Preface
This bulletin is a revision of MAFES Bulletin 1090 (1999) on catfish protein nutrition. It is a bit more comprehensive than the previous bulletin, in that it provides more information on the general aspects of protein nutrition in addition to data from specific studies that we have conducted. The bulletin is intended for fish nutritionists, catfish producers and feed manufacturers, aquaculture scientists, and students.
Introduction
Proteins are organic compounds composed of amino acids that comprise about 70% of the dry weight of fish muscle. Since protein is in a dynamic state, continually being synthesized and degraded, a dietary supply is needed throughout life to provide amino acids and nonspecific nitrogen for maintenance and growth. There is no single level of dietary protein that is optimum for meeting the nitrogen needs of catfish. This is because several factors affect the dietary protein requirement, including fish size, water temperature, feed allowance, amount of nonprotein energy in the diet, protein quality, natural food available, and management practices. Even so, catfish are typically fed the same diet throughout the grow-out phase. This is largely because of logistical issues associated with manufacturing and supplying a number of different diets. Further, there is a general perception that protein quantity equates to feed quality. That is, the more protein the better the feed; thus, the producer may feed a high-protein feed throughout grow out even though it may not be needed. In reality, the actual percentage of dietary protein is not as critical as the concentrations and proportions of amino acids provided in the protein. To ensure that catfish feeds are of high quality, the nutritionist must consider the amino acid composition of various feedstuffs, their digestibility, and the proper balance of protein and energy. The most economical method to achieve the proper balance between protein quantity and quality is to use a mixture of complementary protein feedstuffs and (if needed) supplemental amino acids.
Feeds used for grow out of catfish have traditionally contained 32–35% dietary protein. Commercial catfish feeds contain a liberal amount of relatively expensive, high-quality protein, and feed cost is the major variable operating cost associated with production of catfish; therefore, considerable effort has been expended to determine the quantity and quality of dietary protein necessary to achieve optimum performance of catfish. A short overview of various aspects of catfish protein nutrition is presented in this bulletin, along with data from our studies on protein nutrition.
Methods
The research reported in this bulletin was primarily conducted in ponds, but some data are from studies conducted in laboratory growth trials. Pond studies reported here were conducted in small (0.1–1 acre) research ponds using management practices that reflect those used in the industry. Most of the studies were conducted for a single growing season, but some were multiyear studies. The laboratory studies were conducted using well-established procedures for feeding studies with catfish. Generally, small fingerlings (4–6 grams) were raised in glass aquaria (4–5 aquaria per treatment) in a flowing-water, temperature-controlled system for 8–12 weeks. Additional details on the experimental design of specific studies are described in the footnotes of various tables, and a full description can be found in respective references.
Amino Acids
Although we speak of a protein requirement, it is more precise to formulate fish feeds on the basis of amino acid requirements. Nutritionally, amino acids may be classified as either indispensable (essential) or dispensable (nonessential). An indispensable amino acid is one that the animal cannot synthesize or cannot synthesize in quantities sufficient for body needs; thus, it must be supplied in the diet. A dispensable amino acid is one that can be synthesized by the animal in quantities sufficient for maximal growth. Most simplestomach animals, including catfish, require the same 10 indispensable amino acids (Table1). There are differences between the amount of a specific amino acid required among various species of animals as well as among fish species, but that would be expected since the relative proportion of structural proteins may vary between species as well as physiological needs for certain amino acids.
Catfish can synthesize dispensable amino acids, but there are certain advantages if the nutrients are provided in the diet. For example, energy is saved in their synthesis and some dispensable amino acids can partially replace an indispensable amino acid (cystine can replace about 60% of the methionine, and tyrosine can replace about 50% of the phenylalanine). Practical catfish feeds typically contain liberal amounts of dispensable amino acids inherent in the proteins of various feedstuffs.
In a practical feed, amino acid requirements are best met by feeding a mixture of feedstuffs or by using a mixture of feedstuffs supplemented with amino acids. All 10 essential amino acids have to be present in the diet for growth to occur. If the diet contains an amino acid at a level below the requirement level, growth is reduced; thus, that amino acid is considered the limiting amino acid. Properly formulated catfish feeds with common feedstuffs provide all amino acids in sufficient quantities for maximum growth. If an amino acid deficiency in a feed formulation did occur, it is generally not a problem because it can be overcome by supplementing the diet with the deficient amino acid or by using a feedstuff high in that particular amino acid. Additions of supplemental amino acids are only beneficial if the diet is deficient in that particular amino acid. There appears to be no benefit to adding supplemental amino acids to diets that contain sufficient levels of amino acids from feedstuffs (Table 2). In practice, lysine is currently the only supplemental amino acid used in commercial catfish feeds, although certain other amino acids are commercially available. In reality, if the lysine requirement of a catfish diet is met by using common feedstuffs, all other amino acids are present in amounts that meet or exceed their requirements.
Feedstuffs contain a characteristic level of an amino acid, but all of it may not be available for use by the animal. Thus, it is more accurate to formulate catfish feeds based on available amino acids rather than using the total amount contained in a particular feedstuff. Generally most feedstuffs have relatively high amino acid availabilities (80–90%); however, there are exceptions. For example, only about 66% of the lysine contained in cottonseed meal is actually available to catfish. This is because part of the lysine in cottonseed meal is chemically bound to gossypol, a yellow pigment found in the meal, rendering it unavailable to the animal.
Energy-to-Protein Ratio
Absolute energy requirements for catfish are not known. Estimates of the requirement have been determined by measuring weight gain or protein gain of catfish fed diets containing a known amount of energy. Energy requirements reported for catfish, which have generally been expressed as a ratio of digestible energy (DE) to crude protein (DE/P), range from 7.4–12 kcal/gram. Our work supports previous data in that a DE/P ratio of 8.5–9.5 kcal/gram is adequate for use in commercial catfish feeds (Tables 3–9).
Protein Requirement
Catfish do not require as much dietary protein for maximum growth as has typically been assumed. We examined dietary protein levels from 10–40% in various studies and found no differences in weight gain and feed conversion in fish fed diets containing as low as 24% protein (Tables 3–22). In addition, we found that fish fed 16–20% dietary protein had 80–90% of the growth of fish fed a 32% protein diet. However, as dietary protein decreased, the DE/P ratio increased beyond the recommended range, resulting in an increase in visceral and fillet fat. This effect is quite dramatic in diets containing very low levels of dietary protein. Fish fed a 16%- protein diet grew well but had about twice as much fillet fat as similar sized fish raised on a 32%-protein diet (Table 5). The primary problem with increased fattiness is that too much fat may reduce processed yield. There are many factors other than diet that affect processed yield, but in general fish that are fed 26% or less dietary protein will have reduced processed yield. The data are not as consistent with fish fed a 28%-protein diet, since some studies show a reduction in processed yield and some do not. Overall, there are minimal differences in processed yield of fish fed a 28%- or 32%-protein diet, but the difference may be significant to catfish processors. Diets containing 35% protein or above may result in a decrease in fattiness compared with fish fed a 28%- or 32%-protein diet. This approach to reduce fattiness and increase processed yield may not be economical for the catfish producer, but if the producer is paid a premium for higher processed yield, it may be a more attractive proposition.
Low-protein diets are generally recommended for stockers and food fish, but not for small fingerlings. However, there were no differences in production of fingerling catfish (initial weight: 23 pounds per 1,000) stocked at 100,000 per acre and raised to 100 pounds per 1,000 using either a 28%-, 32%-, or 41%-protein diet (Table 19). It would appear that high-protein diets offer no advantage for fingerlings from about 4 inches in length and larger.
Protein requirements did not differ among catfish strains [Mississippi normal, USDA102, USDA103 (now, NWAC103), and Norris] (Tables 20–22). Thus, it appears that for commonly used catfish strains, dietary protein requirements are similar.
Protein Quality
The quality of protein sources used in catfish feeds must be taken into account to ensure that amino acid requirements are met. Protein quality is dictated primarily by the concentration of indispensable amino acids in a protein and by the biological availability of the amino acids to catfish. Proteins of animal origin, particularly fish meals prepared from whole fish, are considered to be nutritionally superior to proteins of plant origin. This is because animal proteins generally contain a higher level of indispensable amino acids and are more highly digestible by catfish than plant proteins. Commercial catfish feeds typically contain relatively high levels of protein supplied in part by animal protein supplements, but there is a growing body of evidence that animal protein can be reduced or eliminated in feeds for food-sized catfish (Tables 6, 9, 10, 11, 12, and 23). All-plant diets based on soybean meal, wheat, and corn can be formulated to meet the amino acid requirements for grow out of catfish (Table 24). It should also be noted that all-plant diets require additional supplemental phosphorus and the use of a mineral premix. The advantages of using all-plant diets include lower feed cost, milder flavor of fish flesh, and less body fat because of a reduction in dietary energy.
Protein Feedstuffs
Feedstuffs containing 20% crude protein or more are considered protein supplements.Animal proteins used in animal feeds come from inedible tissues from meatpacking or rendering plants, milk products, and marine sources. Those typically used in catfish feeds include fish meal, meat and bone meal, a blend of meat and bone/blood meal, blood meal, and poultry by-product meal.
Fish meal can be replaced (in part or totally) by poultry by-product meal, meat and bone/blood meal, catfish offal meal, hydrolyzed feather meal plus lysine, or a combination of these protein sources (Tables 25 and 26). Care must be used to ensure that these products are of high quality, because processing methods can have a significant effect on their quality. Although these products are good sources of protein for catfish, their use depends on cost and availability. Further, since the perception of using beef products to feed catfish may be detrimental to marketing catfish products (because of its implication of “mad cow” disease), we do not recommend the use of beef by-products in catfish feeds.
The primary plant protein sources used in catfish feeds are oilseed meals, such as soybean meal, cottonseed meal, and peanut meal. Certain other oilseed meals could be used but are not generally available on a timely basis and at an economical cost per unit of protein. Compared with animal proteins, most plant proteins are deficient in lysine and methionine, the two limiting amino acids in catfish feeds. Also, certain plant proteins contain toxins and antinutritional factors that may or may not be inactivated during processing of the meal.
Cottonseed meal, canola meal, and distillers’ grains can be used to replace part or all of the soybean meal (Tables 27–31). If the inclusion rate is higher than 20–25% of the diet, then the diet may need to be supplemented with lysine. The use of these ingredients will depend on their cost per unit of protein. Also, corn gluten feed can be used to replace up to 50% of the corn (Table 32). This will help reduce fattiness because of the decrease in dietary energy resulting from replacing corn with corn gluten feed. Levels of the yellow pigment xanthophyll in corn gluten feed are generally similar to levels in corn grain; thus, it does not result in yellow coloration in catfish fillets.
Dietary Protein and Feeding Rate
Generally, low-protein diets are effective when fish are fed to satiation, but higher-protein diets are required when feed is restricted. It appears that a 28%-protein diet provides for good growth at stocking rates not exceeding 10,000 per acre and feeding rates of 80 pounds per acre per day or more (Tables 13a–15). Also, a 28%-protein diet can be fed throughout the growing season. We find no advantage to begin feeding in early spring with a higher-protein diet and then reducing dietary protein as feeding activity increases. Higher-protein diets (32–35%) may increase net production and weight gain when feeding every other day compared with daily feeding (Tables 17a, b), but the value of this practice may or may not be economical. Regardless of diet, feeding every other day improves feed efficiency and reduces aeration time, but weight, carcass yield, and fillet yield are reduced compared with fish fed daily.
High-Protein Finishing Diets
Finishing catfish on high-protein diets to reduce body fat does not appear to be an effective strategy (Table 16). That is, there is no difference in body fat of fish fed a 28% protein diet for most of the growing season and then fed a high-protein diet for 30–90 days before harvest compared with fish fed the 28%-protein diet for the entire growing season. There were also no differences in fish fed a 28%- or 32%-protein diet for 150 days compared with fish finished for 30 or 60 days on 35%- or 38%-protein diets. Basically, there were no advantages to using high-protein finishing diets.
Dietary Protein and Stocking Rate
There were no differences in fish production using a 28%- or 32%-protein diet when fish were stocked at 6,000, 12,000, or 18,000 per acre (Table 18a). Further, production increased with stocking density, but the fish were smaller. Over two growing seasons, fish weight was 2, 1.5, or 1.4 pounds for fish stocked at 6,000, 12,000, or 18,000 per acre, respectively. Feed efficiency also decreased at the higher stocking densities. Fish fed the 28%-protein diet was slightly fatter than those fed the 32%-protein diet, but there was no effect on processed yield (Table 18b).
Dietary Protein and Disease Resistance
Neither the level of dietary protein nor the source of dietary protein affected the response of catfish exposed to Edwardsiella ictaluri, the bacterium causing enteric septicemia of catfish (ESC) (Table 33). Fish fed diets containing 28%, 32%, or 36% protein without animal protein or with fish meal, meat and bone/blood meal, or a combination of the two sources did not differ in survival after challenge with the bacterium. All diets were formulated to meet all nutritional requirements of catfish; thus, no difference would be expected unless dietary protein was a factor. Basically, this illustrates that the nutrients needed for immune response and growth can be supplied by various feedstuffs.
Summary
Catfish food fish have traditionally been fed relatively high-protein diets (32–35%), but they grow just as fast and convert feed as efficiently on diets containing as low as 24% protein. However, fattiness may increase and processed yield may decrease as dietary protein is reduced.We recommend a 28%-protein diet for catfish stocked at rates not exceeding 10,000 per acre and fed daily at a rate of no less than 80 pounds per acre per day because it provides for excellent growth and there is minimal impact on fattiness and processed yield. If fish are fed less than daily a higher-protein diet may be beneficial. For example, a 32%-protein diet provided for additional gain compared with lower-protein diets when fish were fed every other day. However, weight gain was reduced compared with fish fed daily.
We also recommend a diet based entirely on plant proteins for growing catfish food fish.Although animal protein has been thought of as being indispensable to catfish, data presented herein show that it is not needed for catfish food fish. Some animal protein is still needed in catfish fry and fingerling diets.
There is no single feedstuff that can provide all the nutrient needs of catfish; thus, a mixture of feedstuffs is used. Soybean meal generally provides most of the protein in a typical commercial catfish diet, but various other protein sources can be used to replace a part of the soybean meal. Cottonseed meal and distillers’ grains are two such feedstuffs. If these products are used at levels exceeding 20–25% of the diet, the diet may need to be supplemented with lysine. Most commercial catfish diets are fairly similar, but yet there can be differences in the actual composition of a specific diet. This is because there are many variations on the mixture of feedstuffs that can be used to supply the nutrients and energy needed by catfish.
Increasing dietary protein did not improve fish performance as stocking density increased. This was true for fish fed to satiation and stocked at rates up to 18,000 fish per acre.As stocking density increased, fish average weight decreased, as did the number of market-sized fish.
It appears that catfish strains commonly used in commercial catfish culture have similar protein requirements. Also, increasing dietary protein did not improve the catfish’s resistance to Edwardsiella ictaluri.
To view the tables, please click here (PDF)
June 2007
