Linda Benedict, Reigh, Robert C. | 4/17/2009 9:13:59 PM
Robert C. Reigh
The art of feeding fish is thousands of years old, but the science of fish nutrition began only about 50 years ago.
Fish nutritionists, like the human use and to develop quality feeds for the numerous species produced under highly controlled conditions.
The LSU Agricultural Center’s aquatic animal nutrition program conducts research on the development and improvement of feeds for aquatic animals of commercial importance in Louisiana. These include channel catfish, hybrid striped bass and tilapia.
In catfish farming, feed cost typically constitutes half the cost of production. With such high feed expenditures, even a small reduction in feed price can provide substantial savings.
One way to reduce the cost of fish feed is to reduce its protein content, which is often the most expensive part. Fish feeds tend to contain relatively high levels of protein (usually 25 percent to 45 percent of diet weight) because the species produced in most commercial facilities grow well on such diets. But research with a number of fish species has shown that dietary protein levels can be reduced in properly balanced diets without negative effects on growth or body composition. The magnitude of the protein reduction depends on the dietary requirements of the species and the nutritional balance (energy and amino acid composition) of the diet.
Channel catfish are omnivorous in their natural food habits and will accept a wide variety of ingredients in formulated diets. This makes feeding catfish less expensive than feeding carnivorous species. Catfish producers can substitute lower cost plant products, such as soybean meal and cottonseed meal, for the higher cost animal protein supplements, such as fishmeal and meat and bone meal, which typically constitute a large portion of the diet of cultured carnivorous fishes. Because catfish can use relatively high levels of dietary carbohydrate, which is often poorly digested by many other fishes, large quantities of inexpensive, starchy grains, such as corn and wheat middlings, can be incorporated in catfish diets to reduce cost.
Plant vs. animal diets
Recent research with catfish has demonstrated that balanced, all-plant protein diets can produce weight gains equal to those obtained with high-animal protein (8 percent fishmeal) diets, which were the industry standard just a few years ago. In a three-year study at the LSU Agricultural Center’s Aquaculture Research Station, yields of pond-raised catfish fed an all-plant protein diet (3,257 pounds per acre per year) did not differ significantly from yields obtained with a diet containing animal protein (3,103 pounds per acre per year). In addition to lowering feed cost, the allplant protein diet provided an added benefit of reducing the fat content of harvested fish.
Other studies at the station are investigating the optimum amino acid balance for channel catfish diets. Proteins, which are constructed of chains of amino acids, differ in their nutritional value for fish based on the types and quantities of amino acids they contain. Proteins from ingredients of animal origin, like fishmeal, tend to be better amino acid sources for fish than proteins of plant origin, which are often deficient in one or more essential amino acids. Nonetheless, deficiencies in single ingredients can be overcome by combining ingredients to satisfy dietary amino acid requirements. This approach has been taken in a three-year production study to determine the effect of reduced levels of certain dietary non-essential amino acids on catfish growth. Results of the study, in its third and final year, are not yet available, but weight gains of fish fed the experimental diets in a ninemonth laboratory growth trial (Figure 1) indicate that protein levels below 29 percent do not reduce catfish growth if dietary amino acid composition is balanced properly.
Digestibility affects environment
Digestibility of fish feeds has become an environmental issue. Nutrients in uneaten and undigested feed act to fertilize ponds and can stimulate algal blooms and other negative environmental effects if concentrations are sufficiently high. Phosphorus is usually the most limiting nutrient for aquatic plant growth. It is relatively abundant in fish feeds, and much of it is present in a chemical form (phytic acid) that cannot be digested by fish or other simplestomached animals.
Phosphorus that passes through the fish’s digestive system is eventually released into the pond water by decomposition, where it stimulates the growth of algae and other vegetation.
The most effective way to limit over-fertilization of ponds and the management problems that excessive nutrient loads cause is to reduce the quantity of nutrients entering the system. A project under way at the Aquaculture Research Station is evaluating the feasibility of adding phytase enzyme to catfish diets to release the bound phosphorus in phytic acid so it can be absorbed in the digestive process. Results to date indicate that a concentration of 500 units of phytase per kilogram of diet significantly increases uptake of dietary phosphorus by channel catfish fed an all-plant diet, thereby decreasing the quantity of phosphorus entering the pond environment and reducing the need for dietary phosphorus supplements.
Reducing particle size
Digestive efficiency also is affected by the size of particles passing through the digestive tract. As food particles move through the gut, enzymes break down proteins, fats and carbohydrates exposed on their surfaces. Because small particles have a greater surface-tovolume ratio than large particles, digestion efficiency should be greater for a given weight of small particles than for an equal weight of large particles. Work in progress at the research station involves quantifying the effect of feedingredient particle size on digestibility of protein, energy and dry matter (total organic and inorganic matter) in channel catfish diets. The goal is to quantify changes in digestibility across the range of particle sizes to determine the point at which further reduction in particle size no longer improves digestibility. Application of the results of this research could help to produce diets that are used more efficiently and that provide better conversion of feed to weight gain.
Good pond environment
A good pond environment is essential for fast-growing, healthy fish. At high stocking densities (up to 10,000 fish per acre) and heavy feeding rates (up to 120 pounds of feed per acre per day), maintaining good water quality can become a management challenge. Use of diets with reduced protein content, improved amino acid profiles, greater organic matter digestibility and increased phosphorus availability will result in less metabolic waste, organic matter and phosphorus entering catfish ponds, which in turn will decrease oxygen demand from decomposition processes and reduce problematic algal blooms. Thus, improved diets offer both economic and environmental benefits to catfish producers.