Physiology of Finfish and Shellfish (2+1)

4.3.2.4 Energy Flow in Fish

All food,whether natural or man-made has a total or gross energy value. Not all the gross energy from nutrients is utilized, because some of it is not digested and will be lost as faecal energy material. Of the digestible components of the food, not all can be metabolized completely and further energy is lost through excretion in urine and through respiration across the gills, but these losses are relatively minor compared to the dietary energy excreted in the feces. The remaining metabolisable energy can then be used for maintenance, movement and growth,although a small amount is lost through heat production. The figure below shows this flow of energy along with some approximate percentage values.

It is apparent from the energy balance presented that only a portion of the total chemical energy contained within the food ingested is available to the animal (ie. net energy) for maintenance, activity and growth; 45% of the ingested food energy being lost as undigested food (faeces), metabolic excretion and as heat.Although the energy losses will vary depending on the composition and digestibility of the feed ingredients used, feeding regime, water temperature, fish size and the physiological status of the animal, since all biological systems obey the laws of thermodynamics the energy balance equation can be represented as follows:

Carnivores: 100C = 29P + 44R + 7U +20F

Herbivores: 100C = 20P + 37R + 2U +41F

Where C (for consumption) is the gross energy content of the food ingested, P the energy utilized in growth materials (production), R the net loss of energy as heat (R standing for respiration), U (urinary loss) the energy lost in nitrogenous excretory products, and F the energy lost in the faeces.

Where the figures are expressed as a percentage of the ingested food energy.

Thus approximately one third of a diet is used by fish for growth. As muscle is essentially protein,more than 33% of the diet needs to be in the form of protein to ensure maximum conversion and growth. It should be kept in mind that energy needs for maintenance and voluntary activity must be satisfied before energy is available for growth. Providing the optimum energy level in diets for shrimp or fish is important because an excess or deficiency of useful energy can result in reduced growth rates. For example, excess dietary energy may result in decreased nutrient intake by the fish or excessive fat deposition in the fish, Also, during times of low food intake, fat and protein are withdrawn from the animal body to provide the energy needs for maintenance and the animal loses weight.

(Not all of the gross energy from nutrients is utilized because some of it is not digested and absorbed for further metabolism. Thus,the amount of digestible energy (DE) provided by a feed or feed ingredient is commonly expressed as a percentage of gross energy. A smaller fraction of the DE absorbed by the fish will be lost in metabolic wastes, including urinary and gill excretions, but these losses are relatively minor compared to the dietary energy excreted in the faeces. Because it is hard to collect fish urinary and gill excretions, it is much more difficult to determine metabolizable energy (ME) values for aquatic organisms than for terrestrial animals. Therefore, ME values are not commonly reported for fish feeds or ingredients.

Heat of nutrient metabolism, also called heat increment or specific dynamic action (SDA), which is the increase in heat production following consumption of food by an animal. It includes the energy expended in digestion, absorption, transportation and anabolic activities. It also includes the cost of excretion of waste products.