Fish nutrition - Food for thought or cheap talk

With keeping fish in captivity, sound nutrition and adequate feeding is of the utmost importance to ensure good growth and health of fish. If the costs of keeping and growing healthy fish become unaffordable or unsatisfactory to the keeper, he should identify and address the causative factors such as:

• A long chain of distribution
• High levels of profit taking
• Degradation of quality before use
• Inferior pond conditions, and
• Wrong feeding habits (e.g. infrequent, over or under feeding, etc.)

Based on the abovementioned some background is given to explain why cheap solutions may result in expensive conclusions towards fish nutrition.

The purpose of feeding one’s fish should be to meet it’s minimum nutritional requirements with regard to:

• essential amino acids
• metabolic energy
• essential fatty acids
• vitamins, and
• minerals

in the most cost effective and convenient manner, which is the primary aim and strive of any nutritionist.

If the feed is not consumed by the fish or if they are unable to utilise it because of some nutrient deficiency, there will be no growth. Nutritionally balanced and quality-controlled diets for fish production are of critical concern. Therefore, before attempting fish culture it would be wise to ask the fundamental question “What and how should I feed my fish?”

To grow fish, or any other animal, we must feed them protein. Nothing else will actually put on lean body weight. In order to achieve optimum growth rates, they do require the correct level of the balanced constituents of protein - essential amino acids. Fish are fed higher percentages of protein in their diets than are land animals. Reasons for this are not so much that fish have higher protein requirements than land mammals, but that fish have lower energy requirements. Although fish require the same 10 essential amino acids as warm-blooded animals, they require different concentrations thereof. Younger, small fish with a high specific growth rate, have a requirement for a diet with a higher level of balanced amino acids than older, larger fish. This is because they are undertaking proteinaceous growth, such as the deposition of bundles of muscle fibers within the muscle blocks. The following recommended dietary protein levels is just an indication of the different requirements at different stages of production:

• Fry (Starter diet) 40-47%
• Fingerlings and subadults (Grower) 37-42%
• Adult and broodfish (Maintenance) 28-32%

The protein component represents the largest portion of the cost of a diet and this graded reduction in the amino acid supply is also reflected in the pricing of these products, enabling keepers that use this feed to achieve the goal of attaining the cheapest cost of maximum live weight gain per kilogram of feed fed.

The provision of high levels of balanced amino acids is not the only requisite for fast growth of fish. If this supply is not supported by the correct level of digestible energy, then the fish will catabolize the rich supply of amino acids and use them for their energy needs and not for building protein. The way to reduce food costs is to ensure that the protein is used only for growth and that energy requirements are met by fat and carbohydrates. In other words, the expensive protein should only be used for what it alone can do, namely, to grow the fish. It is for this reason that diets where the protein element is used only for growth are the ones that produce cheaper fish. High-energy diets work on exactly this principle: fat and carbohydrate content of the diet provide the necessary energy. Oil provides approximately twice the energy of that from protein and around four times of that supplied from carbohydrate. This idea is known as protein sparing. The total energy content is of great importance. This is because fish eat to a set energy level. The fish consume food to satisfy their energy requirement, and hence the energy content of the diet also determines the amount of feed consumed. If a food is diluted by a raw material that provides no energy, the fish will simply eat more to compensate for the fact that each unit weight of food contains less energy. It must be remembered that if a higher energy diet is used, the feeding rate should be decreased accordingly. If the feeding rate is not decreased it is unlikely any cost savings will occur. Fish will need to eat much less of a high energy food than a low energy one. Therefore if a food with say 15% extra energy is used, the feeding rates should be reduced by 15%. If this rule is not followed, the food will be wasted.

The main nutritionally active components of dietary lipids are fatty acids. This lipid component of the diet must provide adequate amounts of essential fatty acids as well as large part of required dietary fuel. Fatty acids are derived from the oils contained in fish feed and are an essential structural component of cell walls. Only a small proportion of total oil is used for this purpose. The greater proportion is either oxidised to provide the fish with useful energy and/or is deposited in the carcass as stored fat. As each gram of oil contains about twice the energy of a gram of protein it is important to achieve the correct balance of oil to protein. Too little oil and expensive protein may be catabilised for energy - too much oil and excess body fat may accumulate. Artificial diets are formulated using stabilised, highly palatable oils and careful attention is paid to achieving the right balance of protein and energy.

In addition to the energy content of the diet, the main other factor influencing feed intake of fish is water temperature. It influences metabolic rate and energy utilisation. For most fish species so far studied, when food supply is not limiting, specific growth rate has been found to increase with increasing temperature. This is a function of relationship between metabolic rate and water temperature for fish. In addition it is well established that for any given temperature, specific growth rate decreases with increasing body weight.
To summarise, fish growing at elevated rates when small/ and at higher temperatures have relatively higher protein requirement. Therefore, no single diet composition will be optimal throughout the life of the fish

In addition to highly digestible and pre-digested proteins, which consequently make the diets extremely digestible - therefore reducing the risk of gut diseases such as gut cancer an pancreatic disease, the addition of boosted levels of vitamins, particularly vitamins C and E, are essential. Vitamin C has been closely associated with wound healing and is required in high levels in stressed fish, whilst Vitamin E levels in tissue are known to be depleted during pancreatic disease. Common deficiency symptoms for practically any of the 15 vitamins essential for fish include depressed appetite and reduced growth rate. All these essential vitamins are included in artificially prepared diets to meet the specific requirements or your fish.
Inevitably, fish keeping creates waste including uneaten feed, ammonia, urea (an ammonia based compound), carbon dioxide, non-utilised digestible material (amino acids, fats and carbohydrates) and certain non-digestible “fibre”(a catch-all term for cellulose, grit, ash, including phosphorous, and other non-digestible plant, animal and inert material) and bacteria. These materials are very active, both chemically and biologically. Ammonia, urea, phosphorous and carbon dioxide are readily absorbed by aquatic algae, other plants and bacteria resulting in considerable growth. Not only do fish use oxygen, but the waste and organisms that feed on waste and waste by-products use even more. Eventually, accumulated solids on pond bottoms become anaerobic, allowing the production of more ammonia and highly toxic hydrogen sulphate. Solids can encourage a wide array of disease-causing organisms. Anaerobic conditions on the bottoms of ponds retard fish growth. The growth of algae in freshwater is primarily controlled by relative abundance of dissolved phosphates. Any significant increase in phosphates can lead to algae blooms (which may be toxic) and are the cause of eutriphication. It is therefore essential that ponds minimise the release of such components into the water column. Fish diets are therefore formulated to decrease phosphate levels, using readily digestible raw materials and in particular, low ash fish meals. As a result, a high proportion of this phosphorus is readily available to the growing fish, with less excreted into the environment. All of this waste is derived from the feed ether directly through overfeeding or directly as a result of the metabolism of an unbalanced diet. The aim of a well-balanced nutrient dense diet should be to minimise environmental damage in a number of ways:

• Reduced waste production.
• Reduced oxygen consumption.
• Improved water quality.

Most of us are paying a bit to much for fish feed. Feed being the major cost in fish keeping, such a trend could be alarming. But, we are comforted by advantages in diets and management, leading to even better feed conversion rations (FCRs). FCRs are commonly in the range of 1.5 - 2.0, and sometimes even lower. Do we really know what those numbers mean in terms of fraction of feed that is incorporated in the fish and the fraction that becomes waste? Remember that the contribution of fish waste to the ecology is directly in water. This is the resource we need for our fish to survive, let alone grow.
The most important element is not only the food cost per kg but the cost that the food contributes to each kg of fish. Thus we need to multiply the food cost by the food conversion ratio. A food costing R3.50 per kg and giving an overall FCR of 1.5 will have a “food to fish” cost of R3.50x1.5=R5.25 per kg. A food costing R3.70 per kg and giving a FCR of 1.33 will have a “food to fish” cost of R3.70X1.33=R4.92 per kg. The more expensive food is actually cheaper to use. This is because it uses better-balanced energy and protein sources.

Fish are slow eaters. It generally takes them 30 minutes to an hour to finish eating a dry diet, as compared to 5 minutes for trout. The feed should therefore be able to maintain its physical integrity in order to prevent leaching of nutrient, wastage of feed and possible pollution. Over feeding results in depressed digestibility of the feed due to fast gut passage and elevated levels of faecal waste. It also causes an imbalance in the uptake of nutrients coupled with gross surfeit of amino acids and lipids circulating in the blood stream. The surplus energy is deposited as fat

To conclude: in order to achieve the blend which offers the best possible nutrient specification at optimum cost, diets are formulated on computer using least-costing facilities. This is one of the most significant advantages in recent times. Each diet is carefully formulated to meet the unique nutrient requirements of each animal specie. In other words formulators try to equate nature and often succeed to do better.
The development of artificial feeds aim to achieve three prime objectives:

• To ensure that fish fed diets attain the cheapest cost of live weight gain per kilogram of feed consumed;
• To result in the very highest quality of fish, and
• To reduce the impact on the aquatic environment.

An alternative approach to seek various sources of prepared and live food that will make up a balanced diet and meet the daily nutrient requirements of your fish is a very complex approach and most often impossible. Rather than looking for some “magic” feed formula, quality of feed should be carefully controlled, and sensible feeding practices should be adopted. Then all the fish really require is good attention and a healthy environment.

Lourens de Wet