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Chapter: Aquaculture Principles and Practices: Crayfishes and Crabs

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Culture of Procambarus clarkii(Crayfishes): Pond culture methods, Double cropping

As indicated above, it is only P. clarkii that are actually farmed on a large scale. In nature they are abundant in open waters during wet periods, but the rest of the time they shelter in burrows in swampy and marshy areas.

Culture of Procambarus clarkii


As indicated above, it is only P. clarkii that are actually farmed on a large scale. In nature they are abundant in open waters during wet periods, but the rest of the time they shelter in burrows in swampy and marshy areas. The cray-fish farming practices in the southern USA, particularly in Louisiana, are adapted to the long wet period from autumn to spring when the animal is active. Although crayfish can withstand a wide range of environmental conditions, they thrive best under water temperatures of 20–25°C, 3 ppm dissolved oxygen, salinity of less than 5 ppt, pH of 6.5–8.5 and less than 1 ppm of total ammonia (Avault and Huner, 1985).

 

Pond culture methods

Procambarus clarkii is farmed in rice fields as arotation crop or on a continuous basis in ordinary ponds. For growing crayfish, the levees of rice fields are raised to about 50 cm. The recommended size of ponds is about 8 ha for easy management, but some consider a size of 12– 16 ha optimal. Crayfish ponds are very similar to those used in finfish culture, except that the levees need not be higher than 50–75 cm. Anti-seep collars for drainpipes are recommended to prevent crayfish and rodents from burrowing and causing leaks. It is useful to have baffle-levees in the pond to facilitate water circulation. Wooded and marshy areas can be embanked with a ring levee for the crayfish to establish themselves naturally, with some supplemental stocking if required.

 

Depending on the environmental conditions, it takes three to nine months and a minimum of 11 moults for a crayfish to mature. Brood animals are stocked from April to June at the rate of 20–65 kg per ha, after the pond has been filled to a depth of at least 30 cm. A sex ratio of 1 : 1 is maintained. After about two weeks the water is slowly drained, which may take another two weeks to complete. Mating maZ take place during this period or sometimes even before stocking. Unlike other crayfish which reproduce only once a year,P. clarkii reproduces throughout the year. The male transfers sperm to a seminal receptacle between the female’s walking legs, where they remain viable for up to six months. Eggs are extruded from oviducts and are fertilized by the sperm. A sticky substance that is excreted by glands on the ventral side of the abdominal segments, known as glair, helps to keep the fertilized eggs in place on the paired abdominal appendages (swimmerets). As the water is drained, the crayfish burrows into the levees, and occasionally in exposed portions of the pond bottom. The burrows are sealed with clay plugs to prevent loss of water by evaporation and to protect against predators. Some of the females may lay eggs only after burrowing. The number of eggs varies with the size of the animal, but usually ranges between 100 eggs for a 7.5–8.5 cm crayfish and 600–700 eggs for larger ones of about 12.5 cm length. The eggs develop in the humid burrows and, after hatching, the larvae undergo two moults in the next two weeks. After the second moult they are able to fend for themselves.

Ponds are flooded in about September or October and the rising waters entice the females to release the young and to come out of the burrows. Crayfish are omnivorous, but the bulk of their diet consists of microbially enriched detritus. Vascular plants and epiphytic growths also form highly relished food items. Animal matter, such as worms, insect larvae, molluscs and zooplankton, are specially important food for juvenile crayfish. Though many farmers depend on natural vegetation, including grasses and sedges, in the pond as food for crayfish, it is beneficial to plant rice or millet in the ponds after they are drained in May or June. The decomposing straw covered with micro-organisms, including fungi and bacteria, forms a greater source of nutrients than the green rice plants on which the crayfish feed. By about March, the straw will be depleted and if no additional source of food is provided growth will be affected and stunting of the population may occur. By using additional substrates, such as hay, it has been possible to prevent this and continue the detrital food chain, obtaining yields of up to 4000 kg/ha in stagnant ponds. Analternative is to use commercial feeds after the rice-derived detritus is exhausted.

Unlike the European crayfish, the red swamp crayfish appears to be free from any major disease problems. Two bacterial diseases have been encountered (Ambroski et al., 1975a,b) in red swamp crayfish, but they do not seem to cause any harm in pondraised stocks. Cracked or broken parts of the exoskeleton may be attacked by chitinovorous bacteria, and this may enable other lethal bacteria in the environment to invade the body and uropods. If erosion has not reached the cuticle, moulting will eliminate the affected part. Bacteria producing pathogenic endotoxins occur in gut flora, but do not cause any major problems in low-density cultures. Under crowded conditions in a nutrient-rich environment, blooms of Flavobacterium sp. can occur and the accumulated endotoxins can cause mortality. Besides these, protozoan epibionts can interfere with respiration if dense growths occur on the gills. Spent females emerging from the burrows often suffer from ‘hollow tail’ or ‘wasting disease’, which is probably because they have used up their body reserves during life in the burrows. According to de la Bretonne (1977) these females moult and develop normally succulent tail meat.

Harvesting is usually done with baited wire-mesh traps or with lift nets from boats. Two types of traps are used: a pillow trap, which has one or more funnel openings and is laid sub-merged on the pond bottom; and the stand-up trap, which has two or more funnels at the bottom, with the top open and reaching above the water surface.A metal strip is placed around the inside of the trap to prevent the crayfish from climbing out. Because of the cost and difficulties in obtaining bait, most farmers trap only for about 100 days during the harvest season. Since trapping is labour-intensive and not too efficient, several improved harvesting techniques have been developed, such as a ‘crayfish combine’ and electro-trawls. Avault and Huner (1985) described the equipment and harvesting methods. The crayfish combine or boat has a motor-powered metal wheel mounted on one end, guided by foot wheels, which digs into the pond bottom to propel the boat. One man can operate the boat and at the same time remove the catch from traps andrebait them. The electric trawl uses an electrical current to guide the crayfish into a surface trawl.

Most of the crayfish harvested are sold alive, but about 30 per cent are processed. They are killed by immersion in boiling water and then the tails are hand peeled for sale in the fresh state. When the product has to be frozen and stored before sale, the hepatopancreas, or what is called the ‘fat’, is removed.

 

Double cropping

Following more or less the same techniques as in pond culture, the red swamp crayfish can be produced in rice fields in a double-cropping system in rotation with rice. Crayfish are stocked in the fields in May. When the fields are drained roughly two weeks before harvest around August, the crayfish are forced to burrow into the levees and this facilitates the rice harvest without affecting the crayfish. Four to six weeks after the rice harvest, the fields are reflooded. Crayfish then come out of the burrows and release their young. The rice stubbles left in the field give rise to ratoon growths of green leaves and form the substrate for the growth of periphyton. These, as well as detritus in the field, are fed on by the crayfish. Harvesting is carried out from October to May by means of traps. Many farmers practise a triple-crop rotation, including also a crop of soybeans after the crayfish. One of the main problems with this type of farming is the use of pesticides in rice and soybean growing.

Techniques of intensive culture of crayfish have been developed, and are very similar to those used for culture of other crustaceans. However, the economics of intensive culture of crayfish are not considered attractive at present under conditions in the USA. There is an established market in the USA for soft-shelled crayfish as bait, and recently a market has developed for large soft-shelled crayfish weighing about 10–30 g. Harvested hard-shelled cray-fish can be caught at the pre-moult stage and held in tanks until they moult. Individuals which do not moult readily can be induced to do so by the ablation of both eyestalks at their bases. Another means is to catch the crayfish immediately after they moult, using an active fishing method such as a seine or an electric trawl.

The extensive type of farming as practised in Louisiana, which involves minimum investment and is generally a subsidiary activity of farmers, has proved to be profitable. Competitive returns have been obtained by skilled farmers, and with the introduction of more cost-effective harvesting methods the returns are expected to increase.

 



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