species; in adequate conditions larvae are born after 21 days of incubation (5 mm long at 2 days), growing to 2 cm one month later and up to 80 mm of length in 3 months. It is an eurythermal species (10-22 ºC to >30 ºC) and inhabits all types of water, with a preference for hard water

Although the red swamp crawfish P. clarkii is the target species in most crawfish aquaculture ventures, other species have similar ecological requirements and often co-exist in production ponds. The native range of the P. zonangulus (white river crawfish) overlaps that of P. clarkii in the south-central United States, and introduced P. clarkii co-exists with the native P. acutus acutus (eastern white river crawfish) in culture ponds along the Atlantic coast. Though the abundance of each species can vary among ponds within a region and, to a lesser extent, within a pond during the annual production cycle, the red swamp crawfish usually dominates the catch and is the most desired species in the marketplace, particularly in Louisiana. White river crawfish are commonly found in greatest numbers in ponds that have been in continuous cultivation for several years. No evidence of natural crossbreeding has been observed. The main biological difference is that P. clarkii is capable of spawning year-round in the southern United States and the white river species are considered seasonal spawners.

Because spawning is to a large degree synchronized in pond-reared crawfish, production ponds are routinely flooded in autumn to coincide with peak spawning. Continuous recruitment and differential growth result in a crawfish population of mixed sizes and age classes. Frequent moulting and rapid growth occur in production ponds when conditions are suitable. After a period of growth, both males and females attain sexual maturity and growth ceases. Mature individuals typically increase in abundance in late spring and burrowing in earnest begins. Under favourable environmental conditions, mature crawfish may revert back to a sexually inactive form and continue growing.

Since crawfish populations are self-sustaining, stocking is usually needed only in new ponds, when a pond has been idle for a year or more or after extensive levee renovation. Subsequent crawfish crops rely on holdover broodstock from a previous cycle. Seedstock are simply composed of harvested red swamp crawfish either from natural habitats or culture ponds where a high percentage of the individuals are sexually mature.
 
As noted earlier, the life cycle of this burrowing crawfish is well suited to the annual sequence of spring flooding and summer dry period common to large river systems and floodplains in the southern USA where it is most abundant. Commercial crawfish aquaculture simulates this hydrological cycle, but with precise control over when ponds are flooded and when they are dewatered to optimize recruitment and subsequent crawfish harvests. The dried soil plugs at the burrow entrances become softened when sufficient external moisture is available. Thus, pond flooding, in combination with rainfall, allows the emergence of animals that have been trapped inside the burrows.

Procambarid crawfish aquaculture relies on earthen ponds and extensive methods of production. These methods are little more than limited control of the environmental conditions under which these animals evolved. Sustained periods of open water permit crawfish to feed, grow and mature. Temporary dewatering promotes aeration of bottom sediments, reduces abundance of aquatic predators, and allows for the establishment of vegetation that serves as cover for crawfish and the food resources when water is returned. Crawfish survive the dry intervals by digging or retreating to burrows where they can avoid predators, acquire the moisture necessary for survival and reproduce in safety.

Crawfish are grown in shallow earthen ponds 20 to 60 cm deep. Relatively flat, drainable land with clay soils is required. Water requirements are similar to those for other types of freshwater aquaculture, with the possible exception of water quantity, which can be greater. Ponds are flooded and drained each year, and because of the oxygen demand from decaying vegetation, additional water exchanges are sometimes necessary.

Production strategies for culturing crawfish are categorized by two basic approaches.

One strategy is monocropping (or monoculture) in which crawfish is the sole crop harvested, and production typically occurs in the same physical location for several production cycles or longer. A second strategy is the crop rotation system, in which rice, and sometimes other crops, is raised in rotation with crawfish. The crop rotation approach can be further broken down into two subcategories. Crawfish are either rotated with rice in the same physical location year after year, or crawfish are cultured in different locations each year to conform to normal field rotations of rice. Although these management strategies have many similarities, different production goals dictate different management techniques and have different advantages and disadvantages.

Monoculture (monocropping)

Crawfish monocropping (the 'single-crop' system) is the production method of choice for small farms or where marginal lands are available and unsuited to other crops. Permanent ponds are typically used. Pond size and production input may range from large (greater than 120 ha) impounded wetlands with little management to small (less than 6 ha) intensively managed systems. The main advantage of a monoculture strategy is that producers can manage for maximum crawfish production without the various concerns associated with other crops, such as pesticide exposure, seasonal limitations and other constraints associated with crop rotation.

Crawfish yields in monoculture systems typically range from less than 225 ha/kg in large, low input ponds to more than 1 300 kg/ha with intensive management. Some ponds have yielded in excess of 2 800 kg/ha. Smaller ponds usually have higher yields than larger ponds, especially when marketing of smaller, lower-value crawfish is not a problem. Earlier and more intense harvesting is often justified under this approach because of the dense populations that tend to build after several consecutive years of production. Earlier harvests are almost always associated with the highest seasonal prices.

The disadvantages of the monoculture approach often include:

• The need to construct dedicated ponds, whereas with rice/crawfish rotational cropping, the established rice field serves the purpose.
• Land, overhead and operating costs must be amortized over one crop only.
• Crawfish overcrowding frequently occurs after several annual cycles, particularly in smaller ponds; therefore, yields become composed of small (stunted), low-priced crawfish that are difficult to market.

As noted above, stocking is usually needed only in new ponds. Broodstock are introduced into new ponds at a level of 45-89 kg/ha in the spring. Stocking dates and rates are usually dictated by the availability and cost of mature crawfish.

Ponds are thoroughly drained several weeks after stocking and annually thereafter. Cultivated or volunteer vegetation is established in pond bottoms during the summer when ponds are dewatered. Rice is the standard cultivated crop, and emphasis is on forage (stem and leaf) production. Grain, if present, is not harvested in crawfish monoculture. After re-flooding in autumn, producers monitor the crawfish population with baited traps and initiate harvesting when catch and marketing conditions justify the labour and expense. Harvesting continues (often at intermittent intervals) until ponds are drained the following summer, and the cycle is repeated.

Crop rotational systems

Crawfish may be cultured in two basic crop rotation systems. One is rice-crawfish-rice; the other is rice-crawfish-fallow (or soybeans may be farmed following crawfish). In both strategies, crawfish culture follows the rice harvest, and the forage crop used for growing crawfish is the crop residue and re-growth of the rice stubble after grain harvesting. The advantages of these rotational strategies include efficient use of land, labour and farm equipment. Moreover, some fixed costs and the cost of the establishment of rice can be amortized over two or more crops instead of just one.

Rice-crawfish-rice

This approach takes advantage of the seasonality of each crop to obtain two crops in one year in the same field. Rice is grown and harvested during the summer, and crawfish are reared during autumn, winter and early spring. As with monoculture systems, crawfish are only stocked initially. They are introduced directly into the rice crop about 6 weeks post-planting. Following the grain harvest, the residual rice stubble is usually fertilized with a nitrogen-based fertilizer and irrigated to achieve a ratoon crop of forage. The field is flooded in autumn and management practices are then similar to in the monoculture system, with the exception of a shortened growing and harvesting season to accommodate the establishment of the next rice crop.

A major disadvantage with this rotational strategy is usually that neither crop can be managed to yield maximum production. Rice yields in the southern USA are maximized when rice is planted in early spring. Draining the crawfish pond prematurely to accommodate rice establishment decreases total crawfish yield. Pesticide use is another major management consideration, and it is a particular constraint with this production strategy. Crawfish and rice yields vary and depend on management emphasis. Systems managed mainly for crawfish can expect crawfish yields similar to well-managed monoculture systems but at the expense of rice yield and vice versa.

Rice-crawfish-fallow (or rice-crawfish-soybean)

The other major rotational strategy employs crawfish in a rotational system of rice and, sometimes, soybeans. The major difference in this rotation strategy is that rice is not typically cultivated in the same field during consecutive years, to aid in the control of rice diseases and weeds for maximum rice yield. As with a rice-crawfish-rice rotation, however, crawfish culture follows rice cultivation; therefore, crawfish production does not occur in the same physical location from one year to the next. Under this method, if soybeans or another crop is incorporated, three crops per field can be realized in two years. Depending on a variety of factors, some producers may elect to plant a different crop (hay, pasture or grain sorghum) or, more commonly, simply leave the field fallow instead of planting soybeans after the crawfish season ends.

The field rotational approach requires sufficient land resources to allow staggered crops in different fields within a farm, and it is the preferred cropping system for larger commercial rice farmers. This cropping strategy comprises much of the acreage used to grow crawfish in Louisiana. It has several advantages over rotation within the same field. Each crop can be better managed, and the crawfish production season can be extended. For example, in lieu of draining crawfish ponds in early spring to plant rice, crawfish harvest can continue until late spring or early summer when the pond is drained to plant soybeans (or other crops), or longer if plans are to leave the field fallow. Furthermore, by rotating physical locations each year, overpopulation of crawfish is rarely a problem, and crawfish size often is larger because of lower population densities.

Crawfish yields under this management approach are not commonly as high as in monoculture but, with proper management, yields can routinely exceed 1 000 kg/ha. Some disadvantages of this rotational strategy relative to crawfish production in permanent or semi-permanent ponds are:

• The need to restock every year.
• Routine low-population densities.
• Frequently, a late-season harvest when prices are in decline and marketing is more difficult because of abundant supplies.