Bivalve molluscs (oysters, mussels, clams and scallops) form a significant part of the worlds fisheries production. In 2000 landings of bivalves from capture fisheries and aquaculture operations totalled 14 204 152 tonnes (Figure 1). During the decade from 1991 to 2000 there was a continuing increase in production of bivalves, and landings more than doubled from 6.3 million tonnes in 1991 to 14 million tonnes in 2000.
Figure 1: Production (in millions of tonnes) of bivalves from capture fisheries and aquaculture during the decade from 1991 to 2000 (from FAO Yearbooks of Fishery Statistics).
The global trend in the growth of human consumption of seafood will undoubtedly continue. Seafood constitutes an important and essential part of the diet of many people in the world and the need for increased production in these countries will persist as the human population expands. In some countries, seafood is recognized as an important and healthy part of the human diet and demand for seafood products in these countries will also grow. Most of the demand for seafood is and will continue to be for finfish, but the production and harvest of molluscs, particularly bivalves, will also be important in meeting rising demand. While the harvest of natural bivalve stocks will continue to prove significant, many wild stocks are probably already being harvested at or near maximum sustainable limits, in some places and stocks may even be overharvested. Aquaculture is the alternative to the harvest of wild stocks.
During the period 1991 - 2000, bivalve landings from capture fisheries increased only slightly from about 2.5 - 3.5 million tonnes, while landings from culture operations more than doubled during this same period, increasing from 6.3 - 14 million tonnes (Figure 2). About 75% of the worlds bivalve production in 2000 resulted from some form of culture operation.
Figure 2: A comparison of production from capture fisheries and aquaculture broken down into the relative contributions of the major groups of bivalves in 1991 and 2000.
Bivalves are ideal animals for aquaculture: they are herbivores requiring no additional feeding apart from the natural algae content of seawater and generally minimum husbandry. Altough they have been cultured for hundreds of years advances in culture technology in recent years have led to significantly higher production. Continued improvements in culture methodology and technology will be required to meet increasing demand and also to make bivalve culture more economically attractive to both investors and people who wish to become shellfish farmers. The need for greater efficiency in culture operations will become even more essential in the future as areas where shellfish farming can be undertaken in the world are limited and may become more restricted as human populations and coastal urbanization grow.
A primary requisite in any culture or farming operation is an abundant, reliable and inexpensive supply of juveniles (seed). At present, most bivalve culture operations in the world obtain their seed by collecting natural sets. A substrate (cultch) is placed in breeding areas to collect metamorphosing larvae and the juveniles collected are transferred to growing areas for culture (growout) to market size. In other operations, juveniles are gathered from areas of natural abundance and are transported to growing areas that may be distant from the source of the seed. Collection of juveniles from areas of natural recruitment will continue to be important in bivalve culture operations worldwide and undoubtedly can be expanded in some areas to meet increased seed requirements for culture operations. The importance of these breeding areas must be recognized and every effort made to conserve them.
In many other culture locations, natural breeding areas do not exist to provide seed or, if they exist, they cannot produce sufficient seed to meet growout requirements, or breeding is erratic and an annually reliable source of seed cannot be guaranteed. There are other considerations that may make collection of natural sets unsuitable as the supply for culture operations. Growers in some areas may wish to develop and culture a particular race or strain of bivalve to suit their individual needs and juveniles for such operations are not available from the local or nearby environment. Another consideration is that growers may wish to introduce a non-indigenous (exotic) species but a seed source is not available. The alternative to collection of natural sets of bivalves is to produce seed in a hatchery. Bivalve hatcheries have existed for over half a century and are well-established in several countries. They are an integral part of many culture operations and the major or sole source of seed. In the future, bivalve hatcheries will undoubtedly become more important to culture operations as shellfish farming becomes more specialized and the need for seed increases.
Hatcheries have several advantages over collection of natural sets. They are reliable and can supply growers with their seed requirements when it is convenient to them - often much earlier in a growing season than natural sets would occur. They can supply seed that is not available to farmers from natural sets, for example, genetic strains that have improved biological characteristics for farming operations in local areas or a supply of exotic bivalve seed. Cost is the main disadvantage of producing seed in hatcheries: it is more expensive to propagate seed in a facility than collecting it from natural sets. Although economic factors have probably caused the demise of some bivalve hatcheries in the past, recent improvements in technology have greatly enhanced both reliability and financial viability whereby it is possible to produce seed at competitive prices. In some parts of the world hatcheries are the only source of seed for the commercial culture industry but there is ample scope to make them more efficient and more widely accepted as the preferred source of seed.
Constructing and operating a bivalve hatchery is a large and expensive undertaking and careful thought must be given to the developmental phase of a new venture otherwise it will likely end in failure. There is no single plan to construct and operate a bivalve hatchery; indeed many have begun as a small operation and simply grown as markets for their products expanded. Hatcheries vary greatly in their design, configuration and construction from site to site depending on species cultured, target production levels and, most of all, the local conditions and personal preferences of owners/operators. However, the basics for any bivalve hatchery are the same and include a method to condition and spawn adults, rear and set larvae, rear the juveniles to an acceptable size, together with a facility to produce large quantities of food (algae) to feed all stages of the production cycle. Although these essential elements are the same in every hatchery, there are variations in technology and the efficiency of each phase of operation must continually be improved to make hatcheries increasingly more profitable.
This publication is intended to serve as a "how to" manual for bivalve hatcheries. While other documents also describe bivalve hatcheries many are becoming out-ofdate and do not include the most recent technological improvements. This manual is intended as a practical introduction to the fundamentals of hatchery operation for new entrants in this field. It will also allow a potential investor to assess whether he/she wishes to build and operate a bivalve hatchery and become involved in the business of producing seed for the culture industry. This is not meant to be a scientific publication in the accepted sense and much of the content is based on the authors own experience gained over a collective period of 80 or more years. Although there is an extensive literature pertinent to bivalve hatcheries, many of the more practical publications have had limited circulation and are out of print or are only available through specialist library services. Many readers may not be able to access such written material and so efforts have been made to make this manual as comprehensive and available as possible. Rather than including extensive references in the text, a suggested reading list is given at the end of each major section to provide further sources of information on particular subjects and aspects of operation.