Part III: Nutritional Bioenergetics

Koi nutrition differs from pets like dogs and cats because they are cold blooded animals living in aquatic environment.

(a) Koi do not expend energy to maintain a body temperature like we do

(b) Koi excrete waste nitrogen through gills in the form of ammonia where as humans, dogs and cats need to convert them into urea and uric acid before excreting them in the form of urine. So, Koi require less energy to get rid of nitrogenous waste compared to warm blooded animals.

Energy is lost from the body of a Koi in the feces, urine, gill excretions. In addition, energy is also lost as heat. Heat is generated during:

(a) standard metabolism

(b) physical activity, such as swimming about exploring, staying suspended etc

(c) Heat released by chemical reactions that convert ingested feed into energy.

Energy is required to maintain the body (to breathe, for movements, reproduction and to repair wear and tear). In fact, maintenance of life processes comes first compared to growth and other functions. Only the excess energy will be used for growth!

Therefore, energy must be the first priority while considering optimum diet. Koi prefer protein over carbohydrates as energy source because they can excrete the protein waste using very little energy and therefore net energy gained from proteins is higher than with carbohydrates.

In practice, however, protein is more expensive than other energy sources like carbohydrates and fats. And, detoxifying the ammonia released into water is cumbersome and expensive. So, we feed amount of protein and energy in balance. If large amounts of protein are fed, Koi will simply use protein as their main energy source to maintain their body and use only excess protein for growth. This is a very expensive way to bring up Koi. And these Koi do not grow any larger than the koi fed balanced diet. On the other hand, if Carbohydrates and Fats are high and proteins are low in the diet, then Koi will deposit large amounts of fat in their tissues as in the case of most “Jumbo” koi.

Ratios of digestible protein to Digestible Energy (mg/kcal) for maximum weight gain for Koi is 108mg/Kcal.

Natural diets of Koi contain 50 percent fat. Fat provides 8.5 kcal of usable energy per gram. The fatty acid products of digestion are well utilized by most fish. (There is some evidence that high levels of short chain fatty acids can depress growth).

The energy value of carbohydrates in fish diets depends on the source and type of carbohydrate and how it is processed. Simple sugars provide 3.8 kcal/g of usable energy which is much less compared to fats. Cellulose or plant fiber provides energy near zero and raw starch provides from 1.2 to 2.0 kcal ME/g. Cooking of starch can increase the energy to about 3.2 kcal/g.

Protein provides an energy value of 4.5 kcal/g, which is higher than that for mammals and birds. This is because excretion of ammonia through gills uses less energy than excretion of urea in urine. In general, proteins from animal sources are more digestible than those from plant sources. Protein is used very efficiently by fish as a source of energy but proteins are more expensive and higher ammonia production requires more filters.

Feeding levels must be high enough to supply maintenance needs and still have energy remaining for growth. Generally, people overfeed their fish thinking they can grow their fish into “jumbo” size. But in reality, digestion efficiency in fish decreases as feeding level is increased once it crosses the optimum level.

Why?
i) Heat produced during digestion is proportional to the level of feeding. Production of heat uses a lot of energy
ii) The energy used to excrete urine and ammonia through gills also increases with increased feeding.

The reduced efficiency at high levels of feeding is shown in Fig. 2 by the proportionally large area representing feces at high levels of feeding. The amount remaining for growth is zero at maintenance feeding and becomes proportionately greater as feeding level is increased, until it is balanced by the decreased efficiency, of digestion.

Fig. 2. Distribution of dietary energy intake in a growing fish at various levels of feeding. (DE - digestible energy, ME - metabolizable energy, NE_p - net energy for production, NE_m - net energy for maintenance, H_p - heat production)

The heat production can be measured directly in a calorimeter

There are several factors which can alter the energy requirements of Koi.

• Temperature: As water temperature declines, body temperature of the fish declines and metabolic rate is reduced. The low metabolic rate at low temperatures enables Koi to survive for long periods under ice where little food is available. If temperature gradients exist, the Koi will seek the most favorable temperature. Usually this is the temperature at which highest growth occurs.
   
• Water Flow: Energy which is used for physical activity is not available for growth. Fish which are forced to swim against a strong current are expending energy which would otherwise be used for growth. However, still water allows stratification and the accumulation of waste products. Fish ponds should be designed to obtain maximum use of water without undue stress on the fish. Moreover, Koi are riverine fish and are not used to suction. Bottom drains that are too close to each other or the ones that suck huge amounts of water create stress.
   
• Body Size. Small animals produce more heat per unit weight than do large animals. Small fish should be fed a higher percentage of body weight than large fish. In fish, the metabolic rate is proportional to 0.34 to 1.0 power of body weight. Protein requirements, as a proportion of the diet, decrease as fish approach maturity.
   
• Level of Feeding: The oxygen consumption increases shortly after feeding due to the physical activity of feeding and the heat of nutrient metabolism. The oxygen required per unit weight of feed also varies with feeding level. If food is given only enough for body maintenance (spring and fall), all of the food is oxidized and therefore oxygen requirements are high. At higher feeding levels (in summer) much of the energy is stored for growth and therefore oxygen consumption is less compared to the level of feeding. So, if you feed high protein food in late summer to fall you can satisfy the high oxygen requirement because the water is cooler and nitrifying bacteria are still at their best.
   
• Other Factors: Anything which makes the fish uncomfortable increases physical activity and reduces growth. Crowding, predators, low oxygen and waste accumulation are some of these factors.

Bibiliography:
Kleiber, M., 1961 The fire of life. New York, Robert E. Krieger Publishing Co.

Maynard, L.A. and J.K. Loosli, 1962 Animal nutrition. New York, McGraw-Hill Book Co

Winberg, G.G., 1956 Rate of metabolism and food requirements of fishes. Transl.Serv.Fish.Res Board Can., (194)

Smith, 1989

Wilson and Halver, 1986

R. R. Smith, Tunison Laboratory of Fish Nutrition and Hagerman, Idaho