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Few tropical crustaceans are suitable for farming. Since the systematics of these species are better known than their ecology. only those species with a long tradition of natural exploitation, according to empirical rules based on observation of the animals' behaviour, have - in preference to other species - been the subject of zoological studies which might offer some hope of making their rearing more popular and more profitable.
This book deals with three shellfish. In section 1, the freshwater crayfish, Procambarus.clarkii, is discussed in depth. The other two shellfish, one a chevrette or prawn (Macrobrachium rosenbergii) and the other a shrimp (Penaeus monodon), are marine or euryhaline. Their biology and rearing are examined in sections 2 and 3 in this book.
Section 1 deals with a single crayfish species, not for any lack of freshwater crustaceans although admittedly there are more marine and lagoon species but because only Procambarus clarkii, the Louisiana red swamp crayfish, seems to multiply without difficulty and even quite vigorously in subtropical and tropical waters, It therefore contrasts with the other known, popular species of which propagation in these same regions is limited by strict climatic and environmental requirements, along with disease hazards that currently rule out plans for their profitable introduction or farming.
This robustness has been observed in many tropical and subtropical countries, of which some are traditionally major consumers (Louisiana, USA) while others, such as Kenya where Procambarus clarkii was introduced only recently, now ex.port it. This goes to show the species' usefulness, in the absence of other ways of exploiting wet areas with a view to increasing the value of biomass transformation . . However, despite the good qualities so promptly attributed to a new species, one must not overlook a number of disadvantages. These arise on the one hand, from the high reproductive potential of Procambarus clarkii which makes it a formidable competitor for other species of crayfish and, on the other hand, from a behavioural trait : in certain circumstances, the crayfish is a burrowing animal which can have harmful effect on the environment, especially in the hydro-agricultural field.
These disadvantages shed light on the understandably protectionist and restrictive attitude of temperate countries where other indigenous and foreign species are more suited to the existing biotopes. This is not the case in tropical and subtropical regions; here crayfish are non·existent and there is no lack of swamps. These, particularly in Africa, are unexploitable or unexploited as a source of human proteins, despite the urgency of the task. These areas could benefit from the introduction of Louisiana red crayfish.
Limiting the discussion in this book to only three genera of the farmed tropical crustaceans considered to be of worldwide economic importance is certainly an unwelcome narrowing of the field. Such a limitation can only be justified firstly, by the recent extension as a result of the acclimatisation of these crustaceans, throughout - and even beyond - the tropics, and secondly, because of the amount of work carried out on the methods and techniques employed which has encouraged this increase in crustacean farming.
The choice of three animals requiring different aquatic environments:
• Procambarus clarkii, which lives in fresh water
• Macrobrachium rosenbergii, the giant freshwater prawn or crayfish,
which lives in both fresh and brackish water
• Penaeus monodon, the sea shrimp, which lives only in salt water, involves a development policy which encourages a range of products with high marketable value, on aquatic sites which have been recently abandoned, e.g. salt marshes, or sites which up until now have had a low productivity, such as marshland.
Once there has been significant production over several years so that crustacean farming can emerge from the experimental stage, there must be a transfer of knowledge, techniques and husbandry practices to the operators in the field, farmers and potential farmers. This ensures that the technical and financial knowledge required is fully available and may well prevent failures as a result of ill-informed enthusiasm.
