SALMONID FOODS AND FEEDING

by

Jan Holicky

Anatomy of the alimentary canal in salmonid fishes

The alimentary tract is a relatively simple tube. It starts at the mouth where the teeth are designed only for capture of prey, not for chewing. Ingested food quickly passes down via the oesophagus into the stomach, a V-shaped organ which can expand greatly to accommodate large meals.

In the stomach the food is broken down by the action of digestive enzymes and by the rhythmic contraction of the muscles. At the posterior end of the stomach, where it joins the small intestine, there is a group of blind-ended sacks called pyloric caeca. These usually number 30–80 and are covered with fatty tissue. From the stomach, food passes through a one-way valve into the intestine.

Associated with the digestive tract there are two very important glands. One, the liver, is a large organ situated just in front of the stomach. It is pinky-brown in colour, soft and easily ruptured.

The liver is the main organ of intermediate metabolism in the body. Nutrient molecules are transported in the blood from the intestine for manufacture into the proteins, carbohydrates and fats of the fish's body. On the top of the liver there is a small greenish sack, the gall-bladder. It contains bile, which is secreted into the intestine through bile ducts.

The other important digestive organ is the pancreas. In salmonids this has a very diffuse structure and cannot be seen with the naked eye because it is scattered throughout the fatty tissue surrounding the pyloric caeca. The pancreas has two functions: the production of pancreatic enzymes, which pass via the pancreatic duct into the intestine, and the production of insulin, which controls sugar and protein metabolism and prevents the fish from becoming diabetic.

Physiology of nutrition

The food is not chewed in the mouth. Instead it passes quickly through the oesophagus into the stomach, where the food is really “chewed”.

Digestion

Digestion starts in the stomach. There the food is broken down by the action of enzymes and the crushing contractions of the muscles in the wall of the stomach. The main substances secreted in the stomach are gastric acid and pepsin.

Gastric acid secretion

Gastric acid secretion is triggered by the distention of the stomach. Chemically it is hydrochloric acid, sufficient to maintain the stomach pH in the range 2–4. An acid environment in the stomach is necessary for the digesting activity of pepsin.

Enzymes of the stomach

Pepsin is the major acid protease in fishes. It is secreted by the gastric gland cells. The excretory granules are released by the process of exocytosis. The conversion of pepsinogen into pepsin is brought about by pepsin in an acid environment. During the activation process amino acids are split off from the NH₂ end of the molecule as a mixture of peptides. Pepsin is an endopeptidase which cleaves peptide linkages formed by amino groups of aromatic and acidic amino acids. It attacks most proteins but not mucin, keratin and certain others.

A few non-proteolytic enzymes can also occur in the gastric fluid.

The intestine

In salmonids the intestine is a tube consisting of three major layers: mucosa, submucosa and muscularis.

The mucosa of the intestine is lined by a simple columnar epithelium which possesses the brush border of micro-villi typical of absorptive tissues. The submucosa is thin and contains scattered collagen and elastic fibres, blood vessels and nerves. The mucularis consists of inner circular and outer longitudinal smooth muscle layers. The muscularis is responsible for propelling the food through the alimentary canal. The terminal end of the intestine is differentiated as a wider rectum.

Enzymes of the intestine

The digestive enzymes produced by intestinal cells are located mainly in the brush border of the epithelium. Enzymes produced by the intestinal mucosa include aminopeptidases (formerly called erepsin), alkaline and acid nucleosidases (which split nucleosides), esterases, and various carbohydrate digesting enzymes such as amylase and maltase. Amylase activity in the intestine is higher in omnivorous fish species such as carp than in carnivores like trout.

Related organs of digestion

The liver and gall-bladder

The liver lies anteriorly in the body cavity and receives blood from the hepatic artery and from portal veins which drain the gastric and intestinal mucosa, the swim-bladder, the spleen and the pancreas. It is a complicated structure consisting of epithelial lamellae separating small blood vessels. It is also a storage organ for glycogen. The liver is the main factory of the body, in which food molecules transported in the blood from the intestine are used for manufacture of the body proteins, fats and carbohydrates. The exocrine secretion of the liver, the bile, is stored in the gall-bladder. The bile duct opens into the anterior intestine or into the pyloric caeca. Bile is a detergent-containing fluid and is found in all species of fish. Usually the gall-bladder has contractile walls. By contraction of the smooth muscles of the gall-bladder bile is ejected into the lumen of the intestine. Fish bile contains bile salts, cholesterol, phospholipids, bile pigments, glycoproteins and organic and inorganic ions. It is weakly alkaline. Bile salts are special types of steroids which are synthesized in the liver. In fishes, as in mammals, a large proportion of the secreted bile salts are resorbed from the intestine into the blood and returned to the liver. This so-called interohepatic circulation also occurs with other bile components.

The pancreas

The pancreas of salmonids is diffuse. It consists of ramified tubules or acini scattered in the connective tissue of the intestinal surface. The exocrine cells are strongly basophylic and contain zymogen granules. The secretory cells form acini with narrow lumina. Blood reaches the pancreas from three arteries and drains into the vena portae.

The pancreatic fluid is rich in various types of enzymes and contains bicarbonates which neutralize hydrochloric acid entering the intestine.

• Proteases

Trypsin, chymotrypsin and carboxypeptidases are stored in the pancreatic cells as zymogen granules. On arrival in the intestinal lumen, zymogens are converted into active enzymes.

Trypsin is formed by removal of a hexapeptide from the trypsinogen molecule. Trypsin is an endopeptidase with optimal action at a pH of about 7. It cleaves peptide linkages whose carbonyl groups come from arginine or lysine.

Chymotrypsin is formed by the action of trypsin on chymotrypsinogen. It is an endopeptidase which attacks peptide bonds with carbonyl groups on aromatic side chains.

Carboxypeptidases are exopeptidases which hydrolyze the terminal peptide bonds of their substrates. They are converted from zymogens by trypsin.

• Lipases

Lipases are esterases which split ester bonds. Triglyceride fats, phospholipids and wax esters are hydrolyzed by lipases. Although lipase activity has been demonstrated in various parts of the fish digestive system, the pancreas is probably the major source. Regulation of pancreatic secretory activity is achieved by stimulating secretory cells with peptide hormones produced by cells in the stomach. It is probable that several hormones are responsible for regulation of secretory activities.

Digestion and absorption

By the action of enzymes of the digestive fluids and gut epithelial cells, proteins, lipids, carbohydrates and nucleic acids are degraded into smaller molecules which can be absorbed and assimilated.

• Proteins

Digestion of proteins begins in the stomach. The endopeptidase activities of the gastric juice make proteins more readily digested by pancreatic and intestinal proteases. In the intestinal digestion of proteins, trypsin and chymotrypsin from the pancreas are of major importance. Polypeptides formed by their interaction are further split by carboxypeptidases. Protein digestion produces a mixture of low molecular weight peptides and amino acids in the intestinal lumen.

• Fats

Triglycerides are highly concentrated stores of metabolic energy and are important components of the food. Lipases hydrolyze neutral fat into diglycerides, monoglycerides, glycerol and fatty acid. Products of hydrolysis are rendered soluble by the bile salts, making subsequent absorption possible.

• Carbohydrates

Carbohydrate - digesting enzymes from the pancreas and intestinal epithelium transform poly- and oligosaccharides into hexoses and pentoses.

Absorption

The mechanisms of intestinal absorption in fish are similar to those of mammals. Absorption of the products of digestion takes place by diffusion and by active transport.

• Protein

The degradation products of protein are absorbed from the intestinal contents as amino acids or peptides. Individual acids are readily absorbed against the concentration gradient and their absorption appears to be linked with transport of inorganic ions.

• Fat

In fishes, lipids seem to be absorbed mainly by the epithelial cells in the anterior part of the intestine.

• Carbohydrates

Glucose is absorbed by the intestinal epithelium. This is accomplished by active transport and takes place against a considerable concentration gradient.

Protein, amino acid and fatty acid requirements
of trout and salmon

Proteins

Proteins are large, complex organic compounds which perform an essential role in the structure and functioning of the body. Animals cannot synthesize proteins from simple inorganic materials, and have to rely on digesting them from their diet. Dietary protein is therefore essential for all animals. The optimum dietary level of protein is defined as that which produces maximum growth. However, proteins act as an energy source as well as a tissue component, and excessive levels of dietary protein are used by the body as an expensive supply of energy. The biologically optimum dietary level of protein may therefore not always be the most economic to use in commercial fish culture.

Proteins are composed mostly of amino acids linked by peptide bonds and cross-linked chains with sulphydral and hydrogen bonds.

The proteins are made up of twenty major amino acids. The amino acid composition of proteins varies widely according to their source.

Some proteins lack certain amino acids. Some amino acids can be synthesized by animals. Others cannot be synthesized, and are therefore called essential amino acids.

For fish the following amino acids are essential: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.

Different amino acids contain different amounts of nitrogen. Therefore the figures given for protein level in feed compositional tables is never strictly accurate.

Crude protein is calculated by multiplying the nitrogen level determined in analysis by 6.25.

The quantitative requirements of essential amino acids varies between different fish species (Table 1). Also the content of essential amino acids present in feed ingredients varies even more widely.

The amino acid profile of a feed must be balanced to allow the dietary protein to be used effectively. This can be illustrated in the following way:

Suppose that the exit gate of a fish tank is composed of ten vertical planks of wood, each of which represents one of the essential amino acids, numbered from one to ten.

The level of water in the tank will depend on the height of the shortest plank (plank 7). This plank represents the limiting amino acid. If this plank is lengthened (or the level of the limiting amino acid is increased) then plank 8 would control the water level (or become the next limiting amino acid).

Ideally all the planks should be as high as the level of water desired in the pond (i.e., the quantity of each amino acid in the feed should be at the optimum level for the species being cultured, to avoid wastage of protein).

Table 1

QUANTITATIVE DIETARY AMINO ACID
REQUIREMENTS OF VARIOUS FISHES

New (1986)

As regards total protein, for rainbow trout starter feeds at least 50% is recommended, for grower feeds 40%, for broodstock feeds 35–40%, and in feeds for marketable fish production a minimum of 30%.

Lipids and fatty acids

In feed-stuff chemistry the words lipids and oil are often used synonymously.

The expression “crude fat” means the material which can be removed from the feed by ether extraction. The word lipid is a general term which includes sterols, fats, waxes and sphingomyelins. The fat soluble vitamins may also be extracted with ether.

Fats are the fatty acid esters of glycerol and are the primary source of energy.

Phospholipids are components of cellular membranes, and sphingomyelins are components of brain and nerve tissues. Sterols are precursors of various hormones in fish.

Lipids are also an important factor in the palatability of feeds.

The fatty acids which are the components of lipids are categorized in two ways. They have common names, but are also given specific numerical designations such as 16:0, 18:0, 20:1, 18:3^-3, 18:2n^-6, for example.

The numbers refer to the length of the carbon chain in the molecule, the number of carbon to carbon double bonds present, and the position of the first double bond. Though this system of nomenclature sounds complex to those with little knowledge of biochemistry, it is necessary to understand when fish nutrition is being discussed. Three examples are explained below:

Stearic acid 18:0

Linolenic acid 18:3n^-3

Arachidonic acid 20:4n^-6

In the designation 18:3n^-3 for example, “18” means that there are 18 carbon atoms in the chain “3” means that there are 3 carbon-carbon double bonds, and n^-3 means that the first double bond counting from the methyl (CH₃) end occurs after the third carbon atom in the chain.

Stearic acid (18:0) has eighteen carbon atoms but no double bonds.

Linolenic acid (18:3n^-3) has eighteen carbon atoms and three double bonds, the first of which appears on the third carbon atom.

These fatty acids which have their first double bond on the third carbon atom are known as the n^-3 series, or the linolenic series.

Saturated fatty acids are those without any double bonds. Monosaturated fatty acids are those with only one double bond, while those with more than one double bond are known as polyunsaturated fatty acids. The n^-3 series and n^-6 series fatty acids and the n^-7 and n^-9 fatty acids are all members of the group known as polyunsaturated fatty acids, and are essential for fish. Arachidonic acid serves as a precursor for prostaglandin production. Prostaglandins are very important for various metabolic pathways. They dilate veins, contract smooth muscles and are important for reproduction.

The essential fatty acid requirements of different species vary but are not yet fully understood. Trouts require about 1% of polyunsaturated fatty acids in their diet.

Carbohydrates

The carbohydrates, which include starches and sugars containing only the elements carbon, hydrogen and oxygen, are usually the cheapest source of energy. However, trout cannot digest carbohydrates very effectively. The level of carbohydrates in their diet should thus not exceed 20% of the total feed.

Vitamin and mineral requirements of trout and salmon

Vitamins

Vitamins are complex organic compounds which are required in trace amounts for normal growth, reproduction, health and general metabolism. Many vitamin deficiency symptoms have been described in fish (Table 2). They are most prevalent in intensive production systems. Many problems still occur in practical fish farming since the quantities of vitamins required vary both with the quality and the composition of the compound feeds and with the husbandry conditions under which fish are kept. Very little is known about the requirements for fat-soluble vitamins.

Water-soluble vitamins

Water-soluble vitamins are rapidly metabolized in fish. There is a limit to the extent to which vitamins can be stored in the body, and when ingested in excess surpluses are excreted without being utilized. The water-soluble vitamins mainly function as coenzymes. In some cases significant amounts of water-soluble vitamins can be provided as a result of synthesis by micro-organisms in the intestine.

Table 2

THE WATER-SOLUBLE VITAMINS

Ascorbic acid (vitamin C) is particularly sensitive to heat, rancidity and/or feed processing. This vitamin also tends to leach out into water.

Fat-soluble vitamins

1. Vitamin A

Vitamin A and provitamin A (carotene and carotenoids) are necessary for growth and for the maintenance of the physiological functions in the body. Deficiency of vitamin A also decreases the resistance to infectious diseases. In trout the storage and utilization of vitamin A is very slow. The efficiency of utilization also depends on the source. For example the vitamin A from beef liver is more readily available than that from cod liver oil or from carotene.

2. Vitamin E

Vitamin E is essential for the reproductive functions. The germinal epithelium of the gonads degenerates when the amount of vitamin E is insufficient. As vitamin E in vivo is an antioxidant, it also prevents oxidation of the lipids in the mito-chondria and oxidation of B group vitamins and carotene in the intestine. Vitamin E is present in abundance in natural vegetable oils. Several tocopherols are known and their physiological activities vary.

When rancid fish meal and rancid fats are fed in large quantities, yellow lipoid degeneration develops. Addition of vitamin E to diets has made it possible to prevent this condition effectively.

3. Vitamin D

It is the antirachitic factor. The provitamin of vitamins D₂ and D₃ are ergosterol and dehydrosterol respectively. Fish liver oils contain only vitamin D₃. Deficiency usually does not occur, but the vitamin is added to compound food at a level of about 3 IU/kg.

4. Vitamin K

Vitamin K is concerned with blood coagulation. It is essential for all animals. It is present in green leaves. K₂ is synthetized by bacteria during fermentation processes. Deficiency of this vitamin is signalled by prolonged blood coagulation time.

Table 3

RECOMMENDED VITAMIN MIX FOR TROUT

Piper et al. (1982)

Minerals

Mineral elements are important in many aspects of fish metabolism. They provide strength and rigidity of bones. In body fluids they are mainly involved with the maintenance of osmotic equilibrium with the aquatic environment, and with the nervous and endocrine systems. They are components of enzymes, haemoglobin and other organic compounds, and are essentially involved in the metabolic processes concerned with energy transport. Most of the seven major “minerals” - calcium, phosphorus, potassium, sodium, chloride, magnesium and sulphur, and fifteen trace elements - iron, zinc, copper, manganese, nickel, cobalt, molybdenum, selenium, chromium, iodine, fluorine, tin, silicon, vanadium and arsenic, have been reported to be essential for fish.

Fish can absorb minerals not only from their food, but also from the water across skin or gill membranes. Minerals are therefore probably not as important in fish diets as they are in those of other domestic animals. Calcium can be absorbed by fish from seawater, but freshwater is low in this mineral. However, since most feed-stuffs, particularly those of animal origin, contain high levels of calcium, calcium deficiency in fish is most unlikely.

On the other hand both seawater and freshwater are low in phosphorus, making this element important from a dietary point of view. Some types of phosphorus are unavailable to fish, and an assessment of the availability of phosphorus in the diet is essential. Generally phosphorus from animal sources is best absorbed by fish.

Table 4

RECOMMENDED MINERAL MIXTURE FOR TROUT

To be used at 0.1% in diet

Piper et al. (1982)

Fish bioenergetics and utilization of dietary ingredients

Fish require food to supply the energy needs for movement and all the other activities in which they are engaged, and as the “building blocks” for growth. In this they do not differ from other animals. Fish are poikilothermic organisms, i.e., their body temperature is the same as that of the water in which they live. They do not therefore have to consume energy to maintain a steady body temperature, and consequently they tend to be more efficient users of food than other farm animals. Their metabolic rate depends very largely on the temperature of the water in which they are living. Within the range of temperature of which they are tolerant, metabolic activity and the need for food increases as the temperature rises.

Excess or insufficient dietary energy levels result in reduced growth rates. Energy needs for maintenance and movement must be satisfied before energy can be used for growth. Thus if the energy: protein ratio is too low, protein will be used to satisfy the fish's energy requirement first, and only what is left will be available for growth.

Fish eat primarily to satisfy their energy requirement. Therefore a diet with excess energy content inhibits food consumption and reduces the intake of the protein necessary for growth.

The energy content of diets is often expressed in kcal/kg of dry matter. In feed production the following terms are used:

Gross energy (GE) - Total energy of food measured in a calorimeter

Digestible energy (DE) - Gross energy minus the energy excreted in faeces

Metabolizable energy (ME) - Digestible energy minus energy lost as gaseous products, across the gills and in the urine.

Table 5

CALCULATION OF THE DIGESTIBLE ENERGY VALUE OF VARIOUS
FOOD COMPONENTS FOR RAINBOW TROUT

ADCP (1983)

Lipids, and to a lesser degree carbohydrates, can be used in trout diets to spare proteins for growth. Increased levels of carbohydrates or lipids enable the reduction in the protein level necessary for optimum growth. Providing that the minimum level of n^-3 fat has already been provided, usually from marine oil, the extra lipid required as an energy source alone can be supplied by a cheaper vegetable or animal oil.

Example: Calculation of the digestible energy of soybean meal.

The soybean meal contains:

15.2% moisture, 5.4% lipids, 40.3% protein, 4.3% fibre, 5.4% ash, 29.3% NFE

For the calculation the data concerning dry matter, crude protein, lipids and nitrogen free extract (NFE) must first be converted to a dry matter basis. The digestible energy can then be calculated, as follows:

1. convert to a dry matter basis

   

2. calculate DE contributed by each component

   

3. sum to find total DE

   Total estimated DE of soybean meal =

   0.512 + 1.805 + 0.692 × 1 000 = 3 009 kcal/kg

Utilization of ingredients

Salmonid fishes are carnivores. Because of this their bodies can much more easily assimilate food of animal orgin than that from plant sources. Utilization of the food ingredients also depends on temperature, fish size, light intensity and the chemical composition of the aquatic environment.

Proteins

Utilization of proteins depends to a large extent on their amino acid composition, e.g., on the biological value of the proteins. This value is expressed by comparison with proteins of milk or eggs and is calculated for each amino acid. Differences in the nutritional values of food proteins are evident, due to the high essential amino acid requirements of fish. Most of the methods currently applied for the evaluation of nutritional quality of proteins for fish measure nitrogen retention. The following terms are used:

Protein retention - measure of retention of dietary protein in the body (in %)

Protein efficiency ratio (PER) - the ratio of grammes of dietary protein to grammes of body weight gain

Apparent efficiency of protein deposition - carcass protein at the end of the experiment minus carcass protein at the beginning.

The relationship between energy and protein content in the diets of salmonids has been examined. An increase in the digestible energy:protein ratio leads to an increased depolarization of lipids in the fish. An increase in the energy level at a constant level of dietary protein results in improved feed efficiency. Protein efficiency ratio is negatively correlated to the dietary protein to energy ratio.

There is a case for stating the protein content of the diet in terms of the proportion of energy it contributes. Protein acts both as a nutrient and an energy source. Consequently addition of energy to a diet not only increases energy intake but also lowers the protein energy to total energy ratio. Both factors influence nitrogen balance. It is important to understand that surplus dietary protein cannot be stored, but is immediately catabolized.

Utilization of carbohydrates

Up to a level of about 25% in diets carbohydrate can be as effective as fat as an energy source for some fish species, including rainbow trout. However, even starving fish prefer oxidation of other substrates than glucose, indicating that the capacity of fish to oxidize glucose aerobically is limited.

Utilization of lipids

In common with other animals, fish require lipids as a source of metabolic energy and to maintain the structure and integrity of cellular membranes. The lipids are absorbed in the upper part of the intestine and in the pyloric caeca. Absorption is equally efficient for both saturated and unsaturated fatty acids.

Fats are stored in the fish body in the liver, body cavity or in muscles. Marine fishes are an important source of polyunsaturated fatty acids in salmonid feeds. Fish cannot synthesize polyunsaturated fatty acids, so they must be present in the diets. Salmonids prefer n^-3 series fatty acids at a dietary level of around 1%.

Feeding technology

Feeding rate

An understanding of this topic is very important for the efficient operation of an aquaculture venture. Under-feeding can result in loss of production. On the other hand, overfeeding will cause a wastage of expensive feed and is additionally a potential cause of water pollution, which in turn can lead to the death of fish or expenditure on corrective measures. Thus both overfeeding and under-feeding have serious economic consequences which can affect the viability of the farm. Sometimes such statements as “feed 3%” of biomass per day" can be seen for dry diets. If such a recommendation was applied throughout the growing cycle it would almost certainly result in starvation in the early stages and gross overfeeding with associated water quality problems later. Feeding rate should not remain constant throughout the growing cycle up to market size. On the contrary, it must be modified according to the size and age of fish as well as the water temperature.

The quantity of a feed to be given to a cage or a raceway each day should normally be based on a percentage of the biomass present (i.e., the total weight of animals) and the water temperature. Thus if a tank contains 10 000 trout averaging 10 g, and the recommended feeding rate stated in the food manufacturer's feeding table is 7%/day, the amount of feed to be given daily is

The percentage of biomass to be fed is not constant. It should decrease as the animals grow, to reflect their decreasing metabolic rate. Thus the ratio of weight of feed per day to fish weight (biomass) is high at the start of the growing period and falls towards the time when the animals reach marketable size.

Accurate application of a feeding rate depends on an accurate estimate of the average weight and number of animals in the production units. Average weight can be estimated by weighing samples of fish. Sample weighing to determine average weight should be done weekly or fortnightly. This is easier in tanks or cages than in ponds. Care must be exercised to take samples from several parts of a pond, not only from one feeding point where larger fishes may congregate. An assessment of survival is also necessary to permit accurate calculation of feeding rate. This can be illustrated as follows:

If the receway is stocked with 20 000 trout averaging 10 g, and the feeding rate to be applied is 4% of the fish biomass per day, the amount of feed will be

However, this assumes that all the animals orginally stocked are still present. If in reality there had been a 25% mortality up to 10 g, the correct daily amount of food would be only

Taking the fish mortality into account could not only save 25% of daily feed cost, but also prevent the possible deterioration in water quality caused by overfeeding.

Because a good estimate of survival is necessary for an efficient feeding programme records of mortality should be kept in production units, and fish should be counted during every transport to another tank, cage or raceway.

In practice most farmers apply an arbitrary survival factor based on the number of days since stocking. This factor is derived from experience on the same farm or elsewhere in similar circumstances, modified by observation of actual mortalities, knowledge of water quality and disease problems. The accuracy of the resulting estimates depends on careful measurement of the number of animals originally stocked and the numbers harvested in each cycle. Thus the importance of maintaining accurate farm records becomes obvious.

To facilitate feeding, feeding tables have been constructed by food manufacturers. It must be emphasized that the feeding rates given in tables must not be applied without reference to other factors. Feed must be reduced or omitted during periods of low temperature, increased turbidity, high temperature, etc., based on operational experience in the specific location. Daily feeding rates must also be based on observation of the trout during feeding. At this time the feeding activity of the fish, the water quality, the presence of old feed, etc., must be assessed. All feeding tables are merely a guide which, applied with careful judgement, will markedly improve economic viability. However, applied mechanically without complementary assessment of conditions they can result in disaster.

The basic rules concerning feeding, suggested by Piper et al. (1982), are as follows:

1. For optimum growth and feed conversion each feed should ideally be only 1% of the body weight.

2. In trout, survival rate is not affected by feeding frequency once the initial feeding stage has been passed.

3. Frequent feeding reduces the incidence of starvation and stunting of small fish, giving the group a better uniformity of size.

4. Infrequent feeding results in feed wastage, poor FCR, and water quality problems.

5. Dry food should be distributed more frequently than moist feed.

Very frequent feeding is the best for young salmonids. The daily feed ration should be split up into very small quantities given at regular intervals (often 20–24 times per day), either manually or automatically. Sometimes a 24-hour lighting regime is used for the first few days to encourage the fish fry to take dry pellets. Feeding frequency is gradually reduced to 2–4 times/day as the fish grow. Rainbow trout start to take food about 21 days after hatching when reared at 10°C.

When wet food is used initially (i.e., spleen or liver) a small quantity is given at regular intervals in the inflow of the rearing units to accustom fry to feeding. Thereafter flower pots or other platforms of appropriate size can be used, on which prepared sticky feed is placed. The pots should be hung above the bottom of the rearing unit. After the food has been eaten, pots must be regularly disinfected in permanganate solution.

To facilitate feeding, many types of automatic feeders have been developed. The choice depends on suitability and availability.

Utilization of locally available fish food ingredients

A. Background

Three major factors govern the choice of ingredients for fish food formulation.

1. Suitability for the fish being cultured

For salmonid production only a rather limited range of ingredients is suitable. Trout and salmon require very high quality ingredients from the point of view of their amino acid and fatty acid composition. Choosing the ingredients to be used in feed manufacture on a particular farm is first a matter of matching the types available locally (or that can be imported) with the requirements for formulation of feed suitable for the trout.

Therefore the first step is to prepare a list of available raw materials. This list should contain data on cost, quantity and composition.

Then, an assessment must be made of the quality of the available ingredients; each raw material must be considered according to its composition of proteins, fats, minerals, vitamins and energy. This should be done on the basis of actual analyses rather than theoretical analytical composition. This information is necessary before accurate formulation work can be done. Results of analyses of local ingredients may be available from the suppliers, from local universities, or from surveys conducted by the Government's department of fisheries. In the absence of such local information, tables of ingredient composition must be used. Though this is less satisfactory than the use of accurate local data, it is often the only information available.

Finally reputable suppliers who are known to supply food ingredients of good quality, free from adulteration and toxicity, must be identified.

Quality is an essential part of the feeding programme. If neglected, disastrous consequences can result.

2. Quality and regularity of supply

Ingredients which are regularly available in sufficient quantity should be chosen. Though some ingredients are only available seasonally, the more regular the supply of each ingredient the better. Otherwise much larger quantities that can be immediately processed must be purchased, leading to problems of long-term storage. Alternatively frequent changes in the feed formulation must be made because of the fluctuating supply of specific compounds.

3. Cost

Cost is very important, and is sometimes the decisive factor determining the suitability of ingredients for salmonid feeds.

The cost of an ingredient can vary quite markedly from one location to another, depending on supply and demand. Therefore substances which are very acceptable ingredients in one place may sometimes have to be rejected or used in smaller quantities in another because they are too expensive. Cost obviously interacts with availability and suitability. All these factors have to be reviewed simultaneously in the selection of ingredients for a feed. The summary of data on the raw materials available should be made in a tabular form which shows analytical data and cost per unit at the farm site. This table will be used when feeds are formulated, i.e., when decisions are made on how much of each ingredient is to be used, and potential cost is assessed.

B. Ingredients available in Iran

1. Kilka

The kilka is a small fish of the herring family. The catch in Iran can probably reach 50 000 t/year. This fish can be used as an ingredient for wet feed, but this is only feasible within a very limited distance from the coast of the Caspian Sea. During the summer the air temperature exceeds 35°C. Few facilities are available for freezing this fish, so that it will deteriorate quickly after catching.

A possible way to process the kilka for storage is to make fish silage and this type of food ingredient could become important in local conditions. Fish silage is an excellent food ingredient for growing trout. It is relatively easy to make, and can be stored for long periods without refrigeration. However, it is only economically worthwhile if regular supplies of the raw material can be obtained close to the farm. Fish silage is made by adding 3–4% of an acid to the minced fish. The fish must be either fresh from the sea or from cold storage. The acid can be organic or inorganic, but the best is probably formic acid. The pH of the fish/acid mixture is brought down below 4. This inhibits bacterial decay, but the digestion processes helped by enzymes from the fish guts continue and reduce the mixture to a liquid slurry. An antioxidant is added to prevent the fats from becoming rancid. The liquid fish silage can be stored in tanks for up to 6 months. In use, a 50% mixture of silage with commercially-made dry meal containing vitamins and binders is passed through a simple perforated plate extruder. The oil content can be increased to 20% by adding further fish or vegetable oil to the mixture before extrusion. The moist pellets cannot be stored for more than a few days, and should be used within 24 h during hot weather.

Further ingredients available in Iran are as follows:

2. Ingredient of animal origin

• Wastes from slaughterhouses
• By-products from poultry slaughterhouses
• Limited quantities of fish meal, bone meal, meat and bone meal, feather meal, and silkworm pupae.

3. Ingredients of plant origin

Cereals, soybean meal, sunflower meal, and cotton-seed meal are locally available.

C. Ingredients which will probably become available in the the near future

Single cell proteins: Amongst these are bacterial proteins grown in an aqueous solution of mineral salts in the presence of a nitrogen source, using methane as an energy supply.

Yeasts: Grown on paraffin, or on industrial wastes such as molasses sulphite from the paper industry, are becoming increasingly available as ingredients for animal feeds. Both types of product, but especially bacterial ones, are rich in protein. However, the presence in them of large quantities of nucleic acids limits their use for human consumption. Their amino acid profiles vary according to their type and the media in which they are grown. In particular, some yeasts are deficient in methionine, whilst some are very high in lysine.

Vitamins: Compounded feeds must be supplemented with a sufficient quantity of essential vitamins. They are usually added in premixes designed for inclusion in a specific type of feed. If a special premix for salmonids is not available, it is necessary to investigate carefully the composition of vitamin mixtures for other animals which are available on the local market. The alternative mixes must be compared with the desired composition, and the most suitable mix be selected.

Binders: Many different substances can be used to increase the water stability of fish diets. Some of them are routinely used by the feedstuff industry. Some are special chemicals, whilst others are natural products used either raw or refined. Some binders also have nutritional value.

The selection of a binder depends on cost, availability and trials of the raw material in each particular dietary formulation.

Substances used for improving water stability are as follows: casein, sodium alginate, wheat glutin, gelatin, lignosulphate, hemicelluloses, collagen, seaweed binder, bentonite, agar, corn starch, potato starch, carboxymethylcellulose.

Complete diets for fry, fingerlings and market-sized fish

Fish feeds can be divided into three groups according to their moisture content.

1. Wet feed

Wet diets are prepared at the fish farm by the farmer himself. They are basically minced wet fish or other raw material (e.g., blood, spleen, liver) with added vitamins, minerals and binders. For small-scale production of fingerlings a pulp of spleen or liver can be used. The spleen or liver must be fresh, without any signs of deterioration. Wet foods can be thrown directly to fish in their enclosures, or can be given a variety of types of feeding trays, flower pots, etc.

Once prepared, wet food must be used within a day, or be kept in a cold store.

Composition of wet diets for on-growing: The basic component of wet diets can be industrial fish (e.g., kilka) or waste materials from the fish processing industry or from slaughterhouses. Almost any species of fish can be used in wet diets. Though it is possible to feed salmonids only with fish, it is usual to complete wet diets with binders to improve the consistency of the diet and stop it breaking up into small pieces, and with vitamin and mineral additives to ensure the diet is not deficient in any essential trace element.

The chemical composition of wet food depends mainly on the composition of the basic fish ingredients. However, for salmonids it is desirable that this food should contain 15–20% fat on a dry matter basis. Wet food is usually delivered to the salmon or trout by hand. It is possible to calculate approximately how much food a given weight of fish should eat per day. However, as this varies greatly according to mean size of fish, water temperature and other conditions, most farmers prefer to feed according to appetite. This means as long as fish eagerly accept food the farmer will try to supply it. Wet food tends to float or sink slowly, so it is easy to see whether fish are eating it.

2. Moist pellets

Moist pellets contain 20–50% of water. They are usually produced on the farm. They consist of fresh or frozen sea fish and dry components. Usually fish meal and other protein meals with added vitamins, low melting-point fat commonly in the form of fish oil, and an antioxidant and binding agent are used. Moist pellets do not disintegrate quickly in the water. The manufacture of the pellets is a straightforward operation. The machine used is a modified industrial mixer with a revolving blade mounted in front of the extruder. This cuts off the blended food in short lengths as it comes out of the holes.

Moist pellets should be fed on the same day they were prepared. For storage freezing is necessary.

3. Dry feed

Dry feed is usually produced in large batches in specialized feed mills. This type of feed forms a complete diet. It is easy to handle and store, and it is manufactured in a variety of particle sizes to suit the size of the cultured fish. A wide variety of ingredients can be used according to availability, quality and price. Examples of components and formulations are given in Tables 5 and 6.

Table 5

APPROXIMATE SPECIFICATION OF INGREDIENTS FOR SALMONID DIETS

Hilton and Slinger (1981)

Table 6

EXAMPLES OF PELLET FORMULATIONS FOR SALMONIDS

1. University of Guelph formula

¹ CP = crude protein

2. Trout formula

Piper et al. (1982)

3. Rainbow trout

Sedgwick (1982)

4. Trout

Chew (1982)

Fish feed formulation technology

Formulation is the last step in fish food production. Before it can be undertaken, knowledge of the nutrient requirements of fish and the availability of locally obtainable components are necessary. To facilitate formulation, it is necessary to:

1. Prepare a list of available ingredients, together with data on their chemical composition and cost.

2. Produce a specification for the diet to be made, including protein level, fat level, EAA, EFA, vitamins, etc.

3. Have knowledge of the special suitability of certain raw materials for inclusion in trout diets.

The first step in formulation is therefore to assemble the above information in an organized and accessible way. The second step is to draw up a worksheet for feed formulation. The worksheet includes space for the major nutrients, because it is necessary first to balance these with the specification for the diet before other factors such as EAA and EFA content can be examined. When the approximate basic diet has been defined, other more minor analytical features can be examined and adjustments made to reach the final, balanced formulation.

As a starting point for formulation, previous experience can be drawn on, i.e., already-used diets are examined. In addition, there will be certain predetermined ideas about minimum inclusion rates necessary for certain ingredients, also based on experience. This experience is constantly changing and developing, and is also specific to the individual making the formulation. It is therefore impossible to lay down hard-and-fast rules about the rations for inclusion of various ingredients, especially as there are so many different materials available. The above examples of formulas and the information about nutritional requirements given in this paper are only intended to provide guidelines to assist the student in making his own formulations. The example below illustrates the procedure to be followed.

The approximate basic nutritional requirements of trout are (as percentage of diet):

To decide which of the components available should be used in the diet the approximate composition of a known diet can be followed. To complete this diet, however, it is necessary to calculate the levels of nutrients it contains and if desirable to adjust the formulation. A more accurate but more time-consuming method of diet formulation is to proceed from basic principles as follows:

Based on the supposition that the following ingredients are available locally, an attempt is made to produce a trout diet containing 40% crude protein.

It is known in advance that 30% of fish meal and 5% wheat bran is needed in the diet:

Protein from fish meal = 30% × 0.63 = 18.9%

and Protein from wheat bran = 5% × 0.15 = 1.1%

It is calculated as above that 30% inclusion of fish meal and 5% inclusion of wheat bran contributes 20% protein to the final ration. Thus it is known that the remaining 65% of the diet must contribute 20% (40–20= 20) of protein. This portion can be formulated as if it were a separate diet consisting of the remaining ingredients. To contribute 20% of protein to the final diet, this “sub-diet” needs to contain only 20×100/65 = 30% protein. Therefore the two components used in this portion must be mixed together in a ratio which produces a mixture containing 30% protein to balance the fish meal contribution. The amounts of each of the possible ingredients (or of the five possible pairs of ingredients) which will supply this protein level can be calculated by the “square” or “diagonal” method, as follows:

In this example, the possible combinations are:

1. cotton meal (22%) × wheat meal (12%)

2. cotton meal (22%) × soy meal (40%)

3. cotton meal (22%) × blood meal (80%)

4. wheat meal (12%) × soy meal (40%)

5. wheat flour (12%) × blood meal (80%)

1. Since both ingredients in this pairing contain less than 30% protein, it is impossible to produce the required mixture from them.

Thus for the four usable alternatives, the level of each ingredient in the final diet would be:

Finally, the result of combining each of the four alternative mixtures with fish meal and wheat bran on the composition of the final diet can be checked, and lipid level, NFE, CF, EAA, EFA be calculated. If necessary, the composition of the diet can be recalculated or adjusted to match the original specification or to incorporate new ideas or newly-available ingredients.

Influence of feed on health and body composition

There is a very high correlation between fish feed and health status. This applies especially in salmonid culture, where all the feed is of artificial origin and natural food is negligible. It is therefore very important to maintain the quality of feed as high as possible. One of the most important factors influencing feed quality is storage.

During storage some changes in food quality are inevitable. In particular lipids can break down into fatty acids, making the feed more prone to the development of rancidity. This breakdown can be caused by the damage resulting from fungal growth. Ingredients of high lipid content are of course more susceptible to this type of chemical change. In addition, vitamin potency decreases during storage, particularly in premixes which also contain minerals. It is therefore clear that many problems can occur during feed storage. To minimize them, ingredients must be stored for as short a period as possible, and compounded feeds be used quickly. Vitamins and vitamin premixes are extremely expensive and should be treated with special care. They should be kept in their original containers or in other airtight lightproof containers. They should not be kept in hot, sunny rooms, but in the coolest place available, preferably under air conditioning.

Moist and wet ingredients

These materials must either be used fresh or be kept frozen. Small quantities can be frozen in a cold store, but always in thin layers (maximum 2 cm deep). If large volumes (e.g., drums or bags) of materials are simply put into the cold store, it will take several days before they completely freeze. Rapid deteriorative changes in the feed will take place during this time. Moist diets should be used on the day of manufacture preferably within 2–3 h.

Dry feeds

Buildings used for storage should be secure against vermin and equipped with ventilation. Ventilation points should be low on the wall facing the prevailing wind, and high on the opposite side. Always keep the store clean. Floors and walls should be regularly swept. Spilled material must be removed and the contents of broken bags or containers must be used first. The store must be thoroughly cleaned before new materials are put there.

Make small stacks. Large stacks of sacks generate heat which causes other consequential damage. If possible, sacks should be raised off the ground by stacking them on wooden pellets. All sacks should be clearly labelled and the oldest batch used first. Do not walk on the sacks, as this will break the pellets. Do not allow sacks to rest against the outer walls of the store, but leave a space between the stack and wall.

Toxicity of feeds

Toxic feed will cause reduction in growth and/or mortality. However, poor results with a specific feed may not necessarily be caused by toxicity, but simply by poor formulation or by old feed. If feed is suspected to be of poor quality, the manager should stop using that batch and try another. If the problem resolves itself he will know that the feed was at fault, and can investigate the cause. If the problem continues it is either caused by some aspects of the feed which is common to both batches, or it is not related to the feed at all. Before feeding is stopped, careful consideration must be given to other possible causes of the problem being experienced.

Feed may be toxic for several reasons. It can contain heavy metals, i.e., copper, zinc, nickel, cadmium, or residues of herbícides, fungicides or insecticides. Feed may also be made toxic by substances which are developing in it during storage.

A. Toxic elements

Mycotoxins

The significance of toxic components of the diet was highlighted in 1960 with the finding that aflatoxins, metabolic products of the blue-green mould Aspergilus flavus which grow on various oil seed meals, were markedly carcinogenic for rainbow trout. Aflatoxins at a level of 1 ppb can induce neoplasmatic change in rainbow trout over a six-month period, and higher levels can result in more acute diseases. The rainbow trout is the most susceptible fish species. Manufacturers of commercial feeds are now well aware of the significance of aflatoxin contamination, but aflatoxin can still be produced during improper storage.

Antibiotics and chemotherapeutics

Addition of antibiotics and chemotherapeutics to the diet is widely practised. Where therapy is properly done and limited to a short period of time, minimal damage to the fish ensues, although growth may be checked until the normal bacterial flora of the fish alimentary canal is restored. However, continuous therapy is a very dangerous practice, which can cause toxic changes such as depression of haemopoesis and, especially with sulphonamides, renal tubular necrosis.

Gossypol

Gossypol is a fat-soluble pigment of cotton seeds which is toxic to fish. It accumulates in the liver and kidneys, and can be responsible for severe liver degeneration.

Botulism

This is a very dangerous disease both to fish and man, caused by toxins which can be produced in trash fish when stored anaerobically.

Heavy metals

Heavy metals can also precipitate severe toxicosis if they are present in amounts above the optimal level.

B. Deficiencies or imbalances of certain nutrients

Nutritional diseases are notoriously hard to define in absolute terms, since it is rare for a single deficiency to occur. In most cases the main symptom is only the general one of poor appetite or poor growth. In such a situation it is imperative that all other possible causes connected with infections or husbandry practice are also explored.

Proteins

The main clinical syndrome of amino acid deficiency or imbalance is growth retardation.

Carbohydrate

Excessive feeding with carbohydrate results in liver cell degeneration and excessive glycogen deposition.

Lipids

Fatty acids are nutritionally active components of dietary fat. Deficiency of fatty acids is manifested by depigmentation, fin erosion, cardiac myopathy and fatty infiltration of the liver. In salmonids the liver is not a major lipid storage organ, so that in farmed fish a pathological syndrome, lipoid liver disease occasionally occurs. Lipoid liver disease is usually seen in fish fed on trash fish or pelleted diets in which part of the lipid component has become rancid. Unsaturated fatty acids are prone to oxidation (rancidity) in the presence of molecular oxygen. High storage temperatures increase the rate of oxidation. The higher the fat level, or the lower the level of antioxidants used in the complete diet, the greater the probability that rancidity will develop. All salmonids are susceptible to lipoid liver degeneration, but it is a particularly significant problem in rainbow trout culture. Slightly affected fish are usually capable of complete recovery, but once severe anaemia and hepatic ceroidosis have developed, fish are rarely capable of regaining their previous efficiency in food conversion.

References

ADCP. 1983 Fish feeds and feeding in developing countries. Rome, FAO, ADCP/REP/83/18:97 p.

Hilton, J.W. and S.J. Slinger. 1981 Nutrition and feeding of rainbow trout. Canadian special publication of fisheries and aquatic sciences

New, 1986 M.B. Aquaculture diets of post-larval marine fish of the super-family Percoidae, with special reference to sea bass, sea breams, groupers and yellowtail: a review. Kuwait Bulletin of Marine Science, 7:75–151

New, 1987 M.B. Feed and feeding of fish and shrimp. Rome, FAO, FAO/UNDP, ADCP/REP/87/26, 275 p.

Piper, R.G. 1982 et al. Fishery hatchery management. Washington, DC, US Department of the Interior, Fish and Wildlife Service, 517 p.

Sedgwick, S.D. 1982 The salmon handbook. London, Andre Deutsch, 247 p.