Calculation of growth capacity

The growth capacity was calculated as the maximum of nitrogen deposition by using model from (GEBHARDT 1966).The results show that the daily maximum N- deposition for hybrid Tilapia (O. niloticus X Red Tilapia) from 12 to 42 g body weight at day 28 of the experiment, resulted in 443 mg N/BW _kg ^0,67 for fish fed on diets 1 - 5 containing 16 % to 48

% crude protein with the same energy level of 15.6 MJ ME/kg. 407 mg N/BW _kg ^0.67 were found for fish fed on diets 6 – 10 containing 16 % to 48 % crude protein level with adapted energy levels of 13.6 to 17.6 MJ ME/kg. After 56 days, the hybrid of Tilapia with body weight from 12 g to 70 g, fed on diets 1-5 containing 16 % to 48 % crude protein level with the same energy level of 15.6 MJ ME/kg resulted in 233 mg N/BW _kg ^0.67. 217 mg N/BW _kg

0.67

were found for fish fed on diets 6 – 10 containing 16 % to 48 % crude protein level with adapted energy levels of 13.6 to 17.6 MJ ME/kg, respectively.

At the end of the experimental period, the hybrid of Tilapia with body weight 12 g to 84 g fed on diets 1-5 containing 16 % to 48% crude protein level with the same energy level of 15.6 MJ ME/kg resulted in 250 mg N/BW kg 0.67

. 232 mg N/BW kg 0.67

were calculated for fish fed on diets 6 – 10 containing 16 % to 48 % crude protein level with adapted energy levels of 13.6 to 17.6 MJ ME/kg, respectively. The maximum N-deposition data are presented in table 42.

In general the daily N-deposition capacity decreased with age of fish also some changes were observed corresponding to energy density the diets 6 - 10. The N-deposition curve are presented in figure 6 and figure 7, respectively.

Table 42: Calculation of maximum N-deposition capacity dependent on test diets and age (genotype 3)

Diet 28 days 56 days 70 days

Isoenergetic diets 443 mg N/BW kg 0.67

233 mg N/BW kg 0.67

250 mg N/BW kg 0.67

Adapted diets 407 mg N/BW _kg ^0.67 217 mg N/BW _kg ^0.67 232 mg N/BW _kg ^0.67

Figure 6: N-deposition curve at the end of the experiment for isoenergetic diets (genotype 3).

Figure 7: N-deposition curve at the end of the experiment for adapted energy diets ( genotype 3).

-200 -150 -100 -50 0 50 100 150 200 250 300

0 100 200 300 400 500 600 700 800

Daily N-intake( mg/kg ^0.67) Daily N-deposition (mg/kg0.67 )

-200 -100 0 100 200 300

0 200 400 600 800

Daily N-intake( mg/kg ^0.67 ) Daily N-deposition (mg/kg 0.67 )

5. DISCUSSION

The present investigations were carried out for the description of physiological response of different Tilapia genotypes to different protein supply in combination with different energy density in the diet. Based on the results of these experiments we create basic data for the use in an exponential N-utilization-model and furthermore the estimation of growth capacity of different Tilapia genotypes. The aim of the following discussion is to give a comparable and general explanation about these aspects.

5.1. The effect of dietary protein intake on growth performance

The results of the present study showed that the fish fed on all diets were growing, but in general, growth rate, weight gain and weight gain percentage were strongly related to dietary protein levels and feed intake. As given in table 13, 23 and 33, respectively, the results indicated that final body weight, weight gain and weight gain percentage increased by the increasing of dietary protein levels in the diets. The statistical analysis of the results of the experiments showed that the body weight, weight gain, weight gain percentage and also feed intake increased significantly up to 40 % crude protein with P:E ratio ranged from 22.80 to 29.10 g protein/MJ ME for genotype 1 and 32 % crude protein with P:E ratio from 19.30 to 29.10 g protein/MJ ME for genotype 2 and 3, respectively. The results underline a more pronounced sensitivity of genotype 1 corresponding to a higher protein supply mainly with diets 4, 5, 9 and 10.The daily N-deposition data also show the same pattern. The pattern of growth data is similar to those reported for some Tilapia genotypes. JAUNCEY (1982) found that the optimum protein level was 40 % for O. mossambicus at P:E ratio 27.9 g protein/MJ ME. MAGAUZE (1990) found that the optimum protein level required for producing maximum growth for O.niloticus was found to be 41 % at a P:E ratio ranged from 26.7 - 29.5 g protein/MJ ME. Also ABDELGHANY(2000) reported optimum crude protein level required for producing maximum growth for O. niloticus was 35 % - 40 % at a P:E ratio 13.01 g protein/MJ ME. These studies are in close agreement with that reported in our study mainly with genotype 1. However, the results of experiment 2 and 3 are also similar to those reported

by VIOLA and ZOHAR (1984) who found that optimum growth was indicated with protein level of 30 - 35 % for hybrid (O. niloticus x O. aureus). ROSS (1982) reported the same protein level for O. mossambicus. This pattern is different with the results, which was reported by SHIAU and HAUNG (1989) the authors found that optimum protein level was 24

% for Tilapia hybrid (O. niloticus x O. aureus ) rearing in the sea water, while WANG et al (1985b) reported as optimum of protein at level 29 % and a P:E ratio of 15.4 mg protein /kJ GE. The difference between the results of the studies and those of our study may be attributed to the environmental conditions of the experiments such as temperature, salinity , size of fish, energy density of the diet or to the physiological state of fish and feed intake.

The difference between the results with different genotypes in our study may be due to the different sex ratio between male and female in tank culture, which was obtained mainly for genotype 2 and 3 after 4 weeks of the experimental period. The feed intake which was required for optimum growth was effected by the sex ratio in tank culture. DE SILVA and RADMPOLA (1990) reported that optimal protein level for growth of O. niloticus both male and female was 30% and the greatest percentage of female spawning occurred at 25% crude protein level.

HICKLING (1968), KUO (1969), PURGININ et al. (1975) reported that the all male Tilapia were growing faster than the other Tilapia genotype. In Tilapia low protein rations induce females to mature earlier, produce more eggs relative to their body size and spawn over a longer period of time. This in agreement with MINRONOVA (1978), who showed that reducing feed of uniform quality induced reproduction in female T. mossambicus. When the dietary level is optimal for growth a greater proportion of the population spawns.

The effect of dietary protein level of diets on the growth performance on different Tilapia genotype is shown in figure 8.

Figure 8: The effect of dietary protein level of the diets on growth performance of different Tilapia genotypes (70 days).

The present study indicated also that weight gain and specific growth rate were increased gradually with increasing dietary protein level. As given in table 14, 24 and 34 respectively, statistical analysis showed the same pattern such as weight gain. This pattern is similar to those reported for brown trout (POSTON, 1975), for rainbow trout (LEE and PUTNAM, 1973), for Yellow tail (TAKEDA et al., 1975), for penaeus merquiensis (SEDGWICK, 1979) and for Tilapia (KAUSHIK et al., 1994) where the growth increased with further increase in dietary protein level and P:E ratio above the optimum. Maintenance of growth rate or weight gain at the higher protein levels above the optimum depends to a large extent on the energy content of the diet. When energy level in the diet is enough to compensate the energy losses in catabolizing and excreting the excess of protein growth will be maintained and not decrease. Table 43 shows the N-deposition data of different Tilapia genotypes (see 5. 5.).

0 20 40 60 80 100 120 140 160

1 2 3 4 5 6 7 8 9 10

Diets

Final body weight (g)

Genotype 1 Genotype 2 Genotype 3