Nutrition and Feeding

Under natural conditions in ponds, lakes, rivers and the ocean, fishes rely on natural productivity for their nourishment. Some aqua culturists also use natural food organisms to provide nourishment for the culture species. In China, for example, ponds are stocked with various species of carp that feed on different parts of the food chain. The fish ponds may be fertilized to help promote growth of phytoplankton, rooplankton, rooted aquatic macrophytes and benthic organisms, each of which is fed upon by a different type of carp. In Japan and a few other countries, ground raw fish is often used to feed aquacultured animals (the fish may be fixed with small) amounts of dry ingredients).Oysters, mussels and clams are among the various shellfish that feed by algae and other organic nutrients from the water. The culture of those animals requires the presence of large algae concentrations. Most fishes and invertebrates of aquaculture interest are fed prepared feeds. Such feeds are composed of various ingredients in proper combinations so that the final product will meet the nutritional requirements of the species being fed. Diets vary considerably from one type of aquaculture animal to the next because of differences in nutritional requirements. For example, many crustaceans are unable to synthesize cholesterol, so that chemical must be provided in the feed. Fishes, on the other hand, do not require dictory cholesterol. Determining the nutritional requirements of an aquaculture species can require many years of research. Diets are prepared in which various ingredients are varied with respect to quality and quantity. Then the feeds are presented to the aquaculture species over a period of several weeks to months and the growth response is evaluated. Experimental diets may be prepared to examine the responses of the animals to variations in dietary protein, fat, carbohydrate, minerals, vitamins or energy. Typical aquaculture diets are relatively simple. They usually contain some type of animal protein (fish meal, poultry by product meal, meat and bone meal) and other proteins supplied by plants (soy bean meal, wheat, corn meal, peanut meal and cottonseed meal). The plant products also supply high levels of carbohydrates (sugars and starches). Some species, such as channel catfish, can tolerate levels of 40% carbohydrate in the diet, whereas others, such as trout, tolerate only low carbohydrate levels. Fat is supplied by the various ingredients mentioned, but supplemental fat is often added in the form of corn oil, fish oil or a variety of others. A mixture of required vitamins and minerals is also usually added. In some instances, wet, ground fish is used in the United States as a dietary ingredient. This is particularly true in the Pacific Northwest, where waste products from fish processing plants are readily available. Once a diet has been formulated and the ingredients have been mixed together in the proper proportions, the material is usually made into a pellet. Pellets are made by exposing the material to high pressure in a pellet mill or extruder. Pellet mills may use steam to help bind the ingredients together. Extruders use supplemental heat and extended exposure to high pressure to make pellets. Pressure pellet mills and extruders pass the feed mixture through a small aperture which leads to a product which, is not cut to short lengths, would be much like spaghetti strands. The diameter of the pellets varies, but is typically 0.2 to 0.24 inches (5 to 6 mm). A knife cuts off the strands as they exit the pelleting equipment. Feeds produced by pressure pelleting are denser than water; thus, they sink. During the extrusion process, on the other hand, the high heat used causes changes within the ingredients so that starches expand when the pellets leave the machine and come into contact with air. This rapid expansion of the material traps air within the pellets, which float when placed in water. Because of the higher temperatures and other factors, extruded pellets are more expensive than pressure pellets. Advantages of floating pellets are that the aqua culturist can see that the fish eat feed. By watching the fish eat, the producer can control the amount of feed offered and thereby avoid overfeeding. This can save money. If the fish develop a disease, the aqua culturist may be able to identify the problem by observing changes in the behavior or appearance of the fish and can treat the problem before it progress very far. Floating feeds should not be used on all aquaculture species. Shrimp, for example, feed on the bottom and will not swim to the surface for pellets. However nutrients will be quickly lost from sinking pellets which may dissolve in a few minutes (floating pellets may take 24 hours or more to disintegrate), so valuable sources of nutrition can be lost if the animals do not consume the feed quickly. Also, bacterial and fungal growth on feed particles that are not quickly consumed can lead to disease or toxicity problems.