HOW TO FARM CRAYFISH AND OTHER CRUSTACEANS
Species commonly grown in aquaculture include Penaeid shrimps, crabs, crayfish and lobster. Crustaceans, are the fourth major aquaculture group in the world in quantity and the second in value. The most important cultured groups are Penaeid shrimps and grapsid crabs. The Pacific whiteleg shrimp, cultured in Latin America and the Caribbean, and giant tiger shrimp (Penaeus monodon), grown in Asia, are amongst the most valued, considered luxury foods.
In Europe and USA various other species are farmed some are species exotic to those countries while others are indigenous. In southern USA the species used are the red swamp crayfish (Proccambarus clarkii) and the white river crayfish (P. acutus). These are both indigenous species to these areas but they have been exported (alive) to many other continents, where they are now produced. Procambarus clarkii is now catalogued as an extremely invasive species in Spain (pest), where it is wreaking havoc in the delta plains of the Donana Natural Park and Ebro River.
It is an active burrower, thus destroying channel and rice field walls, and eating young rice shoots and small animals. During late 1986, specimens of the red swamp crayfish entered the RSA (Republic of South Africa) via the pet trade. The crayfish is considered undesirable in the RSA because it poses a threat to freshwater flora and fauna if it should escape or be released into the wild. It is known as an aggressive burrower, causing extensive damage to the walls of irrigation canals and dams which, in time, can lead to their collapse. The species is very hardy, adapts well to a variety of conditions and is a prolific breeder. This crayfish is blue and develops red pincers as it gets older.
In Australia the most farmed species are the marron (Cherax tenuimanus) and the Yabbie (Cherax destructor).
The ideal crustacean for farming should have high fecundity, short larval life, high larval densities and high survival rate.
CRAYFISH CULTURE (Marron and Yabbie)
Australian marron (Cherax tenuimanus) is a species commonly grown both inside and outside of Australia. It is a spiny, non burrowing freshwater crayfish found naturally in Western Australia. It is the highest valued freshwater crayfish farmed in Australia.
Another Australian crayfish that has also received some attention from growers world wide is the yabbie (Cherax destructor). Care has been taken not to import the yabbie into other countries (like South Africa) because it is an aggressive burrower and could become an ecological problem (invasive species) under local conditions, though marron is farmed commercially in that country.
Marron production ponds must be sited near a source of good quality water. The size of the water resource determines the size of the aquaculture farm. Where a farm is sited downstream from other agricultural units, runoff water from these farms into the local rivers and dams can sometimes carry pesticides and herbicides with it. Where these pesticides are degraded very slowly by nature, this can pose a threat to downstream aquaculture. Even small amounts of these toxic run offs can be dangerous for marron and so care must be taken. Freshwater crayfish are particularly sensitive to insecticides. Herbicides and pesticides are less toxic to them but can also be lethal.
With surface water unwanted fish may enter the crayfish ponds. Water should thus be filtered to remove any ova, fingerlings and adult fish or other animals and/or insects that may attack and/or consume young crayfish. Well or borehole water is often low in, or devoid of, oxygen. It is, however, usually free of spray chemicals, if it is not in an industrial basin or downstream an industrial area. Also, well or borehole water doesn’t carry unwanted animals if properly protected. The water can easily be aerated before being used in the ponds.
A general temperature range of 8 to 30 ºC is given for marron. The optimum temperature for growth is 17 20 ºC. Outer lethal limits are 6 and 32 ºC respectively. The latter is particularly important during moulting because then the crayfish are at their most vulnerable. Throughout most of Australia they are unlikely to be killed by very cold water. They are known to survive in ponds frozen over with ice. Water temperature should not exceed 28 ºC for extended periods.
The optimal pH for crayfish is between 6.5 and 7.5. Beyond these limits lower growth rates have been noticed. It is believed pH should not be higher then 7 8.5. This is usually sufficient to maintain water hardness above 100mg/litre. Below a hardness of 50 mg/litre crayfish shells become soft and production falls dramatically. In the southern USA lime is sometimes added to the pond floor soil to bring the soil pH to 6.7 7.0.
In nature marron can withstand salinities up to half that of sea water (seawater is 35 to 38 ppt). Laboratory experiments have shown freshwater crayfish can survive a 20 ppm salinity (measured as sodium chloride). Field experiments have shown the limit to be about 12 ppm. A salinity of no more than 8 ppt is generally recognized as the safest.
With a high stocking rate careful watch must be kept on the Dissolved Oxygen content of the water. As crayfish breathe oxygen, oxygen must be replaced in the tank or pond in order to maintain its concentration constant. A DO of 7 10 ppm (mg/l) is accepted as optimum at the Amanzi Marron farm near George in South Africa. Whatever the DO of the source water, it remains essential for maximizing production to keep water DO above the 5ppm limit. This is considered the minimum for adult freshwater crayfish. Immature crayfish (i.e. below 50 mm long) can survive in water of 1.5mg/litre and grow rapidly in water of 3 mg/litre or higher.
Circulation of the water, or aeration, may be necessary if there is a high stocking density and low DO's are regularly experienced. Electronic DO monitoring devices (with alarms) are available and used in trout farms in the USA. These can be put to good effect with marron as well where water DO's are likely to be a problem. Remember, your production is determined by its most limiting factor.
During the warmer months, particularly when there is decaying organic matter in the pond, oxygen concentrations can drop rapidly. The shallow depth of standard crayfish ponds makes mechanical aeration difficult. Thus, gravity aeration is the most efficient and commonly used in the southern USA crawfish industry. Water flow through ponds is directed to reduce all potential dead areas. This is done using diversion walls inside the ponds, which turns it into a serpentine like raceway. These walls are not as wide as the retaining walls but their crowns must always be a few centimetres above the water level otherwise the circulation effect is lost. Spacing of the diversion walls depends on the size of the pond.
Marron cannot take a very high organic loading in the water. Thus they are not too happy in waters that are enriched (eutrophied) by runoff from agricultural lands, or the continued additions of composts, or have a high organic loading resulting from plant remains in a dam (i.e. such as when rice paddies are used for growing marron).
Marron feed at night and when the light intensity is low. Lightly turbid water is preferred because this creates low light conditions in the ponds and extends the feeding period by a few hours at the beginning and end of each day (particularly in the afternoon).
Pond sizes vary from place to place around the world.
At Amanzi, in South Africa, earth ponds of 20 x 50 m approx are used for market growing of marron. In Australia recommendations (by marron specialist Dr. Noel Morrisy, Department Fisheries, Western Australia) are to build ponds 40 x 25, in total maximum of 1000 m2. Ponds should be shallower, to avoid deep areas low in oxygen more than 1 meter deep and less than 2 meters. They should have compacted clay bottom, over which a layer of compacted gravel is laid. They should have a good drainage system.
Concrete tanks of 4 x 6 m are used for breeding and growing the juvenile marron to 6 months old. Circular plastic pools of 2 3 m diameter are also being used experimentally. A layer of soil, as well as suitable cover, is used on the bottom of the tanks and pools.
In the southern USA red swamp crawfish and the white river crawfish are cultured in shallow ponds with a water depth of 300 600mm. Deeper ponds are sometimes used in very hot areas.
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