Nutrition And Feeding

Problems associated with excessive levels of nutrients and unwanted nuisance species have already been mentioned. In some cases, aquaculturists use intentional fertilization to produce desirable types of natural food for the species under culture. Examples of this approach include inorganic fertilizer applications in ponds to promote phyto-plankton and zooplankton blooms that provide food for young fish such as channel catfish in the United States, the development of algal mats (called lab-lab) through fertilization of milkfish ponds in Asia, and the use of organic fertilizers (from livestock and human excrement) in Chinese carp ponds to encourage the growth of phytoplankton, macrophytes, and benthic invertebrates. In the latter instance, various species of carp with different food habits are stocked to ensure that all the types of natural foods produced as a result of fertilization are consumed.

Provision of live foods is currently necessary for survival of the early stages of many aquaculture species because acceptable prepared feeds have yet to be developed. Algae is routinely cultured for the early stages of mollusks produced in hatcheries. Once the mollusks are placed in grow-out areas, natural productivity is depended on to provide the algae upon which the shellfish feed.

In cases where zooplankton are reared as food for predatory larvae or fry, it may be necessary to maintain three cultures (algae, zooplankton, and the desired aquaculture species). Though wild zooplankton have been used successfully in some instances (eg, in Norway, wild zooplankton have been collected and fed to larval Pacific halibut), it is much more common practice to provide rotifers or brine shrimp nauplii as zooplanktonic food for first-feeding stages of animals being reared for marketing. After being fed rotifers and/or brine shrimp nauplii for periods ranging from several days to several weeks, depending on the species being reared, the aquaculture animals can usually be trained to accept pelleted feeds, though a weaning process during which both natural and prepared feeds are offered is sometimes required. Problems associated with utilizing prepared feeds from first feeding include difficulty in providing very small particles that contain all the required nutrients, loss of soluble nutrients into the water from small particles before the animals consume the feed, and in some cases, the fact that prepared feeds do not behave the same as live foods when placed in the water. For species that are sight feeders, behavior of the food is often an important factor. Feed color may also be a factor, as is texture, in whether a food item is accepted. Some fish, for example, reject hard pellets but will accept pellets that have a more spongy texture.

Some of the most popular aquaculture species accept prepared feeds from first feeding. Included are catfish, ti-lapia, salmon, and trout. All of the fish listed have relatively large eggs (several millimeters in diameter) that develop into fry that have concomitantly large yolk sacs. The nutrients in the yolk sac lead to production of first-feeding fry with well-developed digestive tracts that produce the enzymes required to efficiently digest diets that contain the same types of ingredients used for larger animals. First-feeding fry of catfish and so on have mouth gapes of sufficient size that they can engulf particles that do contain all essential nutrients.

Over the last few decades, fish nutritionists have successfully determined the nutritional requirements of many aquaculture species and have developed practical feed formulations based on those requirements. For species such as Atlantic salmon, various species of Pacific salmon and trout, common carp, channel catfish, and tilapia, sufficient information exists to design diets precisely suited to each species. Aquaculturists are always interested in the development of new species. In each instance, the nutritional requirements of the new species must be investigated. Although there are many similarities among aquatic animals with respect to nutritional requirements, diets that produce the best growth at the least cost will vary significantly from species to species and can only be formulated when precise nutritional requirements are known. Determination of those requirements may require several years of research, although diets based on existing formulations may be at least adequate to produce reasonable growth and survival and can be employed while the research is being conducted.

Requirements for energy, protein, carbohydrates, lipids, vitamins, and minerals have been determined for the species commonly cultured (11). As a rule of thumb, trout and salmon diets will, if accepted, support growth and survival in virtually any aquaculture species. Such diets often serve as the control against which experimental diets are compared.

Since feeds contain substances other than those required by the animals of interest, studies have also been conducted on antinutritional factors in feedstuffs and on the use of additives. Certain feed ingredients contain chemicals that retard growth or may actually be toxic. Examples are gossypol in cottonseed meal and trypsin inhibitor in soybean meal. Restricting the amounts of the feedstuffs used in a particular diet is one way to avoid problems. In some cases, as is true of trypsin inhibitor, proper processing can destroy the antinutritional factor. In this case, heating of soybean meal is effective.

Animals that do not readily accept pelleted feeds may be enticed to do so if the feed carries an odor that induces ingestion. Color development is an important consideration in aquarium species and some animals produced for human food. External coloration is desired in aquarium species. Pink flesh in cultured salmon is desired by much of the consuming public. Coloration, whether external or of the flesh, can be achieved by incorporating ingredients that contain pigments or by adding extracts or synthetic compounds. One class of additives that impart color is the carotenoids.

Prepared feeds are marketed in various forms from very small crumbles to flakes and pellets of various sizes. Pelleted rations may be hard, semimoist, or moist. Hard pellets typically contain less than 10% water and can be stored under cool, dry conditions for at least 90 days without deterioration of quality. Dry pellets should, however, always be stored in a cool, dry place. Semimoist pellets are chemically stabilized to protect them from degradation and mold if they are properly stored (either refrigerated or in a cool, dry place), while moist pellets that contain high percentages of water must be frozen if they are not used immediately after manufacture.

Hard pellets are the type preferred if the species under culture will accept them. Semimoist feeds are most commonly used in conjunction with feeding young fish and species that find hard pellets unpalatable. Semimoist diets tend to be very expensive and can often only be used economically in conjunction with early life history stages where only small amounts of feed are required. Moist feeds, which contain high percentages of fresh fish, are usually available only in the vicinity of fish-processing plants.

Moist and semimoist feed pellets are produced in machines similar to sausage grinders. The most widely used types of prepared feeds, hard pellets, are produced by pressure pelleting or extrusion (crumbles are produced by grinding larger pellets). Pressure pelleting involves pushing the ground and mixed feed ingredients through holes in a die that is a few centimeters thick to produce spaghetti-like strands of the desired diameter. The strands are cut to length as they exit the die. Steam is often injected into the pellet mill in a location that exposes the feed mixture to moist heat just before the mix enters the die. Exposure to steam improves binding and extends pellet water stability. In general, a pelleted diet should remain stable in water for at least 10 minutes so the fish have sufficient opportunity to consume the feed before it dissolves.

Extruded pellets are produced by exposing the ground and mixed ingredients to much higher heat and pressure and for a longer time than is the case with pressure pellets. In the extrusion process the ingredients undergo some cooking that can be beneficial in reducing the levels of certain antinutritional factors, such as trypsin inhibitor. There may be concomitant losses of heat labile nutrients such as vitamin C, so overfortification to obtain the desired level in the final product may be required. In some cases, a heat labile nutrient is dissolved in oil and sprayed on the pellets after they have exited the extruder.

Specialty feeds such as flakes can be made by running newly manufactured pellets through a press or through use of a double drum dryer. The latter type of flakes begin as a slurry of feed ingredients and water. When the slurry is pressed between the hot rollers of the double drum dryer, wafer thin sheets of dry feed are produced that are then broken into smaller pieces. The different colors observed in some tropical fish foods represent a mixture of flakes, with each of the different colored flakes containing one or more different additives that impart color.

Pressure pellets sink when placed in water, whereas under the proper conditions, floating pellets can be produced through the extrusion process. That is accomplished when the feed mixture contains high levels of starch. When heated, starch expands and the pellets produced will trap air as they leave the barrel of the extruder. The trapped air gives the pellets a density of less than 1.0. Floating pellets are desirable for species that come to the surface to feed, because the aquaculturist can visually determine that the fish are actively feeding and can control daily feeding rates based on observed consumption.

Sinking extruded pellets are used for shrimp and other species that will not surface to obtain food and require pellets with high water stability. Shrimp consume very small particles, so they will nibble pieces from a pellet over an extended period of time. Unless heavily fortified with binders, pressure pellets will dissolve before shrimp have adequate time to fully consume them. Extruded feeds, whether sinking or floating, may remain intact for up to 24 hours after being placed in the water. Pressure pellets begin to disintegrate after a few minutes, unless, as mentioned, supplemental binders are incorporated into the feed mixture.

Nearly all aquaculture feeds contain at least some animal protein since the amino acid levels in plant proteins cannot meet the requirements of most aquatic animals. Fish meal is the most commonly used source of animal protein in aquaculture feeds, though blood meal, poultry byproduct meal, and meat and bone meal have also been successfully used. In 1995 more than 15,500,000 tons of prepared feeds were manufactured for use by the world's aquaculture producers. To make that amount of feed, 1,728,000 tons of fish meal were used. That amounted to 25.6% of the world's fish meal production (12).

Commonly used plant proteins include cornmeal, cottonseed meal, peanut meal, rice, soybean meal, and wheat. A number of other ingredients have also been used, many of which are only locally available. Most formulations contain a small percentage of added fat from such sources as fish oil, beef tallow, or more commonly, oilseed oils such as corn oil and soybean oil.

Complete rations contain added vitamins and minerals. Purified amino acids, binders, carotenoids, and antioxidants are other components found in many feeds. Growth hormone and antibiotics are sometimes used. Regulations on the incorporation of hormones along with other chemicals and drugs into aquatic animal feeds are in place in the United States and some other countries (Table 6). Few such regulations have been promulgated in developing nations.

Feeding practices vary from species to species. It is important not to overfeed since waste feed not only means wasted money, it can also lead to degradation of water quality. Most species require only 3 to 4% of body weight

Table 6. Therapeutants and Disinfecting Agents Approved for Use in U.S. Aquaculture

Name of compound

Use of compound

Therapeutants

Copper Formalin

Furanace (Nifurpyrinol) Oxytetracycline (Terramycin) Sodium chloride

Sulfadimethoxine (Romet)

Trichlorofon (Masoten)

Antibacterial for shrimp Parasiticide for various species

Antibiotic for aquarium fishes Antibiotic for fish and lobsters Osmoregulatory enhancer for fish

Antibacterial for salmonids and catfish Parasiticide for baitfish and goldfish

Disinfectants

Calcium hypochlorite (HTH)

Didecyl dimethyl ammonium chloride (Sanaqua) Povidone-iodine compounds (Argentyne, Betadine, Wescodyne)

Used in raceways and on equipment Used in aquaria and on equipment

Disinfection offish eggs

in dry feed daily for optimum growth. Very young and adult animals are exceptions. Young animals are fed at higher rates because they are growing rapidly and consume a greater percentage of body weight daily than older animals. In addition, it is important to have food readily available to them. Food should be spread evenly over the culture chamber area so the young animals do not have to expend a great deal of energy searching for a meal. That is easily possible in tanks and small raceways, but large culture units do not lend themselves to having feed spread across the entire water surface. In those instances the feed can be spread along the pond or raceway's sides. Feeding rates as high as 50% of body weight daily are not uncommon for young animals. Since total biomass is small, even in intensively stocked units such as raceways, the economic cost is not high. Water quality in raceways can be maintained by siphoning out waste feed periodically. In ponds, any un-consumed feed acts as fertilizer, and the quantities used are not high enough to affect water quality adversely. Over time as the animals become increasingly mobile, the feeding rate can be reduced to satiation, which will often be about 10% of body weight daily for early juveniles and will fall to the previously mentioned 3 to 4% as the animals grow. Broodfish and other adult animals, such as ornamental species, being held in captivity are often fed at a maintenance level, which is usually about 1% of body weight daily.

Young animals may be fed several times daily. Examples include the standard practices of feeding fry channel catfish every three hours and young northern pike as frequently as every few minutes. Keeping carnivorous species such as northern pike satiated helps reduce the incidence of cannibalism. Animals stocked in growout culture cham bers may be fed several times a day, but it is common practice to feed only once or twice to satiation. In warm weather, feed should be provided early in the day (but after making certain that the dissolved oxygen level is within the optimum range) and late in the afternoon. As the water cools in the fall in temperate climates, feeding rate is often reduced. Channel catfish farmers often feed during winter on warm days or every three days (unless there is ice cover) at a rate of no more than 1% of body weight at each feeding. The fish will overwinter without being fed but will lose weight. Weight loss can be averted by providing a modest amount of feed during winter.

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