Different sizes and species of fish and the diverse environmental and management conditions used in aquaculture require different feeding strategies. Diet characteristics, such as source (living or nonliving feed), particle size, texture, density, and palatability, must be carefully considered for size and species of fish. Feed allowance and frequency of feeding are important for growth rate and feed efficiency. Type of feed (floating or sinking) used and method of feeding will depend on the fish, the culture system, and the equipment and personnel available. These strategic factors are as important as meeting the nutrient requirements of the diets and have been addressed in a number of recent publications. This chapter will focus on feeding young fish and some key points on production feeding of several commercially important species; reference will be made in appropriate places to more detailed sources of information.

The larval stage is defined by the metamorphosis of external and physiological characters from hatch until the juvenile stage is attained. External characteristics and major organ functions of juveniles match those in adult fish. For practical purposes, larval fish can be divided into three groups according to alimentary tract morphology and the enzymes secreted into the gut (Dabrowski, 1984a). The first group includes such fishes as salmonids and channel catfish, which appear to have a functional stomach before changing from endogenous to external feed. The second group includes fish, such as striped bass and many marine species, that at the larval stage have a very rudimentary digestive tract with no functional stomach or gastric glands and which undergo complex metamorphosis of the digestive system. The third group of larval fish are those that develop a functional digestive tract but remain stomachless throughout life, such as carps. Species that at the time of first feeding have structurally and functionally differentiated alimentary tracts pose less of a problem with initial feeding. Those with immature digestive systems at first feeding are more difficult to feed and usually require live feeds as a part of their diet.

Larval fish undergo different phases of larval metamorphosis and at a certain phase can be weaned to dry, prepared diets. For example, striped bass, which complete metamorphosis in 21 to 42 days, cannot use dry diets at day 5, when initial feeding begins, but they can at day 15 (Baragi and Lovell, 1986). Common carp can be transferred to commercial dry diets at the size of 15 to 30 mg (Bryant and Matty, 1980), whereas larval whitefish must obtain a size of 50 mg to be weaned to dry diets (Dabrowski and Poczyczynski, 1988a). The transition from live to dry diet is a gradual process.

One reason for the poor ability of some larval fishes to utilize prepared diets at first feeding may be the low affinity of the proteolytic enzymes in the immature digestive tract for the proteins offered in formulated diets (Dabrowski, 1984b). Several fish species have been shown to be unable to digest the protein from prepared diets at larval stages (Bremer, 1980). Also, the relatively high volume of feed consumed contributes to increased passage speed and results in low digestive efficiency (Kaushik and Dabrowski, 1983). Larval fish ingest more feed per unit weight than adult fish, consuming 50 to 300 percent of their body weight daily compared to 2 to 10 percent of body weight for subadult fish fed to marketable size (Bryant and Matty, 1980, 1981). The inability of some larval fish to immediately use prepared diets may also be due to the absence of enzymes, hormones or their regulators, or growth factors that are provided in live feeds (Lauff and Hofer, 1984; Baragi and Lovell, 1986). A lipid-soluble growth factor extracted from zooplankton was shown to be active for coregonid larvae (Rembold and Fluchter, 1988); however, the same fish species were grown successfully on a prepared diet that did not contain the extract or the live zooplankton (Dabrowski et al.,

(1984). Other dietary factors in live feed might inhibit or stimulate hormone action in larvae. Thyroid hormones, which play an important role in larval fish metamorphosis and growth (Collie and Stevens, 1985) may be influenced by diet components (Miwa and Inui, 1987; Specker, 1988; Inui et al., 1989).

The preferred live food organisms for larval fish are those in their natural diets; however, rotifer (Brachionus plicatilis) and brine shrimp (Artemia) are the only zooplankters produced in mass quantities. Variations of nutritional quality, primarily (n-3) polyunsaturated fatty acid (PUFA) content, exist among sources of zooplankton from different geographical origins and culture conditions (Watanabe et al., 1983). Many fish larvae are very sensitive to a deficiency of n-3 PUFAs (Watanabe et al., 1983; Gatesoupe and Le Millinaire, 1985); thus, the composition and amounts of fatty acids in zooplanktonic food affects growth and survival. Enrichment of live zooplankton with essential fatty acids may be accomplished by two procedures (Sorgeloos, 1980): (1) the newly hatched zooplankton can be fed for a period of 24 hours on marine algae (Chlorella spp.) or yeast containing a high concentration of (n-3) PUFA, or (2) the zooplankton nauplii can be exposed to a suspension of lipid rich in (n-3) PUFA, such as fish oil, and an emulsifying compound for 3 to 12 hours before being offered to the larvae.

Microparticulate diets for larval fish have to meet the nutritional requirements of the species, be of a size appropriate for ingestion; have the desired physical properties with regard to buoyancy, texture, and color; and, in many cases, simulate movement. Nutritional components of prepared diets for fish larvae should be determined on the basis of juvenile fish requirements, although outogenetic differences are possible. Scant information exists on the differences in quantitative nutrient requirements between larval fishes and juveniles, except for the common understanding that larval fish have a higher metabolic rate and thus benefit from a higher concentration of nutrients and energy in their diet (Dabrowski, 1986).

Optimum diet particle size increases in proportion to fish size and should not exceed 20 percent of the month opening (Dabrowski and Bardega, 1984). Frequent feeding is important in all larval fish; food can be offered 10 to 24 times per day or almost continuously and in excess (Charlon and Bergot, 1984; Charlon et al., 1986).

Diets containing 70 to 80 percent good-quality fishmeal support good growth in starter feeds for salmonids (Reinitz, 1983) and channel catfish (Winfree and Stickney, 1984); however, severe mortality occurred in larvae of sturgeon (Dabrowski et al., 1985), channel catfish (Winfree and Stickney, 1984), and common carp (Dabrowski et al., 1988), if casein was the major ingredient of the diet. Larval diet formulation based on single-cell protein and freeze-dried animal tissues (Dabrowski et al., 1984; Dabrowski and Poczyczynski, 1988b) proved successful with the stomachless larvae of common carp, grass carp, and silver carp. Microparticulate diets have been prepared for marine fish larvae composed of micropulverized meals from fish, crab, yeast, chicken egg yolk, short-necked clam, and krill (Kanazawa, 1986; Kanazawa et al., 1989).

Channel catfish, like salmonids, have a relatively well-developed digestive system and consume and assimilate prepared diets satisfactorily at the time the fish begin feeding. The time to initiate feeding is when the yolk sac reserves have been significantly reduced and the fish "swim up" to the water surface in search of feed. This phenomenon appears to be synchronized with histogenesis of the taste buds and mucous cells of the oropharyngeal region (Twango and MacCrimmon, 1977). Swim-up channel catfish are fed at hourly intervals by automatic feeders at rates of 25 percent of body weight per day. As fish size increases, these rates are reduced to four to two feedings of 5 to 10 percent of body weight daily while in the hatchery (Dupree and Huner, 1984). They usually begin on meal-type diets 0.5 mm in diameter and change to crumbles of 1 to 3 mm in size as they reach a fingerling size of about 10 g.

A popular commercial practice is to transfer the fish from the hatchery to prepared nursery ponds within a few days after feeding begins. The nursery ponds should have a good population of feed organisms and be free of predators (Dupree and Huner, 1984). The fish are fed prepared diets in the pond twice a day at the rate of 10 percent, decreasing to 3 percent, of body weight per day for the remainder of the growing season. Initially, nursery pond diets are 2- to 3-mm crumbles; later, small pelleted or extruded particles of 3 to 5 mm diameter are fed.

Most catfish produced in the United States are fed extruded diets in large ponds, 5 to 10 ha in size. Extruded diets float on the water surface and allow observation of the fish during feeding. Fish can, therefore, be fed closer to their maximum rate of consumption without overfeeding, and disease and water quality problems can be detected more easily. The use of pellets that sink can reduce feed costs by 10 to 20 percent when compared with feeding floating feeds; however, management skills must be better when feeding sinking pellets. Using sinking and floating feeds in combination (85 percent sinking and 15 percent floating) saves 10 to 15 percent in feed costs and still allows the management benefits of the floating feed (Mgbenka and

Lovell, 1985). Most commercial diets fed in production ponds contain 32 percent crude protein and 2.8 to 3.0 kcal of digestible energy/g. Li and Lovell (1991) showed that 24 percent crude protein diets allowed maximum weight gain in production ponds with satiation feeding, but 32 or 36 percent protein diets gave highest gains with restricted feeding. Most catfish farmers do not feed completely to satiation in large ponds to minimize wasted feed.

Traditionally, catfish are fed once daily, 6 or 7 days per week. Feeding twice daily when the water temperature is above 25°C will allow for a 20 percent higher rate of consumption and a comparably faster rate of growth (Andrews and Page, 1975). Feeding 7 days per week allows for 17 percent more feed to be consumed and 19 percent more growth than in a 6-day regimen (Lovell, 1979). Catfish should not be fed late at night or very early in the morning when dissolved oxygen (DO) in the pond water is low, because the increase in oxygen requirement of the fish should not coincide with the decrease in oxygen in the pond.

Table 6-1 shows daily feed allowances for channel catfish in the southern United States stocked in earthen ponds (8,800 fish/ha) in the spring and fed to near appetite capacity for a 6-month growing season (Stickney and Lovell, 1977). Factors such as temperature, water quality, fish density in the pond, and size of fish at different periods during the growing season will affect feed consumption by catfish. Therefore, the values in Table 6-1 are presented as a guide

and other values may be appropriate for other conditions. Generally, catfish do not feed consistently in ponds when the water temperature drops below 21°C. A recommended guide for winter feeding of catfish in ponds is to provide feed-size fish at a daily rate of around 0.75 percent of their estimated weight when the water temperature at 1 m depth is equal to or greater than 13°C. Fingerling fish can be fed 1 percent of body weight three times per week or daily with extended periods of warm weather. Low-protein diets (25 percent) are recommended for winter feeding of marketable-size fish (>0.25 kg) (Robinette et al., 1982).

More detailed information on feeding channel catfish is found in the comprehensive publications by Dupree and Huner (1984) on feeding practices for warm-water fishes and by Tucker and Robinson (1991) and Robinson (1991) on feeding channel catfish.

Culture systems and husbandry methods used for producing tilapia (Oreochromis and Tilapia spp.) are diverse. Because tilapia are efficient feeders on natural aquatic feed organisms, they are often produced in ponds with low-cost supplemental diets. When natural feed constitutes an important source of nutrients, nutritionally balanced feeds are not necessary; impressive pond yields have been obtained by feeding only rice bran, brewery waste, copra meal, coffee pulp, or animal manures (Lim, 1989). Natural pond feed contributes a significant amount of protein, so 24 percent crude protein is sufficient for pond diets for tilapia (Lim, 1989). The importance of micronutrient supplementation of pond feeds for tilapia is not well known.

Intensive culture of tilapia has gained popularity in recent years. Nutritionally complete feeds are needed when the fish are stocked at high densities in tanks, raceways, net pens, and ponds, and natural feed is absent or insignificant. The nutrient requirements of tilapia appear to be similar to those of other warm-water fishes (Luquet, 1991). Commercial diets formulated for channel catfish and common carp have been fed successfully to tilapia (Lim, 1989).

Physical properties of pelleted tilapia feeds, especially size and water stability, are important. Tilapia prefer smaller pellets than channel catfish and salmonids of comparable size (Kubaryk, 1980). The most common pellet size for feeding tilapia to a marketable size of 0.5 kg is 3 to 5 mm in diameter. Tilapia respond to more frequent feeding than channel catfish and salmonids because of their continuous feeding behavior and smaller stomach capacity. Kubaryk (1980) found that Nile tilapia grew faster when fed four times daily rather than twice but did not grow faster when fed eight times. Suggested feeding rates and frequencies for various sizes of tilapia in commercial cultures are given in Table 6-2.

More details on feeding tilapia can be found in publications by Jauncey and Ross (1982), Hepher (1988), and Lim (1989).

Striped bass and striped bass × white bass hybrids are important sport fish, and they are rapidly becoming a significant aquaculture fish in the United States. Larvae and juveniles must be supplied from hatcheries because inland waters do not provide essential spawning requirements (Stevens, 1966). These larvae have rudimentary digestive systems and are usually started on small brine shrimp nauplii or rotifers at day 4 to 5 posthatch. The concentration of nauplii in the rearing container is maintained at 10 to 100 nauplii per milliliter of water. Incorporation of dry larval diets of appropriate size may begin on day 5 to 8 and gradually replace all of the live food by days 14 to 28, depending on the acceptance of the dry diet and the size and condition of the larvae. A popular practice, where nursery ponds are available, is to release the larvae into prepared ponds with heavy zooplankton populations as early as 5 days after the larvae begin to feed. Clawson (1990) reported that the larvae of hybrid bass were 50 percent larger after 14 days in ponds than companion fish fed brine shrimp and a prepared diet in the hatchery.

Striped bass and hybrids are voracious feeders. They respond to multiple daily feeding and can grow rapidly (Hodson et al., 1987). They respond to diets high in crude protein (36 to 45 percent) that contain a high percentage of fishmeal (Klar and Parker, 1989). Little is known about their preferences for energy sources. Young striped bass and hybrid bass require eicosapentaenoic or docosahenaenoic acid in the diet for normal growth (Clawson, 1990; Webster and Lovell, 1990). Commercial trout and salmon diets can be successfully used for the rearing of these fish from juveniles to marketable size.

More information on the practical feeding of striped bass and hybrid bass can be obtained from publications by Hodson et al. (1987) and Brandt (1991).

The feeding of rainbow trout fry must start as soon as fish deplete their yolk sac and begin to swim up. At this time the fish are capable of consuming dry, prepared diets. Swim-up fry should be fed at least once every hour during the normal light hours, and may even be slightly overfed to ensure adaptation to dry feed as long as uneaten feed is removed regularly. Water temperature should be kept above 6°C for swim-up fry. Fish must be fed with appropriately sized granules or pellets, and, when necessary, the feed should be screened to remove particles that are too small for the fish to consume and to prevent fouling of the water. Optimum size of diet particles are 0.5 to 1.5 mm granules for 1 to 10 g fish, 2 to 3 mm granules for 20 to 40 g fish, 3 to 4 mm pellets for 50 to 100 g fish, and 5 to 7 mm pellets for fish over 200 g (Cho, 1990). Fish should be fed carefully in production units to allow all fish to have the opportunity to obtain sufficient feed for maximum growth but to avoid overfeeding. Overfeeding reduces feed efficiency and increases the concentration of nutrients in the discharge from the culture operation. Close observation by the feeder or use of an appropriate feeding guide compatible with the energy requirement of the fish without feed wastage is required to prevent overfeeding.

Daily feed allowance for rainbow trout varies with fish size, strain, water temperature, feeding frequency, and energy concentration of the diet. A number of feeding guides have been developed for rainbow trout under various management and environment conditions. In some areas of North America there is a trend toward higher concentrations of energy and nutrients in salmonid diets for economic and environmental reasons. For these diets of higher nutrient density, less feed is recommended than some of the early feeding charts recommend. Cho (1992) prepared feeding guides based on energy requirements for weight gain of fish of various sizes at different temperatures. This approach allows for an adjustment of the feed allowance for dietary energy and nutrient density. Table 6-3 shows an example of a feeding guide calculated from energy requirements of fish of various sizes at different water temperatures and to be used with a contemporary, high performance trout diet (4.06

kcal of digestible energy/g and 92 mg of digestible protein/kcal of digestible energy).

Feed may be dispensed by hand, usually twice daily, or by mechanical devices at predetermined amounts with appropriate feeding frequencies. Hand feeding offers human contact with the fish. However, mechanical feeders have the advantages that they can reduce labor costs and allow fish to feed many times during the day. These feeding devices must be carefully designed for the culture system and diet, and adjusted and serviced frequently for optimum feeding allowance.

More detailed information on the practical feeding of trout is given in publications by Hardy (1989, 1991) and Cho (1990, 1992).

Most of the Pacific salmon culture in the United States involves rearing the fish to smolt stage in freshwater hatcheries and then releasing them to migrate to the Pacific Ocean where they grow to adulthood and return to near-shore areas where most enter capture fisheries. Some Pacific salmon are grown from postjuvenile to marketable size in net pens on the Pacific coast of North America, in South America (Chile), South Australia, and Japan.

The fry are started on a meal diet (<0.6 mm), and as size increases they go to crumbles (0.8-2.0 mm), then to pellets (>2.0 mm). Fry should be fed frequently; automatic feeders that dispense feed at approximately hourly intervals are often used. Starter diets contain at least 40 percent crude protein with whole fishmeal being at least one-half of the formula. Many hatcheries feed moist pellets instead of dry pellets for better feed consumption. The Oregon Moist Pellet (OMP), the standard for moist diets, contains 30 percent pasteurized wet fish or fish hydrolysate and 28 to 32 percent moisture (Hardy, 1991). Dry compressed (pelleted) or extruded diets have replaced moist diets in many hatcheries because of reduced cost, and it eliminates the need for frozen storage. The Abernathy salmon diets have served as the basis for many commercial pelleted and extruded formulations. The Abernathy salmon diet 19-2, which is presented in Chapter 5 (Table 5-2) as a natural ingredient reference diet, contains 50 percent fishmeal. This formula has been modified to replace some of the fishmeal with soybean meal, cottonseed meal, or poultry by-product meal for a cost reduction.

Extruded feeds that float or sink slowly are frequently fed in net pens. These porous, low-density feeds will absorb more oil on the surface than compressed pellets, they will give the fish more time to consume the feed before it sinks through the net, and they will give the feeder a better opportunity to see how much feed the fish consume. Extruded feeds, however, are more expensive than compressed pellets. Semimoist diets (18 to 22 percent moisture) that do not require frozen storage are also used to feed Pacific salmon. Processing of these diets is discussed in Chapter 5. Many commercial feeders and hatchery managers claim that intermediate moisture diets, like the OMP, are also more palatable than dry diets to young salmon.

Salmon diets must contain carotenoid pigments to give the flesh a pink-red color. Traditional commercial feedstuffs do not contain these pigments, so sources of astaxanthin and canthaxanthin must be included in feeds for salmon that are grown for human food. Crustacean meals or oils, dried Phoffia yeast, and certain algae contain astaxanthin. Astaxanthin and canthaxanthin are also available as synthetic products, but are restricted-use compounds in the United States. A concentration of 40 to 50 mg of carotenoid per kg of diet fed for about 6 months is required to obtain satisfactory flesh color.

Daily feed allowance and feeding frequency vary with fish species size, temperature, economics, and moisture content of the diet. Several feeding charts have been developed for various conditions. These feeding guides are based on air-dry weight of the feed and should be adjusted when feeding moist or semimoist diets. Hand feeding twice daily is commonly used in growing Pacific salmon from postjuvenile to food-size. This offers human contact with the fish, and it also provides for more growth than once-daily feeding. Automatic and demand feeders have advantages in that they can be labor saving and they allow the fish to be fed many times throughout the day. However, these feeding devices must be carefully designed and managed because malfunction can result in wasted feed or the fish being underfed and frequent servicing can negate the economics.

Hardy (1991) and Halver (1989) present more detailed information on feeding strategies for Pacific salmon.

Atlantic salmon farming is a relatively new industry, but it provides for a major portion of the salmon cultured for human food in the world. It involves two phases: the juvenile freshwater stage and the finishing saltwater stage. The freshwater stage, during which the fish grows from fry to smoltified postjuvenile, lasts approximately 1.5 years. The saltwater phase may last for 2 years when the targeted market size is 4 to 6 kg.

Atlantic salmon fry can use prepared, dry diets as their first feed. They begin on a finely ground (<0.6 mm) mash diet and are changed to crumbles and then to small compressed pellets during the freshwater stage. The starter diets contain over 50 percent high-quality fishmeal and 10 to 12 percent marine fish oil (Storebakken and Austreng, 1987).

In spite of the commercial importance of Atlantic salmon, relatively little information is available on the nutrient

requirements for this species. Functional commercial diets have been formulated from nutrient requirement data for rainbow trout and Pacific salmon and from feeding trials with natural-ingredient diets. Both moist and dry grower diets are used. Moist diets, containing dry ingredients supplemented with raw fish parts or fish ensilage, are highly palatable but are generally more expensive to feed. Atlantic salmon accept dry, compressed, or extruded diets satisfactorily. Slowly sinking extruded diets have become popular because they absorb more oil than compressed pellets, and they give the feeder a better opportunity to observe feeding activity of the fish. Fishmeal has traditionally been the primary source of protein; however, as much as 20 percent soybean meal is used successfully in the diet formula (Helland et al., 1991). Over 20 percent lipid, as fish oil, is commonly used in commercial grower diets that contain 40 to 45 percent crude protein. This is more lipid than is used in production diets for other fish species, and it produces fish with a high concentration of body fat. The effect of such a high amount of fat on the consumer-quality attributes of the processed fish has been questioned. Approximately 50 mg of carotenoid, as astaxanthin or canthaxanthin, is added per kg of diet and fed for 1 year for satisfactory flesh pigmentation.

Austreng et al. (1988) contend that Atlantic salmon have a smaller stomach than rainbow trout and must be fed more often. They recommend 5- to 10-minute feeding intervals for fry and 30-minute intervals for parr. Automatic feeders are often used in hatcheries. Fish in net pens are usually hand fed at least twice daily.

More information on feed preparation and feeding practices for Atlantic salmon is presented by Helland et al. (1991).