Pd/a Crsp Nineteenth Annual Technical Report

Stable carbon and nitrogen isotopes were used to obtain estimates of the contribution of natural and supplemental feeds to the nutrition of Oreochromis niloticus in ponds (free-swimming or caged) receiving different inputs in Sagana, Kenya. Three dietary treatments were employed in the pond study: 1) the test diet; 2) a pig finisher diet; and 3) a rice bran diet. Feeding rates and fertilization regimes are detailed in the report for 9FFR2. For isotope analysis, samples of Oreochromis (free-swimming and caged) and plankton were taken from ponds in Sagana three times (January, March, and May) during the study. The carbon and nitrogen isotope ratios of the diets were analyzed once. Modest fish growth during the study on all dietary treatments (the fish acquired ≤ 50% of their final weight between January and May) limited the application of the stable isotope technique for determining the relative assimilation of plankton and the different diets. The patterns of change in the δ13C and δ15N of free-swimming and caged Oreochromis and plankton over time and their possible interpretation were described within and between treatments. NINETEENTH ANNUAL TECHNICAL REPORT 18 RESULTS AND DISCUSSION Proximate composition of the diets is shown in Table 1. The test diet and the pig finisher diet were similar to each other in proximate composition, and the rice bran was lower in protein and higher in ash than the other diets. Several batches of each diet were formulated during the study, and rice bran varied the most in proximate composition between batches. The test diet and the pig finisher diet have similar δ13C and δ15N values (Tables 2 and 3). Rice bran had much more negative δ13C values and positive δ15N values than the other two diets (Tables 2 and 3). Oreochromis and plankton samples collected 5 January 2000 were used as the initial samples for isotope data. By January there were already significant differences in the δ13C values of the free-swimming fish and plankton in ponds receiving rice bran (Table 2) and in the δ15N values of the plankton in ponds receiving rice bran (Table 3). By contrast, there were no differences in δ15N of free-swimming or caged tilapia in ponds receiving any of the diets in January. There were also no differences in δ13C of caged tilapia in ponds receiving any of the diets in January (Table 2), probably reflecting their lower rate of weight gain and thus assimilation of different nutrients, as reflected by their isotope values. By March the δ13C values of the free-swimming and caged tilapia as well as the plankton were all significantly more positive in ponds receiving the test diet or the pig finisher diet compared to ponds receiving rice bran (Table 2). This trend continued in the fish and plankton samples taken in May (Table 2). The δ15N values of free-swimming tilapia, by contrast, were distinctly different among all three diets (Table 3), while there were no significant differences in δ15N of caged tilapia or plankton among diets in March (Table 3). This trend in the δ15N values continued into May (Table 3). Figure 1 shows the change in δ13C values in ponds receiving each of the diets over time. The test diet (Figure 1a) was isotopically distinct from the plankton (both Plankton and Plankton 2) for the whole period. The values for plankton and both free-swimming (Pond) and caged fish stayed within a fairly narrow range throughout the study, and there was no large decrease in δ13C of the fish, which would have indicated a high rate of assimilation of the test diet. However, freeswimming fish gained only about 50% of their final body weight during the period of observation. Generally, a weight increase of 250% is needed to determine whether a diet will be Diet Protein Lipid Dry Matter Ash Test 11.52 ± 0.00 6.33 ± 0.04 87.80 ± 0.00 8.10 ± 0.07 Pig Finisher 11.59 ± 0.49 4.18 ± 0.32 87.82 ± 0.00 9.04 ± 1.16 Rice Bran 8.10 ± 0.61 4.12 ± 0.18 90.59 ± 0.00 19.41 ± 0.08 Table 1. Proximate composition (%) of diets used in a feeding trial with free-swimming and caged tilapia in ponds in Sagana, Kenya.1 1 Values are means of two replicates ± SD for the first batch of diets used in the experiment. 2 Protein in subsequent batches of the test diet was within 1% of the first batch. Protein ranged from 9.8 to 12.4% for subsequent batches of the pig finisher and 4.7 to 8.1% for subsequent batches of rice bran. 3 Lipid in subsequent batches of all diets was within 1% of the values for batch 1. 4 Dry matter in subsequent batches of all diets was within 2% of the values for batch 1. 5 Ash in subsequent batches of pig finisher was within 1% of the value for batch 1. Ash in subsequent batches of rice bran ranged from 15.5 to 21.1%. Diet Free-Swimming Tilapia Caged Tilapia Plankton