Pd/a Crsp Sixteenth Annual Technical Report

Stable carbon isotope analysis is a useful technique to obtain quantitative estimates of the relative contributions of different food sources to the nutrition of aquatic animals in ponds. This technique is being used to obtain quantitative estimates of the contribution of natural and supplemental feeds to the nutrition of tilapia in ponds in Sagana, Kenya. Results can be used to adjust feeding/fertilization practices and minimize feed costs while maximizing fish production. Samples of Oreochromis niloticus, Clarias, chemical fertilizers (DAP and urea), rice bran, plankton, and mud taken from ponds in Sagana at three times during the study (initial, midpoint, final) have been submitted to a commercial lab for carbon isotope analysis. Results for initial and some of the midpoint samples are summarized and discussed in this report. The most distinct trend in the isotope data was the more positive values for plankton, Clarias, and O. niloticus found in Treatment 1 versus Treatments 2 through 4 for both initial and midpoint samples. Possible reasons for this trend are discussed in light of experimental and non-experimental variables. A more comprehensive discussion of the effects of various nutrient inputs on the production of O. niloticus and Clarias will be possible once the remaining isotope data are obtained. SIXTEENTH ANNUAL TECHNICAL REPORT 44 Treatment δ13C (‰) ± S.D. Tilapia Clarias Plankton Mud 1. Chemical fertilizer: Urea (16 kg N ha-1 wk-1) and DAP (4 kg P ha-1 wk-1) -17.88 ± 0.86a -22.40 ± 2.08a -22.43 ± 1.79a NAc 2. 1/2 of treatment 1 -22.83 ± 0.72b -24.67 ± 1.27ab -26.97 ± 1.16b NA 3. Rice bran only (120 kg ha-1 d-1) -22.93 ± 2.39b -26.42 ± 1.23b -29.55 ± 1.23b NA 4. Rice bran (as treatment 3) and Chemical fertilizer (as treatment 2) -24.03 ± 1.17b -26.83 ± 0.61b -27.83 ± 1.75b NA the mean isotope ratios of DAP and rice bran were very similar: -26.8 and -27.8‰, respectively. The mean δ13C of midpoint plankton samples in Treatment 1 was significantly more positive than that of plankton in Treatments 2 through 4 (Table 2), as observed in the initial samples. The mean isotope ratios of O. niloticus and Clarias followed the same pattern as the plankton, although statistical differences were less pronounced for Clarias (Table 2). The mean isotope ratios of plankton were significantly more negative (P = .0001) in all treatments in the midpoint than in the initial and samples (Tables 1 and 2). The mean isotope ratios of O. niloticus and Clarias followed the same trend (Tables 1 and 2).