Dependence on live foods for feeding larvae of a number of aquaculture species has been referred. One important constraint to the formulation and preparation of appropriate artificial diets for larvae is the lack of suitable techniques for determining their nutritional requirements. Measurements of food intake, weight increment, digestibility, etc., are extremely difficult at the larval stage. So, the only solution at present appears to be the extrapolation of data for juveniles and young adults, which is obviously less reliable. Because of this, aquaculturists very often resort to feeding with a combination of live food and compound feeds, which generally gives better results.
Meals, crumbles and small pellets are used as starter feeds for certain species such as salmonids and channel catfish, but many others do not accept such feeds. Pollution of water in larval tanks due to accumulation of disintegrated feed is a common problem when intake is slow. Experience shows that prepared feeds can be used for larval rearing of a number of finfish only if the level of acceptance is good and the particle intake per unit time per litre of water is high enough to prevent rapid disintegration of the feed. Flake diets, extruded diets and micro-encapsulated diets have been tried to overcome some of the problems, particularly water stability.
Flake diets can be formulated with commercially available feedstuffs. Readily metabolizable vegetable binders are used to achieve the water stability needed to optimize the residence time of the flake in water. A double drum dryer unit, consisting of steam-heated or electrically heated rollers, is used to dry homogeneous wet suspensions of the feed into thin platelets. They can be reduced to smaller particle sizes without reducing the basic stability. According to Meyers (1979) a major advantage is the comparative ease with which definableformulations can be made using natural or synthetic products. Flavours and colours can easily be introduced as required. The main disadvantage is the relatively high cost per unit weight of product. It is necessary to avoid over-heating in the drying process and thereby loss of nutrients.
Extruded larval diets can have good water stability, if readily digestible vegetable binders are used. Extrusion can be run at temperatures between 60 and 100°C. Small feed particles of various shapes can be extruded using material of different densities. It is claimed that specific shapes or particle configurations provide some irregular or erratic motion that simulates prey movement and thus attracts the larvae to feed on them. Low-temperature extrusion and elimination of post-extrusion drying protect heat-labile components and lower costs of production.
Another type of larval feed under investigation is a microencapsulated diet. The micro-capsule consists of a liquid or particulate dietary component, enclosed within a suitable shell or wall. The selection of the wall matrix depends on the material to be capsulated. It can be made of a biodegradable polymer, such as modified gelatine or zein, so that the nutrients within the capsule can be released by enzymic processes of the animal or by microflora present in its gut. It is claimed that a whole range of components can be encapsulated to constitute a complete diet, facilitating a wide range of sizes of nutritionally diverse capsules to feed different growth stages of aquaculture species.
One major advantage of the encapsulated diet is that the specified nutritional requirements of the larva, if known, can be met with a high degree of precision, since there will be minimum nutrient loss due to leaching. Its use will facilitate the maintenance of better water quality, which is of special importance in intensive culture conditions. Unlike natural or live foods which may not be nutritionally complete, an encapsulated diet could have consistent nutrient composition, be free from contaminants and have a good shelf life.
Chow (1980) describes simple methods of preparing an encapsulated egg diet. According to this author, the whole chicken’s egg contains all the necessary nutrients (48.8 per cent protein, 43.2 per cent fat, 0.2 per cent calcium and 0.9 per cent phosphorus, with a gross energy of 5830 kcal/kg and metabolizable energy of 4810 kcal/kg) required during the first 10 days of life of most species of fish. Egg yolk alone, which is often used in feeding larvae, is nutritious, but as a diet for very young fish its high energy/protein ratio may result in an inadequate intake of protein for maximum growth. The suggested method is to encapsulate the whole egg (white and yolk).
Cracked whole egg is beaten vigorously with a fork or homogenized with a mechanical blender. Boiling water (approximately 150 ml for each egg) is poured rapidly into the homogenate with constant stirring. A fine opalescent suspension is obtained, which may be made up to the desired volume with cold water. The suspension can be introduced directly into larval tanks. The opalescent protein coat of the microcapsule reflects enough light to attract larvae to it. Wholeegg diet can also be made in larger particles by controlled stirring to feed fry, or the homogenate made into a custard for feeding adult fish such as eels. When egg diets are made for fry or adults for longer-term feeding, vitamin supplementation may be needed as they are lacking in water-soluble vitamins, especially ascorbic acid. If the protein content has to be lowered, finely ground carbohydrate ingredients like wheat flour or cassava flour can be added.
A larval feed that is often used in shrimp hatchery work in India is a crustacean tissue suspension. Small-sized shrimps of low commercial value are processed into wet tissue suspensions. Different feed particle sizes and dosages are used for successive larval stages. Larval survival rates vary according to culture conditions (Hameed Ali et al., 1982). The survival rates can possibly be improved by using dried crustaceans ground into free-flowing powder of appropriate particle size (Tacon, 1986).