58 March 2009 Table 1. Experimental design of a tank study with two commercial diets (30% and 36% CP) when fed at different ration sizes. Treatment ID % CP in the Diet n % Protein in Ration 30 - 100% 30 6 30 36 - 84.2% 36 5 30 30 - 84.2% 30 5 25.3 36 - 70.9% 36 6 25.3 Table 2. The effects of two commercial diets (30% and 36% CP), when fed at different ration sizes, on daily water parameters in an outdoor tank study with Litopenaeus vannamei when operated under limited water discharge. Treatment ID DO (mg/L) Temperature (oC) pH (% CP-Ration) am pm am pm am pm 30% CP-100% 6.6±0.42 7.0±0.20 27.1±1.46 28.1±1.81 7.7 8.0 36% CP-84.2% 6.6±0.37 7.0±0.22 27.1±1.47 28.1±1.68 7.7 8.0 30% CP-84.2% 6.6±0.38 7.0±0.20 27.1±1.48 28.2±1.74 7.7 8.0 36% CP-70.9% 6.7±0.38 7.1±0.20 27.1±1.50 28.2±1.91 7.8 8.0 diet fed, shrimp on the higher protein diet were fed only 84.2 g of 36 percent CP diet. To make sure the shrimp offered the lower protein diet were not overfed, the study also evaluated the effect of offering that diet at a reduced ration size, 84.2 percent of the full ration. That dietary protein was matched by feeding the higher protein diet at further reduced rate; for each 84.2 g of the 30 percent CP diet, the shrimp were fed only 70.9 g of the 36 percent CP diet. Feed was distributed manually four times daily. Growth was monitored weekly in one tank per treatment. Rations were calculated weekly based on the observed growth, estimated survival and an assumed FCR of 1:1.5 for all treatments. Dissolved oxygen temperature, pH and salinity were measured twice daily in each tank. Ammonium-nitrogen nitrite-N and nitrate-N were monitored weekly in selected tanks. Shrimp production and water quality data were analyzed using SPSS software (SPSS Inc., Chicago, Illinois). The Repeated Measures ANOVA test was used to identify differences in water quality indicators between treatment means. One way ANOVA served to identify differences between treatments in final weight, survival, yield, FCR and growth. The Student Newman Keuls test was used as a mean separation tool. Results There were no significant differences in daily mean water quality indicators among treatments (Table 2). Salinity varied between 25 and 28 ppt. The means for ammoniumnitrogen and nitrite-nitrogen were 0.26 ± 0.46 and 0.46 ± 2.31 ppm, respectively. Mean daily water renewal, including rain and added freshwater, varied between 0.85 and 0.90 percent. Mean shrimp survival varied between 87.7 percent and 94.9 percent with no significant difference among treatments (Table 3). The yields varied between 0.98 kg/m3 and 1.14 kg/m3 with the latter representing shrimp fed the higher protein diet at the 84.2 percent rate of the lower protein diet. This yield was significantly higher than the one obtained when the shrimp were fed the lower protein diet at the reduced rate. No significant differences were found among treatments in yields. No differences were found among treatments in average weight, yield, survival and growth when shrimp were fed the higher protein feed on an isonitrogenous basis to the 30 CP feed. This was also the case when the shrimp were fed on an isonitrogenous basis aimed to provide 25.3 percent protein; however, those shrimp were significantly smaller than those fed the diets at the 30 percent protein level. Of importance is the fact that the FCR of shrimp fed the higher protein diet at both levels, 30 percent and 25.3 percent, were significantly lower than those obtained when shrimp were fed the lower protein diet at full and at reduced rations. Although digestible energy and the protein were not determined in our study, the good performance of the higher protein feed, when offered at a lower ration rate that met the nitrogen level offered by the lower protein ration, suggests that the ratio of digestible energy to protein in the higher protein feed was adequate. The results from this study showed that feeding the higher protein diet on an isonitrogenous basis to the lower protein diet resulted in significant improvement in FCR. Although one can expect to pay more for the higher protein feed, the lower ration size used in the case of the higher protein diet can easily offset the differences in feed cost. Furthermore, the uses of smaller amounts of feed translates to less pollutants being released into the environment. Pond Study A concurrent 138 day growout study was conducted in two HDPE membrane-lined ponds, each with working water volume of 2,450 m3, using the same commercial diets, 30 percent and 36 percent CP, made by the same feed mill. That study evaluated the effect of feeding the higher protein feed at reduced ration rate, which provided the shrimp with same nitrogen level offered by the lower protein diet. Both ponds were stocked with juvenile (0.8 g) L. vannamei at a density of 74/m3 (106/m2). Shrimp in one pond were fed the lower protein feed, while those in the other pond were fed the higher protein feed. The higher
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