1. INTRODUCTION
2. ENCAPSULATED DIETS
3. OTHER NOVEL FORMS OF FISH DIETS
G. L. Rumsey
Tunison Laboratory of Fish Nutrition
Cortland, New York
It is rather well agreed that a major problem in commercial aquaculture is the feeding of larval forms to, the juvenile stage. This particular problem is common to all species presently utilized and includes both fresh and saltwater teleost fish, crustaceans and mollusks. Many of these species are now the subject of intensive investigations to establish optimal conditions for profitable husbandry from egg to marketable size. To date, culture of the more difficult to rear species has been dependent on live natural food such as artemia, algae, etc. However, mass culture of these natural live foods is both tedious and expensive; therefore it is necessary to develop synthetic or artificial diets (using natural readily available feedstuffs) to rear the larval forms.
It has been conjectured by researchers at Louisiana State University (LSU) that micro-encapsulation, in which liquids and particulate dietary components are enclosed within a carefully engineered wall with release under specific environmental conditions, has broad applications in fish and crustacean culture.
The technology associated with micro-encapsulation has not been described in scientific literature but has been the subject of several patents. Capsules consist of a wall and an internal phase. Release of the internal nutrients at active sites within the target biological system is a very interesting part of the capsulation technology. It is reported that a variety of mechanisms are possible to effect this release by modification of the wall material (composition, permeability, thickness, method of manufacture) during capsule production. Wall or shell dissolution can be accomplished by rupture (as in grinding), enzymatic action, pH change or bacterial action. The wall could be a bio-degradable polymer i.e., modified gelatin, wherein the nutrients within the capsule could be released by the enzymatic processes of the animal or by micro-flora present in its gut. Ultimate selection of the particular wall matrix will depend on the material(s) to be encapsulated and specific properties desired in the finished product. It is reported that the range of components for the internal phase can constitute a complete diet for vertebrate/invertebrate culture. Capsules smaller than 20 microns in diameter are reported to be possible, allowing the culturist a wide range of sizes of nutritionally diverse capsules to satisfy the different growth stages of the particular species.
Advantages of encapsulated diets would be many. One major advantage is that specific nutritional requirements can be met with a high degree of precision since there will be minimal nutrient loss through water leaching. Along the same lines, encapsulated rations would facilitate more rigid control over water quality, minimizing BOD and adverse effects on oxygen, pH, etc. This is especially important under intensive culture conditions where good water quality is essential. Unlike natural or live foods which are by no means nutritionally complete, an encapsulated food could be thoroughly quality controlled, have good shelf-life, be consistent in nutrient composition, and may be developed completely free of contaminants.
The technology of fish feed encapsulation has not yet been demonstrated to be economic or technologically feasible - at least not in the scientific literature. The costs of micro-encapsulation could be considerable. However, Dr. S. P. Meyers of LSU believes that where survival and moulting rate in larval and young stages may dictate the success or failure of the aquacultural enterprise, great latitude is conceivable in cost projections of the support diet. Encapsulated systems might be far less costly and more dependable than currently used natural live food sources. Obviously, much research work is needed to ascertain the technological and economic feasibility of feed encapsulation.
Flakes and extruded diets should be also mentioned as other forms of processed feed that will afford reasonable water stability.
Flake diet formulation can be made with certain commercially available feedstuffs. The double drum dryer processing units are readily adapted to large-volume production. Vegetable binders can be used to achieve the water stability needed to optimize residence time of the flake in the aqueous system. Flakes can be reduced to smaller particle sizes without reducing the basic stability characteristics. Extrusion and extrusion-expansion of both intermediate-moist and dry type diets has also been suggested as a means of achieving some degree of water stability. Again, however, the most important consideration will be economic feasibility.
