The essential thrust of studies on bioenergetics of animals is to provide a basis for diet formulation and evaluation, and develop convenient and accurate systems to predict the energy balance of animals based on body weight, sex, activity, physiological state, environment, and amount and nu- tritive value of feed consumed (Baldwin and Bywater, 1984).
Several bioenergetics models have been developed to predict energy re- quirements and growth of fish under a variety of conditions (reviewed by Cui and Xie, 1999). In several bioenergetics models developed mostly by fish ecologists, FE, UE+ZE, HiE, HeE, and the GE content of the carcass are considered a fixed fraction of IE regardless of the composition of the feed and performance of the fish. A basic understanding of nutrition should indicate that these are unrealistic assumptions.
It is common to observe energy requirements expressed as the absolute amount of DE required per kilogram body weight per day for maximal pro- duction or energy expenditure, and deposition expressed as a proportion of the maximum feed consumption (Cmax)in numerous fish bioenergetics studies (e.g., Gatlinet al.,1986; McGooganet al.,1998; Ohta and Watanabe, 1998; Cui and Xie, 1999; Elliott and Hurley, 1999; Watanabeet al.,2000). It is important to recognize that the maximal production andCmaxof an animal are factors of genetics, diet, environmental conditions (e.g., temperature), husbandry practices, health status, and other factors. Maximum production andCmaxare, therefore, highly variable parameters. Consequently, the en- ergy requirement for maximum production calculated in some studies (i.e., energy requirement expressed as an absolute term such as kJ fish−1day−1) can be valid only for the specific conditions (diet composition, strain, tem- perature, culture conditions, etc.) encountered in the study. Fish growing at different rates will deposit nutrients at different rates and, consequently, have different energy and feed requirements. Energy requirements should therefore be calculated for explicitly expressed levels of performance (e.g., expected or achievable level of performance), feed composition, and life stage (Cho, 1991, 1992; Cho and Bureau, 1998; Kaushik, 1998). In addition, this should be done using factorial approaches (Cho and Bureau, 1998, Lupatschet al.,1998), i.e., approaches that divide energy requirements into different components or fractions, as opposed to lumping them into one estimate as is commonly done.
Cho (1991) proposed factorial models to determine energy requirements of fish based on expected level of performance, diet composition, and ex- pected body composition. These models were updated by Cho (1992) and Cho and Bureau (1998). Using this approach, calculation of the total energy requirement and, consequently, the feed requirements (or allocation) can be accomplished as follows.
1. Characterization of diet (including DE content) 2. Calculation of expected live weight gain and RE
3. Allocation of HeE based on fish size and water temperature 4. Allocation of HiE for maintenance and energy deposition 5. Allocation of UE+ZE
6. Calculation of minimum DE requirement 7. Calculation of feed requirement
Determination or estimation of DE, HeE, HiE, and UE+ZE can be done using the approaches described above or by carefully analyzing the litera- ture. It is imperative to take into account the composition of the diet and type of fish used (species, life stage, etc.) rather than blindly applying values reported in the literature.
Accurate prediction of the growth potential of a fish stock under given husbandry conditions is an inevitable prerequisite to estimation of the en- ergy or feed requirement (e.g., weekly ration). The formula most commonly used for fish growth rate expression is the instantaneous growth rate, known as the “specific growth rate” (SGR), which is based on the natural logarithm of body weight:
SGR=(lnFBW−lnIBW)/D. (4)
where FBW is the final body weight (g); IBW, the initial body weight (g); and D, the number of days.
The SGR has been widely used by most biologists to describe the growth rate of fish. However, the exponent of the natural logarithm underestimates the weight gain between the IBW and the FBW used in the calculation and it grossly overestimates the predicted body weight at weights higher than the FBW used. Furthermore, the SGR is dependent on the IBW, making comparisons of growth rates among groups meaningless unless the IBW are similar.
A more accurate and useful coefficient for fish growth prediction in re- lation to water temperature is based on the exponent 1/3 power of body weight (Iwama and Tautz, 1981).
Thermal-unit growth coefficient (TGC)
=[FBW1/3−IBW1/3]/[T D] 100 (5) Predicted final body weight
=[IBW1/3+(TGC/100T D)]3 (6)
whereTis the water temperature (◦C).
This model equation has been shown to represent very faithfully the actual growth curves of rainbow trout, lake trout, brown trout, chinook salmon, and Atlantic salmon over a wide range of temperatures. Figure 1.6 shows the growth curve of rainbow trout fed to near-satiation and reared at 8.5◦C. Live weight increases curvilinearly, whereas the cubic root of live weight increases in a highly linear fashion, supporting the observations of Iwama and Tautz (1981) and the validity of the TGC model.
Since these TGC values and growth rate are dependent on species, stock (genetics), nutrition, environment, husbandry, and others factors, it is
(Live weight)1/3 = 0.019x + 5.38 R2 = 1.000
0 100 200 300 400 500 600 700
0 28 56 84 112 140 168
Days
Live weight (g/fish)
0 10 20 30
Live weight (g/fish)1/3
FIG. 1.6
Live weight and cubic root of live weight of rainbow trout fed to satiation and reared at 8.5◦C for 168 days. Data from Bureauet al.(unpublished observations, 2000).
essential to calculate the TGC for a given aquaculture condition using past growth records or records obtained from similar stocks and husbandry con- ditions. Once the weight gain is known, RE can quite easily be predicted using simple models (as shown by Figs. 1.3 and 1.4, for example). Devel- opment of such models can be done relatively easily, as it may quite simply involve sampling animals at different sizes and determining their chemical composition.
Using an approach similar to that of Cho (1991), the energy, oxygen, and feed requirements and expected feed efficiency of fish of different sizes reared under different conditions or rearing periods can be calculated (Cho and Bureau, 1998). Table 1.6 lists the energy and oxygen requirements of rainbow trout reared at 12◦C and fed a diet with 44% DP and 20 MJ DE at different sizes or growing from 1 to 1000 g with a TGC=0.220. The DE to produce a 1-kg biomass (e.g., 1000 10-g fish gaining 1 g each) varies from about 9.5 MJ for 1-g fish to 23.6 MJ for 1-kg fish. It is of utmost importance to understand that these estimates are valid only for the given set of conditions (species, water temperature, TGC, diet composition, etc.) and should not be applied blindly.
Data from Azevedoet al.(1998) show that the DE requirement is largely independent of water temperature, since as the temperature increases, DE and RE increase but the efficiency of ME and DE utilization does not change.
Energy and Oxygen Requirementsaand Expected Feed Efficiency of Rainbow Troutb
GE REd HeEe HiEf UE+ZEg DEh Oxygeni
Live weight (kJ g−1) (MJ kg−1 (MJ kg−1 (MJ kg−1 (MJ kg−1 (MJ kg−1 (g kg−1 Feed (g fish−1) live weight)c weight gain) weight gain) weight gain) weight gain) weight gain) weight gain) efficiencyj
1 4.4 4.4 1.1 3.7 0.3 9.5 357 2.10
5 4.8 4.8 1.6 4.3 0.3 11.1 433 1.81
10 5.2 5.2 1.8 4.6 0.3 11.9 472 1.68
50 6.8 6.8 2.4 6.2 0.5 15.8 623 1.28
100 6.9 6.9 2.7 6.5 0.5 16.6 675 1.21
500 8.1 8.2 3.5 7.9 0.6 20.2 840 1.00
1000 9.8 9.8 4.0 9.2 0.7 23.6 968 0.85
1–1000 — 8.7 3.6 8.2 0.6 21.1 869 0.95
aMJ or g kg−1weight gain.
bAt various sizes or growing from 1 to 1000 g, based on the assumption that the fish are reared at 12◦C, growing with a TGC=0.220, and fed a diet with 20–22 g DP/MJ DE and 20 MJ/kg DE.
cGE, gross energy content of carcass. Calculated from experimental data (Bureauet al.,unpublished) as follows: for fish 30 g or less: GE (kJ g−1)= −0.0006 (live weight)2+0.0948 (live weight)+4.31; for fish more than 30 g, GE (kJ g−1)=0.0031 (live weight)+6.61.
dRE=(live weight gain; g fish−1)(GE content).
eHeE=[−1.04+3.26(T)−0.05(T)2](0.0200.824)−1day−1(Cho, 1991).
fHiE=0.67 (HeE+RE) (Azevedo, 1998).
gUE+ZE=0.03(HeE+RE+HiE) (Kaushik, 1998).
hDE requirement=(RE+HeE+HiE+UE+ZE).
iOxygen requirement=(HeE+HiE)/13.6 kJ g−1O2.
jExpected feed efficiency=weight gain/feed.
The total energy requirement should ideally be expressed as DE, since FE and, consequently, IE are highly dependent on the composition of the diet fed. FE is mainly from undigestible starch, fiber, and some protein in the diet and is influenced by the quality of ingredients. Less expensive commercial diets tend to contain higher levels of undigestible plant products, diluting digestible nutrients and increasing the amount of FE.
1.16