Bioenergetics - Application in Aquaculture Nutrition

Bioenergetics describe the flow of energy and nutrients within a biologicalsystem in our example a fish or shrimp. It describes the biological process of utilisation and transformation of absorbed nutrients for energy, for own body synthesis.The feed, that is consumed, is transformed in the body, complex chemical compounds are broken down into simpler components -protein into amino acids, carbohydrates into glucose, lipids into fatty acids and with this process energy is released - which is used for maintenance, for renewing worn out tissue and building new tissue - for growth. The major organic compounds in feeds such as lipid, protein and carbohydrates are the sources of energy but they also supply the building material for growth.

There are different types of energy, chemical energy, electrical energy, mechanical energy and heat. These different forms of energy can be transformed into each other but only at a cost, the transformation is not 100 percent efficient. What is lost is mostly in the form of heat. Heat is also the only form of energy, into which all the others can be transformed and measured. The chemical energy stored in feed and animal tissue is measured using a bomb calorimeter. The amount of heat produced by complete oxidation of feed or tissue is known as the heat of combustion or gross energy (GE). Heat energy is usually expressed in kilocalories (kcal) or kilojoule (kJ). One kcal equals the energy needed to raise the temperature of one kg of water by one degree Celsius (°C). One kcal equals 4.184 kJ.

For the bio-energetic model, the two laws of thermodynamics can be applied:

1. Energy cannot be created or destroyed within a system but many be changed into different forms (what goes in must go out)

2. In a system where energy is transformed (from feed to flesh) there is a degradation and loss of energy in the form of heat (nothing is 100 percent efficient)

The flow of energy from feed to growth in an animal is illustrated in Figure 1. Not all the energy from the feed is digested, substances such as fibre and cellulose from plant ingredients pass through the digestive system without being available to the fish. The consumed GE minus faecal energy losses (FE) is called the digestible energy (DE) which is then available for the metabolic processes of an animal.

The next major losses occur, when energy containing compounds (on DE basis) are transformed by the fish, broken down to smaller units and then used to build its own energy reserves or to deposit protein as growth. As mentioned above, this process of transformation is never 100 percent, there are always losses and they are mostly in the form of heat. In poikilotherms such as fish this heat is lost to the surrounding water, in homeotherms it is partly used to keep the body temperature constant. Only the net energy (NE) is now available for maintenance and for growth. Maintenance requirement represents energy needed for movements, osmo-regulation, blood circulation, first this energy has to  be supplied before the remainder can be channeled into growth - the main product in fish culture. 

By quantifying the energy budget - the energy input on one hand and the various energy losses on the other hand, valuable information can be gained in order to optimise feeds and guarantee optimal fish growth. By defining demands for maintenance and growth (Figure 1) and anticipating certain losses beforehand, feeds can be formulated and feeding tables established.  

Fish require energy for maintaining basic processes of life such as blood circulation, osmo-regulation, excretion and movement, regardless of whether or not feed is consumed. An animal deprived of feed continues to require energy for those processes and will obtain it from the catabolism of own body reserves. Depending on the activity, several metabolic levels can be distinguished: basal, standard, routine and active metabolis.

Metabolic rate (Q) at all levels of activity, depends largely on the size of the fish and the water temperature, and is (at constant temperature) proportional to the metabolic body weight in the form of  

Where (kg)^b: Metabolic body weight; a is the constant for given conditions (species, activity, temperature); bis the scaling exponent of the metabolic body weight

Most metabolic studies on fish are carried out via indirect calorimetry. This is based on the assumption, that energy production in an animal is an aerobic process and requires oxygen for oxidising nutrients either from the food or from the tissue. In this case it is assumed that the amount of oxygen taken up by respiration will release an equivalent amount of energy which can be calculated from the oxy-caloric value. Another method is the comparative slaughter technique which measures the caloric value of the tissues utilised during fasting. 

Figure 2 illustrates the relationship between metabolic rate of a fasting fish (gilthead sea bream) and weight. 

The relationship between fasting metabolism and fish weight is not linear and results (Figure 2) were fitted to ln - ln functions as have traditionally been used by animal nutritionists to express metabolic body weight. The antilog of these functions describes the allometric relationship common in biological measurements.  

With an exponent of b = 0.80 for the metabolic body weight, the implication is that metabolic rate is increasing with increasing fish weight in absolute terms (kJ/fish/day), but smaller fish spend more energy per unit size than bigger fish. This concept of metabolic body weight will be clarified further on. It should be noted that the fasting metabolism is only an approximation of the maintenance requirement; allowance must be made for the efficiency of utilisation of the dietary energy. This can be achieved by feeding fish graded levels from zero feed up to maximum intake. Energy gain or loss in fish is then determined by comparative slaughter technique. The following Figures 3 and 4 describe the relationship between energy fed (DE) and energy retained for sea bream of two different sizes. (at 210C). 

It is obvious from Figure 3 that as more energy is consumed the more energy is gained, until the fish refuse to eat more. Figure 3 also demonstrates that the relationship between daily DE consumed (x) and energy retained (y) is linear and can be described by the following equations for each the two fish sizes:  

During fasting the fish would lose energy as expected - 2.2 kJ per fish of 30 g and 4.6 kJ per fish of 100 g per day. The DE requirement for maintenance (no energy gain or loss) can be found where energy gain (y) is set at zero. According to the equations above, the maintenance requirement per day would amount to 2.2 / 0.66 = 3.33 kJ for the 30 g fish and 6.86 kJ for the 100 g fish. As mentioned before, absolute maintenance requirement is increasing with increasing fish weights, but regarded per unit of weight gain it is decreasing. Energy requirement of the smaller fish is 110 kJ / kg and for the larger fish only 69 kJ / kg. 

The slopes of the lines are nearly identical at 0.67; they can be regarded as the efficiency of utilisation of energy. Per unit of DE consumed 67 percent is retained as growth, the remainder is lost as heat to the water. 

In Figure 4 the same data set is used but daily energy retention in fish is presented referring to the metabolic weight of kg^0.80. By expressing DE intake and the subsequent retention of energy per metabolic weight (kg^0.80) the resulting regressions of the relationships for both fish sizes can be combined.

Thus the relationship between DE fed (x) and energy gained (y) both expressed in kJ / kg^0.80 / day is as follows: 

According to the equation (4), the maintenance requirement per day would amount to 33.7/0.67 = DE_maint = 50.3 kJ x kg^0.80 (at 21ºC). Again the slope of the line, the efficiency of energy utilization for growth remains the same at 0.67. The reciprocal of 0.67 is 1.49 (1/0.67), which means that 1.49 kJ of DE have to be invested to produce 1 kJ of energy as growth, in other words, the energy cost to deposit one unit of energy as gain is close to one and a half units of energy from the feed (based in DE). 

Besides fish weight, water temperature is one of the major factors to determine maintenance requirement. Adding data of an additional trial with sea bream performed at 27ºC provides the following equation for the relationship between DE fed and energy gained per (kg) ^0.80 (Figure 5): 

According to equation (5), the maintenance energy requirement would amount to DE_maint = 78 kJ kg^0.80 at a temperature of 27ºC, while at 21ºC the maintenance requirement was calculated as 50.3 kJ kg^0.80 as shown before. However in both instances the slope of the line (efficiency) remains the same even at the higher temperature. 

To be able to estimate feed requirements it is essential to predict the growth potential of the target species. In contrast to terrestrial animalsfish seem to grow continuously, growth does not cease and reaches an asymptote, which in aquaculture however might never be attained. As growth is affected by temperature, it increases with increasing temperatures up to an optimum above which growth decreases, until the upper lethal temperature is reached. 

Together with the anticipated increase in weight, the energy content of this gain is another factor determining the subsequent total energy demand of fish.

The following equations describe the daily weight gain of gilthead sea bream for water temperatures ranging between 20 and 28ºC and the energy content per unit of weight gain. 

The calculation of daily energy and consequently the feed demand (based on digestible energy DE, i.e. the amount absorbed through the gut) for fish can then be described as follows: 

The expected live weight gain, which is dependent upon fish size and water temperature, can be predicted with the following common equation, where again a, b, and c are constants typical for a fish species:

The average energy content of the weight gain for a fish is dependent on the fish size and can be described as: 

The expected daily energy gain is therefore:

For the quantification of daily maintenance requirement which is the energy requirement at zero growth:  

The cost of production as DE intake (in units of kJ for energy) for one unit of energy deposited as fish energy (as growth) is for many fish species around 1.50 or 1 / 1.50 = 0.67 = efficiency for growth.

Combining those equations suggests that the feed allowance based on energy intake can be calculated as follows: