High consumer demand and market prices have led aquaculturists to devote considerable attention to the culture of crayfishes (craw fishes), lobsters and crabs. However, it is only the crayfishes that presently account for any significant production through culture. Some small-scale production of crabs is also reported from tropical countries. For a number of years, scientists have been involved in investigations on the possibilities of culturing the homarid lobsters, Homarus americanus and H. gammarus. Preliminary efforts have also been madein rearing the post-larval stages of the spiny lobsters, Panulirus spp. and Jasus spp. The protracted larval development, the nature of the food required by the different stages of the phyllosoma larvae, the long time the juveniles take to grow to market size and pronounced cannibalism at both larval and adult stages, have made available culture technologies uneconomical for commercial culture. Efforts are presently directed towards large-scale production of juveniles for stocking protected open waters.
Crayfishes belonging to the families Cambaridae and Astacidae are widely distributed over all the continents, including Africa (where they have been introduced in Uganda and Kenya), and are highly priced in several countries of Europe. Besides being a delicacy, small crayfish are also in demand as bait for anglers in the USA. Among the 300 or so species of crayfish, only four appear to have been used in some form of aquaculture. The most important among these is the red swamp crayfish Pro cambarus clarkii (belonging to the familyCambaridae (fig. 26.1). The other is the signal crayfish, Pacifastacus leniusculus (belonging to the family Astacidae) (fig. 26.2). The red swamp crayfish cultured in the southern USA is reported to form at least 85 per cent of all cray-fish raised in the country. The other species cultured in the USA are the white river cray-fish P. acutus and the paper-shell crayfish Ore-onectes immunis. Pacifastacus leniusculus is notyet cultured on a commercial scale under confinement, but has been introduced into Europe from the USA to replace stocks of the European noble crayfish Astacus astacus, which were decimated by the crayfish ‘plague’ caused by the fungus Aphanomyces astaci. As P. leniusculus is immune to the disease, it has establisheditself in Scandinavian lakes (which provide favourable environmental conditions). It has also found a ready market, as it was already known in European markets through regular imports from the USA.
Small-scale farms do exist in France for growing the European crayfish, and hatcheries produce juvenile P. leniusculus to stock the lakes in Scandinavian countries. In the cold northern waters, the noble crayfish mature when four to five years old. Both the signal and noble crayfish mate in the autumn and the eggs hatch out the following spring. By rearing in warm water at 13–16°C, the hatchlings grow to a length of 3–4 cm in less than half a year. They are generally stocked in lakes at this stage, but in lakes with high densities of predatory species stocking with larger juveniles is preferred. Experimental culture of this species to adult size in tanks has also been attempted. The optimal temperature of water in the tanks is
13–16°C, and the food used is a mixture of fresh fish (herring), liver and chalk. The main cause of mortality in culture tanks is cannibalism of the newly moulted soft animals by the hard-shelled ones.