Different species of fish have various preferences or habits in consuming food. The position in the water (e.g., bottom, midwater, or surface), color, odor, texture, shape, and size, and other factors control their desire and/or ability to consume feed and flourish.
Fish exhibit diversity in food gathering but may be classified into a limited number of feeding types (Hyatt 1979). Salmonids prefer floating or slowing
1 Throughout this chapter the term “fish” refers to both finfish and shellfish.
falling particles, whereas eels and crustaceans are bottom feeders. Channel catfish are normally bottom feeders but can be trained to consume salmonid- type feeds (Mgbenka and Lovell 1984). Although bottom feeding is natural and beneficial for some species, others, such as salmon, waste feed and grow poorly when forced to feed off the bottom of tanks and pens.
Of particular importance to the future of aquaculture, especially shrimp production, is the feeding of larvae to eliminate reliance on the capture of berried females and postlarval forms. Although many larval species have been successfully grown on live food organisms, this source of feed is ex- pensive and sometimes inadequate. Research to develop water-stable diets for larvae continues (Pigottet al.1982; Pigott 1999c).
11.2.1.2. Nutritional Components
A properly compounded diet must contain adequate energy as well as all of the essential amino acids, vitamins, minerals, and other required growth factors. Quite often the quantitative requirements for these compounds are not known, and therefore, a trial-and-error approach to formulation of diets has been employed. A diet is formulated to give maximum growth and yet be economically available to commercial operators. A major problem with selecting components for feed formulation is balancing the nutritional re- quirements of the fish with the availability and cost of the ingredients. This is the essence of feed formulation, and the balance between optimum nutri- tion and economy will change with the life-history stage and the purpose of fish rearing, e.g., food production, broodstock, and fish raised for stocking.
11.2.1.3. Aquaculture Environment
Many environmental factors must be considered in preparing feeds. This is particularly true in formulating and manufacturing “special feeds,” where these factors have a major impact on the cost and efficiency of feeding fish, regardless of the diet. The water temperature and temperature gradient, water flow rates, salinity, turbidity, depth, bottom and side conditions, ma- terials of construction, fish density, fish size and size distribution, and state of the growing cycle all have a bearing on the requirements for specific physical and chemical properties of a feed.
Ecological preservation has become a worldwide concern. Aquaculture is the focus for many groups opposing pollution of the environment. Metabolic wastes and feces, as well as uneaten feed, contribute to and are blamed for environmental pollution. Additionally, wasted feed increases production costs and pollution affects animal health. Several possibilities exist and are being researched to reduce the pollution associated with aquaculture. As more accurate data are assembled regarding actual nutrient requirements for each species, feeds can be formulated to increase the retention of specific
polluting ingredients (e.g., nitrogen and phosphorus), thereby lowering the amount not retained and discharged into the aquatic environment. Increas- ing palatability will decrease feed waste, as will improvement of management techniques. Use of feeding trays, for instance, is one recommended method.
More careful attention to feeding times and appropriate feeds for specific stages of maturation is also ecologically friendly (Lee and Lawrence 1997).
The best possible feed will yield poor results if it is not handled, stored, and used properly.
Much of the research on reducing the use of polluting ingredients has been with salmonid feeds. Partial replacement of fish meal with corn or wheat gluten yielded a low-phosphorus feed which created no flavor changes in the flesh of rainbow trout (Skonberget al.1998). Maximum retention of both phosphorus and nitrogen from commercial feeds will reduce both in effluent. Sugiura (1998) found that citric acid increased the availability of many minerals in fish meal-based diets and also increased the effect of supplemental phytase in soybean meal-based diets, thereby reducing the phosphate pollution to the environment. In Tahiti and New Caledonia, intensive shrimp culture has increased the need for a “low-pollution” diet.
Research to decrease nitrogenous waste by reducing crude protein levels in practical diets is being conducted (Bailletet al.1997).
The high cost and dwindling supply of fish meal necessitate the use of alternative protein feed ingredients. Nutritionally, the best feed ingredients have a composition similar to that of the animal being fed. This is especially important with essential amino acids. Numerous studies report partial sub- stitution of fish meal in diets of finfish and crustacean species. Salmonid diets with varying percentages of plant products such as wheat gluten meal, corn gluten meal, soybean meal, and soy protein concentrate have been reported (Hardy 1996; Haardet al.1996; Krogdahlet al.1994; Kaushiket al.
1995). Growth responses of shrimp at various maturation stages for feeds containing different protein sources have been measured (Sadhana and Neelakantan 1997; Che 1992; Stickneyet al.1996; Akiyama 1991; Limet al.
1997).
11.2.1.4. Physical and Chemical Properties of Feeds
Many of the feed components being used in conventional fish diets are marginal contributors to the nutritional value because of the chemical or physical state in which they are fed. Most of these limiting conditions can be altered by judicious selection of processing techniques used to prepare feed ingredients. Considering the relatively few basic processes and operations available for preparing feed components or formulations, it is important to combine the various methods of manufacturing to maximize the feed qual- ity. Not only is it important that fish can efficiently metabolize the ingested
feed, but the nutrient components must be stable under the conditions that are encountered during shipping, storing, and feeding. Some of the factors that limit the efficacy of a fish feed are as follows.
(1) A particle size larger than the mouth or esophagus of a fish or larva that cannot tear or chew the particles to reduce the size.
(2) Leaching of nutrients by the water before the fish can eat the feed.
(3) Selective leaching of specific important nutrients by the water.
(4) Severely reduced nutritional properties of a diet or its components due to uncontrolled processing.
(5) A short shelf life of feeds.
(6) Poor binding characteristics of feeds.
(7) Improper density, odor, taste, appearance, etc.
As is true of all animals, sensory responses control the acceptability of a particle of fish feed. These include visual perception, electrodetection, mechanoreception (sound and turbulence), chemoreception, odor, and taste (Hyatt 1979). Affected stimuli are size, movement, shape, color and contrast, and odor. These factors in a feed are related to the feed composi- tion and manufacturing techniques.
Search behavior is adaptive and related to the particular sensory capac- ities of each species. For example, coprophagy is an important element in feeding many caridian and decopod crustaceans. Panaeid shrimp feed most readily on small-diameter, ribbon-like feeds resembling fish and invertebrate fecal materials (Costa-Pierce and Laws 1985).
11.2.1.5. Water Stability
One of the most important requirements for a feed is that it remains intact, without losing any nutrient components, for a specified period after it enters the water. Since the length of time for which the diet should re- main intact in the water depends on the fish species and growing facility, requirements for stability vary for different feeds. An extreme case can oc- cur with crustaceans, which are nibblers and intermittent feeders and often allow food to remain on the bottom for several hours before consumption (Goldblattet al.1979).
11.2.1.6. Particle Density
The density of a diet particle controls its position in the water, an impor- tant consideration depending on whether the species being fed is a surface or bottom feeder or prefers to consume feeds in the water column. The conventional compacted pellet is dense and quickly settles to the bottom.
Hence, special density control is an important factor in the preparation of diets. One type of floating ration is produced by extrusion during which
starch binders gelatinize and trap air. Utilization of this bulky extruded feed is more efficient (Mgbenka and Lovell 1984).
11.2.1.7. Color and Contrast
Adding color pigmentation to simulate natural foods can control the visual perception of a feed particle by the fish. Colors can also be used to cause light refraction that simulates motion of live prey (Meyers 1979).
Color can be added as an inert additive or can be from a material that also includes volatile compounds to give additional attraction. Krill acts in the latter manner, supplying both the reddish attractant color and the natural krill odor.
11.2.1.8. Odor
In a symposium review, Meyers (1979) summarized the effects of feed odor on the attraction of various species. Catfish are attracted by amino acids, especially alanine. Turbot show a preference for inosine and inosine monophosphate. Fish oil attracts some but not all fish. Carp are less respon- sive to odor and more responsive to color and shape.
Some attractants for crustacea are powderedArtemia,shrimp by-products, glucosamine, and glycine. Feeding stimulants are low molecular weight com- pounds such as certain amino acids, peptides, and betaine (Teshimaet al.
1993; Tidwellet al.1998). Certain amino acids and nucleotides added to feed improve shrimp flavor and increase market desirability (Kanazawa 1997).
Adult shrimp seem to respond to chemicals released by prey, while larvae, which use new food sources with each stage, respond to a broader range of feeding cues (Lee and Meyers 1997). Intensive culture of shrimp carries with it an increased risk of disease. Since antibiotic use is limited, alter- natives such as antioxidant nutrients and omega-3 highly unsaturated fatty acids (ω-3 HUFA) can increase the immune response and disease resistance.
Carotenoid pigments, especially astaxanthin, must be included in shrimp diets not only to provide coloration, but also to provide a metabolically ac- tive antioxidant. The stress of intensive and semiintensive shrimp culture requires antioxidants for disease resistance, optimal growth, and coloration (Meyers and Latscha 1997).
11.2.1.9. Digestibility
Obviously, no matter how well formulated nutritionally, feeds are of no use unless they are digestible and appealing. Additionally, due to environ- mental needs for low-pollution feeds, they must be highly assimilated by the animal being fed. Poor digestion is the most important factor limit- ing nutrient availability. A number of factors influence digestibility such as species and age, environment (salinity, temperature, hypoxic conditions),
palatability, fiber content, and feed processing. How the feed is processed is significant in relation to nutrient availability. Some binders contribute compounds which act as negative feeding stimulants.
11.2.1.10. Particle Texture, Size, and Shape
A major problem with formulated or synthesized diets is the optimization of certain physical properties of the individual feed particle to ensure that it is not only attractive but also ingestible by the fish. Texture is important to the control of density, water absorption, and leaching, as well as other physical factors that alter the acceptability of a feed particle. Larvae can consume larger particles of live zooplankton than those in a dry diet, which are neither elastic nor compactable after swallowing (VanLimborgh 1979).
This property of deformation or collapsibility is most difficult to incorpo- rate into a manufactured feed particle, particularly a dry particle that has a “toughened” texture due to drying. Many fish larvae have esophagus dia- meters of 50μm or less. A larger particle may be attractive to the fish as a food but not available to the individual that cannot chew or tear the particle apart. Conventional machine reduction of particle sizes to less than 50μm is difficult, especially in dry feeds. However, homogenization of a slurry (e.g., hydrolyzed fish protein) followed by drying and sizing of the resulting flakes can enable the manufacture of complete feeds for larvae (Pigottet al.1982;
Tucker 1999).
The geometry of the particle, as well as the texture and composition, is important as related to the amount of leaching that takes place in the water.
Extremely small particles have a much larger surface area per unit weight of the feed. Also, those of an extremely irregular geometry have more surface area than round particles. Physical parameters that increase surface area also increase leaching of unstable ingredients, unbalancing the nutritional value of the portion of the diet that is consumed.
11.2.1.11. Storage Stability
The best diet for fish is, like that for humans, one that is prepared fresh every day. However, this is not practical in most situations involving the rearing of large quantities of fish. Therefore, feeds that must be purchased and stored for periods of time must be stable under the storage conditions.
These, of course, vary in time, temperature, and humidity with both the time of the year and the geographical location.
The economic viability of commercial aquaculture is dependent on the use of inexpensive rations. This is best accomplished in the mass manu- facture of feed by dry diets that are easy to transport and store. Also, the nutritional components must be stable over the shelf life of the products.
These requirements can be met by a combination of obtaining the best
ingredients for a feed, using good manufacturing procedures, and packag- ing the final product properly. A dry diet stored in a cool, dry environment in a package that allows minimal exposure to the air will normally be stable for long periods of time.
Improper handling and storing of feed by on-site hatchery personnel can negate good processing methods. Maintaining the nutrient quality of the feed during on-site storage is equally as important as all other factors involving the quality and effectiveness of a given formula.