Chapter 22. Practical Fish Diets

1. WET DIETS FOR RESEARCH AND PRACTICAL FEEDING
2. CATFISH FEEDS AND FEEDING
3. CRUSTACEAN DIETS
4. REFERENCES

R. T. Lovell
Auburn University
Auburn, Alabama

1. WET DIETS FOR RESEARCH AND PRACTICAL FEEDING

1.1 Research Diets
1.2 Practical Wet Diets
1.3 Feeding Lactic Fermentation Preserved Wet Diets to Catfish

Advantages of wet diets are:

(i) a pelleting machine is not needed, a food grinder will suffice;
(ii) many fish species find soft diets more palatable than dry, hard diets;
(iii) heating and drying is avoided which may prevent nutrient losses.

Disadvantages are:

(i) wet feeds are susceptible to micro-organism spoilage unless some preservation measure is followed;

(ii) oxygen sensitive nutrients such as ascorbic acid are subject to deterioration unless the wet diet is kept frozen; and

(iii) wet, non-heated animal tissues may contain an anti-thiamine enzyme.

1.1 Research Diets

Purified or semi-purified research diets are often processed as moist diets and stored frozen. In preparing laboratory research diets, the following specifications should be followed as completely as possible. All diets in an experiment should be:

(i) alike in all respects except the variable being tested;
(ii) palatable;
(iii) feedable (water stable, optimum particle size and texture);
(iv) nutritionally complete; and
(v) as far. as possible made from purified ingredients.

Casein and gelatin (5:1) are a good protein combination for purified diets. Vitamin-free casein is available for use in vitamin experiments. Blood fibrin is a desirable protein for mineral studies. Egg protein is good for use in experiments where quantitative protein requirement is a variable. All of these protein sources are available in highly purified forms.

Dextrin is traditionally used as a carbohydrate source. Starch is satisfactory for warmwater fishes, but it is not utilized as well as dextrin by coldwater species. Some of the lipid should be from fish oil to provide omega-3 fatty acids; some may come from soybean oil or, for warmwater fish, animal fat.

Purified cellulose is used as a non-nutritive binder. Research has shown that up to 20 percent cellulose fibre is beneficial in purified fish diets, and levels above this are not deleterious provided the diet contains adequate amounts of the nutrients.

Moist diets must contain a binding agent that will hold the diet particle together for a reasonable time in water. Gelatin, agar, carboxymethylcellulose (CMC) and pregelatinized starch are good binding agents. Agar and CMC have no nutritional value and are used at levels of 2 to 5 percent.

Formulations for research diets for channel catfish and salmonids that are usually fed in moist form are presented in Table 1.

Table 1 Test Diets for Fish

Ingredient	Channel catfish diet	Test diet H440 for salmonids
Vitamin-free casein	29	38
Gelatin	6	12
Dextrin	30	28
Cellulose flour	20.25	8
Fish oil	3	3
Soybean oil	3	-
Corn oil	-	6
Carboxymethylcellulose	3	-
Mineral mixture 1/	4	4
Vitamin mixture 2/	1.5	1
Calcium propionate	0.25	-

^1/ A salt mixture supplying all essential minerals for laboratory animals

^2/ Should provide the recommended vitamin allowances presented by NRC for warmwater fishes (1977) and for coldwater fishes (1973)

These diets have been used successfully with other fish species. Steps for preparing moist diets such as these are as follows:

(i) weigh each ingredient and place in separate containers; do not mix until everything is weighed for one diet;

(ii) mix the dry ingredients well before adding oil; then add oil and mix;

(iii) add approximately 350 ml of water per kg of diet mixture and stir with a dough mixer (Hobart); the moist mixture should have a stiff plastic consistency when compressed; if it does not stick together add more water; too much water will cause sticking when the extrusions come out of the food grinder;

(iv) extrude through a food grinder with proper diameter holes in the grinder plate; and

(v) break extrusions into short lengths by hand or with a sharp implement; package and freeze quickly.

Moist diets should be stored frozen until one or two days prior to feeding and should be kept in a refrigerator during this period in order to minimize loss of ascorbic acid.

1.2 Practical Wet Diets

In many areas pelleting equipment is unavailable, but the fish should receive their supplemental food in pellet form. Moist diets can be mixed and extruded through a food grinder into satisfactory pellets for feeding in water if a suitable binding agent is used. Natural feedstuffs that make good binders are fresh animal tissue and cooked starch. Blood, slaughter waste from fish, poultry or livestock, trash fish, and cooked grains or grain byproducts have been used successfully to bind moist fish feeds.

Wet, pelleted diets are made by mixing well-ground dry ingredients with ground wet materials and subsequently extruding the mixture through a food grinder with the optimum size holes in the grinder plate. Moisture content should be 35 to 40 percent; too much makes the extruded material too sticky to break up for feeding and too little reduces binding property. The combination of dry and wet ingredients depends on their availability and binding properties. For example, a combination of 50 percent trash fish and 50 percent rice bran will produce a diet mixture containing 63 percent dry matter and 23 percent protein (dry basis), with very good physical properties for feeding as wet pellets.

Wet feeds must be fed fresh or preserved against microbial spoilage. Some wet diets are frozen or refrigerated, but this is very expensive. Lactic fermentation and storage in air-tight containers is a satisfactory preservation method.

1.3 Feeding Lactic Fermentation Preserved Wet Diets to Catfish

A study was conducted at the Auburn University to evaluate lactic fermented catfish diets made to contain 60 percent catfish processing waste. The wet diets were stored in sealed containers for up to five months during the summer season. The wet diet mixture (40 percent moisture) decreased in pH to 4.5, due to lactic fermentation, within 48 hours, which prevented further bacterial decomposition. Exclusion of oxygen prevented mould growth (mould will grow at pH 4.5). The wet diet was removed from the container and extruded and broken up into pellets immediately before feeding.

Pour experimental diets were fed to channel catfish for a 5-month period (Table 2). Diet 1 contained non-pasteurized fish waste, diet 2 contained pasteurized fish waste, diet 3 was similar to 1 and 2, but was dried and pelleted, and diet 4 contained marine fish meal instead of catfish waste and was in dry pelleted form.

Average percentage weight gain and food conversion for channel catfish fed the test diets were as follows:

Diet	Percentage gain	Food conversion
A Moist, nonheated waste	377	2.1
B Moist, heated waste	460	1.6
C Dry, catfish waste	462	1.5
D Dry, menhaden fish meal	624	1.1

The fish fed the diets containing the heated or dried catfish waste gained more than those fed the raw fish waste. Thiamine analysis of the diets at the time of feeding showed diet A to be almost devoid of this vitamin, which was assumed to be the reason for the poorer growth rate of the fish. Menhaden fish meal produced greater weight gain than catfish waste because of superior protein quality.

Table 2. Composition of Experimental Diets Containing Catfish Waste, Fed as Wet or Dry Pellets, and Fish Meal, Fed as a Dry Pellet

Ingredients	Experimental diets c/
	1	2	3	4
	%	%	%	%
Catfish waste	7.60.00	60.00	60.00	-
Commercial fish meal	-	-	-	15.17
Soybean meal	8.5	9.5	8.5	8.5
Peanut meal	8.5	8.5	8.5	8.5
Wheat flour	18.7	18.7	18.7	18.7
Lard	-	-	-	5.11
Dicalcium phosphate	-	-	-	1.04
Salt	2.0	2.0	2.0	2.0
Binder (carboxymethylcellulose)	2.0	2.0	2.0	2.0
Vitamin mix a/	0.42	0.42	0.42	0.42
Water	-	-	-	38.56
Protein b/	36.9	36.1	34.7	34.5
Ether/extract b/	5.9	8.3	10.0	10.8
Ash b/	9.1	13.7	13.1	11.9
Ca b/	0.8	1.0	0.9	1.0
p b/	1.0	1.3	1.2	1.2

^a/ Vitamin mixture (mg/kg diet): vitamin A (30 000 I.U./g), 183.3; micro D, 73.7; alpha tocopherol, 119.9; ascorbic acid, 550; vitamin B₁₂, 41.6; riboflavin, 124.8; menadione sodium bisulfite, 13.7; calcium pantothenate, 187; niacin, 198; thiamine hydrochloride, 22; pyridoxine, 5.5; folic acid, 5.5; biotin, 0.44; choline chloride, 3 300; ethoxyquin, 198

^b/ Average values as percentages of dry matter, determined by chemical analyses of the diets

^c/ Diet 1 contained non-pasteurized catfish waste, fed wet,

Diet 2 contained pastuerized catfish waste, fed wet
Diet 3 contained pastuerized, dried catfish waste, fed dry
Diet 4 contained marine fish meal, fed dry;

A concurrent study was conducted to measure the rate of thiamine destruction in the moist fish food containing non-heated fish waste. After 12 hours storage at room temperature, the moist food contained 65 percent of the original thiamine level; after 7 days it contained 8.2 percent of the original level. This indicates that moist diets containing non-heated fish waste can be fed with good results if fed within several hours after mixing. If fed after periods of storage, another source of thiamine must be provided for the fish.

2. CATFISH FEEDS AND FEEDING

2.1 Nutrient Requirements of Catfish
2.2 Manufacture of Catfish Feeds
2.3 Catfish Feeding Practices

Catfish feeds represent the largest amount of commercially manufactured fish feed produced in the United States. The estimated 35 000 metric tons of farm-raised catfish produced in 1977 (Feedstuffs 1977) consumed approximately 60 000 tons of supplementary food. Almost all of these fish were produced in ponds and at least two-thirds of the feed fed was the extruded (floating) type. Feed conversion ratio in practice is near 1.7. Technology of formulating, manufacture, and feeding practical diets to catfish is advanced to the stage that highly productive and cost-efficient feeds are being produced; however, there are several areas which are still amenable to refinement.

2.1 Nutrient Requirements of Catfish

2.1.1 Protein and amino acids

The qualitative amino acid requirements of channel catfish have been determined. Catfish require the same ten essential amino acids as other finfish (salmonids, eel, carp). Quantitative requirements of essential amino acids have been partially determined for channel catfish and generally agree with values derived for other species. A notable difference is an apparent lower sulphur-amino acid-requirement for catfish than salmonids. Recommended levels of essential amino acids are presented in Table 3. The values which were not actually determined with catfish are estimated from values determined with other warmwater fish.

Table 3. Guidelines for Formulating Diets for Channel Catfish

Item		Quantity in diet
Protein, %		30-36
Amino acids, % of the protein:
	Methionine + cystine	2.8
	Lysine	5.1
	Arginine a/	3.9
	Tryptophan	0.5
	Threonine	2.2
	Valine	2.9
	Histidine	1.6
	Leucine	3.4
	Isoleucine	2.3
	Phenylalanine + tyrosine b/	5.1
Digestible energy, kcal/g of protein		7-8
Available phosphorus, %		0.5
Vitamins		c/
Trace minerals		d/

^a/ Argenine requirement determined for eel
^b/ Phenylalanine + tyrosine requirement determined for ee1
^c/ See Table 4
^d/ See Section 2.1.5

The optimum percentage of protein in fish diets is influenced by several factors such as the following:

(i) Size of fish. Fish, like land animals, have higher protein requirements during early life than during later phases of growth.

(ii) Physiological function. Less protein is needed in a maintenance diet than in one fed for a rapid growth rate.

(iii) Protein quality. A protein that is deficient in one or more of the ten essential amino acids will produce less growth than a protein that is balanced in the essential amino acids, or, more of a low quality protein is needed in the diet for maximum growth than a high quality protein.

(iv) Non-protein energy in the diet. If the diet is deficient in energy, the fish will use part of the protein to meet energy needs thus reducing the amount of dietary protein available for growth.

(v) Feeding rate. Fish fed to less than satiation, as frequently occurs in intensive pond culture of food fish, will benefit from diets containing higher percentages of protein than fish diets fed at or near the satiation rate.

(vi) Natural foods. If natural aquatic organisms contribute significantly to the daily food intake of the fish, the protein level in the prepared diet may be reduced. For example, aquatic fauna that are consumed by various fishes contain from 60 to 80 percent protein, thus, if in abundance, the supplementary diet would need only a very low percentage of protein.

(vii) Economics. The cost and availability of protein sources is a major factor in determining how much protein to use in commercial diets.

Channel catfish of small size (10 to 20 g) require 9 to 10 g of high quality protein per kg of fish per day for maximum growth when fed a nutritionally balanced diet. As channel catfish approach harvestable size (0.5 kg) the daily protein requirement decreases to 7.5 to 8.0 g per kg. If channel catfish fingerlings will consume, or are fed, at the rate of 3 percent of their weight per day, a 33 percent protein ration will provide all of their protein requirement (10 g of protein per 30 g of feed per kg of fish). However, if fed at the rate of 2.5 percent of their weight per day, they would need a 40 percent protein diet. Most warmwater fishes have protein needs similar to channel catfish, thus protein levels of 30 to 36 percent will probably be adequate for most warmwater fish diets.

2.1.2 Energy sources and requirements

Energy requirements and-availability of energy from various feedstuffs for catfish are areas in need of more research information. Digestibility studies with channel catfish have indicated that they digest uncooked carbohydrate (starch) much better than salmonids but poorer than farm animals. Digestibility of calories in fats and in proteins is high. Cooking of feedstuffs, such as occurs in extrusion processing, improves the digestibility of calories in most materials, especially those high in starch. Fibrous materials, such as alfalfa meal and rice hulls., are poorly digested by catfish. With the exception of feedstuffs high in starch and highly fibrous, digestible energy values in livestock and poultry feeding tables are generally useful for catfish.

Energy needs of fish are less than those of warm-blooded animals because:

(i) fish do not have to maintain a constant body temperature;

(ii) fish require less energy for muscle activity to maintain their position in water than animals on land; and

(iii) fish require less energy to excrete nitrogen waste products than warm-blooded animals.

According to recommendations on the nutrient requirements of farm animals by the National Research Council, USA, the optimum amount of metabolizable energy for each gram of protein in the ration is 14 to 16 kcal for poultry and 15 to 24 for swine. This compares with values of 6 to 10 kcal per gram of protein which have been found adequate for catfish rations.

Many commercial fish feeds, because of high protein percentages, are probably deficient in energy in relation to protein. The ratio of digestible energy to protein in several commercial catfish feeds was calculated and found to range from 6.6 to 7.4 kcal for each gram of protein.

Research at Mississippi State University, University of Georgia and Auburn University, has shown that the ration of energy to protein in channel catfish rations markedly affects growth. The values below represent the optimum ration of kcal of digestible energy per gram of protein for maximum weight gain by channel catfish between 30 and 500 grams in each study. Differences in the values may be due to variation in energy source or fish size.

Ratio of digestible energy (kcal) to protein (g)	Station
9.6:1	Mississippi State University
8.3:1	University of Georgia
8.4:1	University of Georgia
7.9:1	Auburn University
7.3:1	Auburn University

Catfish eat to satisfy their metabolic energy requirement and, consequently, cease feeding when their calorie needs are satiated. Because of this phenomenon, fish will eat less of a high energy diet than a low energy diet. Therefore, too much energy in relation to the percentage of protein in the ration can prevent fish from consuming enough protein to meet their daily need for optimum growth rate, even though the fish are allowed to eat as much as they will consume.

High energy diets, especially when a high percentage of the calories is from fat, will produce fatty fish which have a lower dressing percentage than fish fed a lower energy diet. Experience with channel catfish has shown that body fat content only varies with very high energy diets if the non-protein calories in the ration are mostly from carbohydrates. High carbohydrate diets have not produced the adverse effects when fed to channel catfish that have been found with salmonids. In an experiment at Auburn University, when diets fed to channel catfish contained high percentages of protein (42 percent and above) and very low amounts of non-protein energy (less than 1.5 kcal per g), growth was suppressed. When the protein level was reduced to 36 percent and the non-protein energy level remained the same, growth increased. When the non-protein energy in either the 42 percent or the 36 percent protein diets was increased, growth improved. This experiment indicated that when too many of the calories in a fish diet came from protein, the efficiency of utilization of the ration is suppressed, probably due to metabolization of the protein for energy purposes by the fish.

2.1.3 Vitamins and essential fatty acids

Channel catfish require dietary sources of thirteen of the fifteen vitamins listed in Table 4. The vitamins B₁₂ and inositol have been found to be unnecessary in diets otherwise nutritionally balanced. Qualitatively, the thirteen other vitamins have been demonstrated as dietary essential nutrients; however, the quantitative needs of all have not been elucidated. The values in Table 4 have been determined for channel catfish or other warmwater fish.

Table 4 Recommended Amounts of Vitamins in Channel catfish Diets

Vitamin	Amount mg/kg of diet
Vitamin A (I.U.)	5 000
Vitamin D3 (I.U.)	1 000
Vitamin E (I.U.)	50
Vitamin K	10
Choline	550
Niocin	100
Riboflavin	20
Pyridoxine	20
Thiamin	20
D-calcium pantothenate	50
Biotin	0.1
Folacin	5
Ascorbic acid a/	50
Vitamin B12	none
Inositol	none

^a/Approximately 20 to 30 percent is lost in pelleting and 50 to 60 percent is lost in extrusion processing. Also, the half-life of ascorbic acid in fish feed is approximately 2.8 months in a warm environment

Essential fatty acid requirements for catfish have not been determined. Although trout require omega-3 fatty acids instead of omega-6, there is speculation among researchers that warmwater fish require both omega-3 and omega-6 fatty acids. Channel catfish grow poorly on fat-free diets; they grow only moderately well on corn oil diets, but well when soybean oil, fish oil and beef tallow is the lipid source. Practical diets containing 4 to 5 percent fat from natural feedstuffs have not produced effects indicative of a fatty acid deficiency.

2.1.4 Minerals

Calcium and phosphorus: Fish absorb large amounts of calcium from the water unless the calcium content of the environment is unusually low, i.e. less than around 5 ppm as CaCO₃. Common carp, rainbow trout, and red sea bream were found to absorb enough calcium from the environment to meet their needs for growth, provided their diets were adequate in phosphorus. Eel were found to need a dietary source of calcium. As indicated in Figure 1, channel catfish benefited only slightly from dietary calcium. The data were obtained in aquarium studies with a level of 35 ppm of CaCO₃ in the water. The graph indicates that the catfish showed a marked increase in growth response to the addition of phosphorus to the diet.

Fig. 1 Weight gain of channel catfish fed calcium-deficient diet

Minimum requirements of "available" phosphorus in diets for channel catfish have been determined to be 0.4-0.5 percent using purified diets (Auburn University), and 0.8 percent using practical diet ingredients (University of Georgia). This difference is apparently associated with the availability of phosphorus to channel catfish from various dietary sources. If 0.5 percent is used as the minimum available phosphorus requirement for satisfactory growth of channel catfish, the following availability percentages are suggested for phosphorus from various common sources:

Source	Availability of Phosphorus (percent)
Dicalcium phosphate	8
Fish meal, meat, and bone meal	50
Soybean meal	40
Grains	33

Pond raised channel catfish fed an all-plant diet needed additional phosphorus in their ration for maximum growth rate, as indicated by the data in a feeding study at Auburn University. The amount of non-phytate phosphorus in the control diet was 0.22 percent. The addition of 0.3 percent phosphorus in the form of dicalcium phosphate increased the non-phytate phosphorus level to 0.55 percent which was adequate for maximum growth by the fish.

2.1.5 Other minerals

Dietary requirements for most of other minerals have not been established for fish. Natural feedstuffs are usually adequate in K, Mg, Na, and Cl for normal growth of animals unless there is a high rate of mineral loss such as in sweating. These elements are probably available in sufficient quantity in practical fish feeds without mineral supplementation. However, fish feeds low in animal products (fish meal, meat and bone meal, etc.) may be deficient in trace minerals. When less than 15 percent of the ration is composed of animal products, a trace mineral supplement is recommended to provide the following amounts of minerals in the ration (mg/kg): Mn, 115; I, 2.8; Cu, 4.3; Zn, 88; Fe, 44; Co, 0.05.

2.1.6 Guidelines for meeting nutrient requirements

The values in Table 3 will serve as a guide in meeting nutrient requirements in practical catfish feeds.

2.2 Manufacture of Catfish Feeds

2.2.1 Complete versus supplemental feeds

In many cases it is unnecessary and uneconomical to balance diets fed in ponds according to the absolute nutrient requirements of the fish. In this case, supplemental diets, which may be deficient in some vitamins or minerals, are often used to feed fish at low densities in the pond on the premise that the diet deficiencies will be made up by natural food organisms in the pond.

In cases where the fish has limited or no access to other nutrient sources, such as in heavily stocked pond cultures or in artificial culture systems (cages, raceways and the like), all of the essential nutrients must be provided in the prepared diet in adequate quantities. Such complete diets are being used more frequently in catfish culture ponds because fish density has been steadily increasing in recent years and dietary deficiencies such as the broken backs syndrome have been observed in pond culture.

Examples of practical diets for channel catfish are presented in Table 5. These formulations have been used with good results in commercial culture.

2.2.2 Formulation restrictions

Soybean meal and fish meal are used as major protein sources in all practical fish feeds in the United States. Soybean meal is the highest quality (best balanced in essential amino acids) plant protein available. Fish meal is much more expensive than plant protein sources, but a minor amount of fish meal is necessary in all practical fish feeds. A minimum of 7.5 percent fish meal has been recommended in catfish feeds. Trout and salmon diets contain much higher percentages of fish meal; closer to 20 or 25 percent of the formula.

Cottonseed meal containing gossypol is toxic to some fish when fed in large amounts. Its toxicity has not been adequately evaluated with most fishes; therefore, when gossypol-containing cottonseed meal is used in fish diets, the level should probably not exceed 15 percent of the formula.

Table 5. Practical Diets Fed to Channel Catfish in Commercial Culture

Ingredient	Skidaway, Ga. formula a/	Auburn No. 4 extruded diet formula b/
	(%)	(%)
Fish meal (menhaden or anchovy)	10	10
Soybean meal (44% pro.)	35	51.2
Corn	29	22.7
Distillers dried solubles	-	7.5
Wheat	-	5
Corn gluten	20	-
Animal fat	2.5	2
Dicalcium phosphate	3	1
Vitamin premix	0.5	0.5
Trace mineral premix	0.05	0.08

^a/ 32 percent crude protein
^b/ 36 percent crude protein, 2.88 kcal digestible energy/g

Highly fibrous feeds tuffs like alfalfa meal, soybean mill-feed and rice hulls should be avoided since they are very poorly digested by fish. Fibre is beneficial for digestibility and absorption of nutrients in purified research diets, but studies with channel catfish indicated no benefit of fibre in practical diets.

Some ingredients affect the binding or pelleting property of fish feeds. Fat reduces the binding quality in both pelleted and extruded diets; consequently, supplemental fat should be applied after the feed has been polluted or extruded. Usually, 2 percent or less of fat can be added to the ingredient mixture prior to processing without seriously reducing the binding property of the processed feed particles. Starch is an important ingredient in fish feeds for binding ingredients together. Generally, a minimum of 20 percent grain is necessary in extruded fish feeds for proper expansion during processing. Organic binding agents such as lignin sulphonates or hemicelluloses (both wood pulp byproducts) are often used in pelleted feeds, at a level of 2 to 2.5 percent, to increase water stability.

2.2.3 Pelleting and extrusion

Pelleting, through compression, produces a dense pellet that sinks rapidly in water. Extrusion is a process through which the feed material is moistened, precooked, expanded, extruded and dried, producing a low-density feed particle which floats in water. Pelleting is less expensive and generally costs 10 to 12 percent less than extruded fish feeds. However, extruded or floating feeds are very popular with catfish farmers.

Pelleting involves the use of moisture, heat, and pressure to agglomerate ingredients into larger homogenous particles. Steam or hot water added to the ground feed mixture (mash) during pelleting gelatinizes starch, which aids in binding ingredients. Generally, an amount of steam is added to the mash to increase its moisture content to approximately 16 percent and temperature to about 85 C before passing through the pellet die; however, ingredient composition will influence these conditions. The moisture must be removed by proper cooling and ventilation immediately after the pellets leave the pelleting apparatus.

Pellet quality refers to resistance to crumbling and water stability. The amounts of fat, fibre, or starch in the formula can influence quality of the pelleted feed. Some ingredients, because of chemical or physical properties, do not have desirable pelleting quality and can be used only in limited quantity in pelleted feeds.

Additives that serve primarily as pelleting aids are frequently used in fish feed formulas to reduce fines and increase water stability, although research in fish feed technology has demonstrated that high-quality fish feeds can be made without binding materials by following good pelleting procedures. However, use of compounds such as hemicellulose and cellulose derivatives, lignosulphonates, bentonites, and others does allow the processor greater variation in ingredient selection and processing conditions to produce pellets of satisfactory quality.

Extrusion requires higher levels of moisture, heat, and pressure than pelleting. Usually, the mixture of finely ground ingredients is conditioned with steam or water and may be precooked before entering the extruder. The mash, which contains around 25 percent moisture, is compacted and heated to 135 to 175 C under high pressure. As the material is squeezed through die holes at the end of the extruder barrel, part of the water in the superheated dough immediately vaporizes and causes expansion. The low-density extruded particles contain more water than pellets and require more drying. Heat-sensitive vitamins are usually added topically after extrusion and drying. Extruded feeds are more firmly bound due to the almost complete gelatinization of the starch and result in less fines than pellets.

Extruded or expanded fish feeds have two definite advantages over pelleted feeds: the particles float and are more resistant to disintegration in water, and a floating feed allows the fish culturist to observe the condition of the fish and the amount of food consumed. A large percentage of the catfish farmers in the United States use expanded feeds.

2.3 Catfish Feeding Practices

2.3.1 Pond feeding

Most catfish produced in the United States are grown in ponds 10 to 40 acres (4 to 16.1 hectares) in size. The fish are usually fed once daily, six or seven days per week. Most catfish (approximately 67 percent or more) are fed extruded (floating) feeds. This allows the feeder to observe the feeding activity of the fish which prevents over-feeding and can serve as a check for disease or water quality problems. The fish are usually fed in the morning after dissolved oxygen (DO) level in the pond has begun to rise. (Figure 2 shows the typical diurnal variation in DO in highly enriched catfish ponds caused by the respiration and photosynthesis of phytoplankton).

Feeding should not be done late in the evening, so that maximum oxygen consumption of the fish will not coincide with a decrease in DO level in the ponds, as occurs when photosynthesis stops.

2.3.2 Water temperature

The optimum temperature for food consumption and growth of channel catfish is near 86°F (36°C). Economical feeding can be done at temperatures above 70°F (21°C). Figure 3 illustrates the effect of water temperature on feeding activity of channel catfish in ponds during various parts of the season. When minimum (morning) water temperature was above 26°C, feeding two times daily resulted in maximum food consumption and growth; when morning temperature was 22-26 °C, once per day feeding was optimal; and when morning water temperature was below 20°C, alternate day feeding resulted in the highest food consumption. These data indicate that catfish should be fed twice daily as soon as water temperature warms up in the early summer and fed on this schedule until the water temperature begins to decrease in the fall.

2.3.3 Winter feeding

Several studies have been conducted on feeding regimes for catfish held in ponds to overwinter. Winter feeding can effect a small weight gain and will increase disease resistance of overwintered fish. Results of several studies at Auburn University indicate that feeding 1 percent of fish weight on days that water temperature is above 12°C is the best programme for fish 0.4-1.0 pound (454 g) size. Fingerlings may be fed at a rate of 1 percent of body weight, three times weekly throughout the winter.

Fig. 2 Typical daily variation in dissolved oxygen content in intensively fed channel catfish ponds

2.3.4 Feeding rate in ponds

Fingerling catfish will consume 4 percent or more of their weight in food daily, whereas catfish near 1-pound (454 g) size will consume only approximately 1.5 percent of the weight per day. Data in Table 6 represent the daily amount of food consumed (percent of body weight) by channel catfish fed to satiation with two extruded (floating) diets. The high energy-protein diet was less bulky than the lower energy-protein diet and the fish were able to consume more of it when appetite was greatest (during the early part of the season). The data in Table 6 show the decrease in food consumption as fish size increases.

A study at Auburn University revealed that feeding catfish to 87 percent of satiation level produced as much growth and was more economical than feeding the fish to satiation when a 36 percent protein diet was fed, but not when a 30 percent protein diet was used. Apparently, feeding at slightly below the satiation level and using a high protein diet is the most beneficial practice, since it results in more efficient food consumption.

Fig. 3 Food consumed per 2-week period by channel catfish according to three feeding schedules (Horizontal bars, once daily; solid line, twice daily; diagonal bars, alternate days) for 16 weeks in earthen ponds

Table 6 Weight (G) and Volume (Cm) of Food Consumed per 100 grams of Body Weight by Channel catfish Fed Diets of Two Nutrient Densities to Satiation

Date	6/30-7/13		7/14-7/27		7/28-8/11		8/12-8/25		8/26-9/8		9/9-3/22		9/23-10/6		10/7-10/17
Temp. °C 1/	29.2		31.1		30.5		29.8		28.1		24.7		23.6		19.16
Diet	g	cm3	g	cm3	g	cm3	g	cm3	g	cm3	g	cm3	g	cm3	g	cm3
High energy 2/	3.3	3.7	3.0	3.2	2.4	2.6	2.1	2.3	1.9	2.1	1.7	1.9	1.8	1.9	0.9	1.0
Low energy 2/	3.3	3.9	2.8-	3.4	2.1	2.5	2.1	2.6	2.0	2.4	2.0	2.4	2.0	2.4	1.3	1.5

^1/ Average afternoon temperature at 1 metre depth
^2/ Mass density of the diets was 0.91 g/cm³ for the high energy diet and 0.83 g/cm³ for the low energy diet

3. CRUSTACEAN DIETS

Crustaceans, such as marine shrimps and freshwater prawns (Macrobrachium sp.), are becoming important species for commercial culture. The nutritional requirements of these animals appears to be similar to that of finfishes in many respects; however, some differences have been noted. For example, higher dietary levels of cholesterol are beneficial for crustaceans, and intestinal synthesis of amino acids appears to be significant.

Because shrimp and prawns are slow feeders, diets that remain stable in water for several hours have been developed for feeding tests with these animals. Pregelatinized starch (in extrusion cooked feeds), alginates, carboxymethylcellulose and other hydro-colloidal materials with good binding properties have been used in these diets. Extrusion-processed shrimp diets are commercially available.

In spite of the apparent need for water-stable diet particles for feeding crustaceans, a fair degree of success has been achieved in feeding freshwater prawns in ponds with pelleted poultry diets which have very poor water stability. A study at Auburn showed that a pelleted diet, which was water stable for no longer than 15 to 20 minutes, produced almost the same weight gain in densely stocked freshwater prawn ponds as the same diet formula in extruded form, which had a stability time of 6-12 hours. It is speculated that the prawn receives a significant nutrient contribution from consuming microorganisms growing on the surface of the food particles of the disintegrated pellets in the pond.

In a recent study at Auburn University, a formula similar to that used for a practical catfish diet was processed in three forms for feeding freshwater prawns. The diet mixture was extruded into high density (non-floating) cylindrical particles 3 mm diameter × 5 mm long and dried. Another mixture was extruded as described, but was not dried; it was coated with anti-caking and anti-mould compounds and fed as a soft diet. The diet was also fed in pelleted form. The dry and moist extruded forms have good water stability and the pelleted form disintegrated quickly in water.

The prawns were fed in plastic lined pools for 89 days and grew from an average size of 0.9 g to 6.4 g. There was no significant difference among weight gains of prawns fed the diet processed in the three forms. Prawns fed a commercial extruded prawn diet gained slightly but not significantly (P>0.05) more. Average food conversion for all treatments was 2.5 g weight gain/g food fed.

These results indicated that soft food particles or pellets with a high degree of water stability had no advantage over conventional pellets for prawns.

Table 7. Average Growth Rate and Food Conversion of Freshwater Prawns Fed a Diet Processed in Three Forms and a Commercial Diet

Diet 1/	Gain		Food conversion, g gain/g fed
	Weight	Length
	(g)	(cm)
Extruded, dry	6.41	7.24	2.58
Extruded, moist	6.35	7.00	2.61
Pelleted	6.35	7.21	2.61
Commercial diet	6.97	7.56	2.37

^1/ Ingredient formula of the experimental feeds was: corn-wheat flakes, 47.6 percent; soybean meal (49 percent protein), 20.6 percent; anchovy fish meal, 18 percent; meat and bone meal, 4.6 percent; dried whey, 1.9 percent; animal fat, 6 percent; dicalcium phosphate, 0.7 percent; and a vitamin-trace mineral premix, 1.3 percent.

4. REFERENCES

National Research Council, 1973 Subcommittee on Fish Nutrition, Nutrient requirements of trout, salmon and catfish. Washington, D.C., National Academy of Sciences, (Nutrient requirements of domestic animals), 11:57 p.

National Research Council, 1977 Subcommittee on Warmwater Fishes, Nutrient requirements of warmwater fishes. Washington, D.C., National Academy of Sciences, (Nutrient requirements of domestic animals), 78 p.

Stickney, R.R. and R.T. Lovell (eds), 1977 Nutrition and feeding of channel catfish. South.Coop.Ser.Bull., (218):67 p.