Fish Nutrition and Feed Technology (2+1)

Unit 11 - Different forms of feed

11.4.Larval feeds

The major problem in hatchery is the feeding of larval stage. Currently the marine fish and crustacean seed production depends on live natural food such as artemia, algae, etc. However, mass culture of these natural live foods is both tedious and expensive; therefore in order to reduce the reliance on live food, it is necessary to develop synthetic or artificial diets to rear the larval forms.

There are several types of compound larval diets. Their range includes from simple egg custard to complex microparticulate feeds. For early weaning, young larvae require feed of smaller size and must be prepared as microbound, microcoated or in microencapsulated form. For late weaning crumbles can be prepared by grinding and sifting of pellet feeds.

Microencapsulated feeds
Micro-encapsulation is a process in which liquids and particulate dietary components are enclosed within a carefully engineered wall. Capsules consist of a wall and the internal nutrients are released at the active sites within the target biological system of fish or shrimp larvae. The wall or shell dissolution can be accomplished by rupture, enzymatic action, pH change or bacterial action in fish or shrimp larval body system.

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. 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, 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.

Microbound feeds
In microbound feeds, the finely powdered ingredients are bound within the water stable matrix of hydrocolloid binders. The binder used may be of starches (inulin, chitin, chitosan, pectin, gum Arabic, agar, carrageenan or calcium alginate) or proteins (zein, casein or gelatine). The microbound particle lack distinct coat, with ingredients haphazardly distributed throughout matrix. This type of binder affects particle characteristics, such as water stability, palatability and digestibility.

The main disadvantage of microbound particle feed is quick sinking which makes it unavailable to larvae and results in nutrient leaching. This can be overcome by lipid coating.

Microcoated feeds
In a normal microparticulate an extra coating of any encapsulate like alginic acid, gelatin, lipids etc. is given to enhance its stability by a simple process like soaking or spraying. Even a coating of cholesterol-lecithin or modified corn gluten or zein makes them microcoated feeds. Unlike microencapsulated particles these do not possess entire coat.