Cite as: [Author(s), 2002. Title.] In: K. McElwee, K. Lewis, M. Nidiffer, and P. Buitrago (Editors), Nineteenth Annual Technical Report. Pond Dynamics/Aquaculture CRSP, Oregon State University, Corvallis, Oregon, [pp. ___.]

Fish Yields and Economic Benefits of Tilapia/Clarias Polyculture in Fertilized Ponds Receiving Commercial Feeds or Pelleted Agricultural By-Products

Ninth Work Plan, Feeds and Fertilizers Research 2A (9FFR2A)
Final Report

Rebecca Lochmann
Department of Aquaculture and Fisheries
University of Arkansas at Pine Bluff
Pine Bluff, Arkansas, USA

Abstract

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 ¹³C and ¹⁵N of free-swimming and caged Oreochromis and plankton over time and their possible interpretation were described within and between treatments.

Introduction

Stable carbon and nitrogen isotope analyses are useful techniques to obtain quantitative estimates of the relative contributions of different food sources to the nutrition of aquatic animals in ponds (Schroeder, 1983; Anderson et al., 1987; Yoshioka et al., 1989; Lochmann and Phillips, 1996; Lochmann et al., 2001). Presumably, isotope ratios of the fish will resemble those of the food(s) they assimilate most. The carbon isotopes are used to delineate the directions of carbon flow throughout a food web, and the nitrogen isotopes can be used to indicate the trophic level of different organisms in a pond. The double tracer technique is usually more powerful than the use of a single isotope in determining patterns of nutrient flow in aquatic systems, particularly when organisms of multiple trophic levels are present. In this study stable carbon and nitrogen isotope ratios of diets and plankton were compared to those of Oreochromis (free-swimming in ponds or confined to cages in the same ponds). Although Clarias was included in the study as a predator, insufficient data were collected on this species to determine its nutrient sources during the study. The caged Oreochromis did not receive any prepared feed and were expected to consume mostly plankton, while the free-swimming fish in the same pond had access to both an artificial input (one of the three diets) and plankton. Acomparison of the stable carbon and nitrogen isotope ratios (¹³C and ¹⁵N, respectively) between the two groups was expected to reveal the additional benefits, if any, of adding supplemental inputs (diets) to ponds containing plankton. We anticipated using the results to optimize nutrient utilization and minimize feed and fertilizer costs for Oreochromis production in Kenya and possibly other locations.

Methods and Materials

Pond production data such as stocking rates, feeding rates, and fertilization regimes are detailed in the final report of Liti et al. (see "Growth performance and economic benefits of Oreochromis niloticus/Clarias gariepinus polyculture fed on three supplementary feeds in fertilized tropical ponds") and will not be repeated here. The three diets tested in this study were a test diet, a pig finishing ration, and rice bran. Each diet was fed to fish in four ponds, for a total of twelve ponds, for approximately seven months. Because some of the experimental samples were lost in Sagana prior to analysis, this report includes data for only five months of the study (January through May). Dry matter and ash analysis of the diets was conducted at the University of Arkansas at Pine Bluff using standard methods (Association of Official Analytical Chemists, 1984); protein analysis was by the Kjeldahl method, and lipids were analyzed using a Folch extraction (Folch et al., 1957).

Plankton was the main natural component of the pond system assumed to contribute to the nutritional status of Oreochromis and Clarias. Therefore, samples of the plankton and the fish (Oreochromis, both free-swimming and caged) collected in January, March, and May were subjected to carbon and nitrogen isotope ratio analyses. There were not sufficient numbers of Clarias to sample more than once, so isotope data for Clarias were included for qualitative comparison only. Plankton was treated two different ways at the March sampling period only. Some of the plankton (called Plankton in the Results section) was collected by net and not treated further, like at the other sampling periods. The rest of the plankton collected by net was centrifuged to concentrate the phytoplankton. This phytoplankton concentrate was called Plankton 2 in the Results. The isotope values of both samples were analyzed. Diets, fish, and plankton were prepared for stable isotope analysis as described previously (Lochmann and Phillips, 1996). The analyses were conducted at the Stable Isotope Laboratory at the University of Arkansas, Fayetteville, Arkansas, using a micromass isotope ratio mass spectrometer.

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 ¹³C and ¹⁵N values (Tables 2 and 3). Rice bran had much more negative ¹³C values and positive ¹⁵N 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 ¹³C values of the free-swimming fish and plankton in ponds receiving rice bran (Table 2) and in the ¹⁵N values of the plankton in ponds receiving rice bran (Table 3). By contrast, there were no differences in ¹⁵N of free-swimming or caged tilapia in ponds receiving any of the diets in January. There were also no differences in ¹³C 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 ¹³C 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 ¹⁵N values of free-swimming tilapia, by contrast, were distinctly different among all three diets (Table 3), while there were no significant differences in ¹⁵N of caged tilapia or plankton among diets in March (Table 3). This trend in the ¹⁵N values continued into May (Table 3).

Figure 1 shows the change in ¹³C 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 ¹³C of the fish, which would have indicated a high rate of assimilation of the test diet. However, free-swimming 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 assimilated to a great degree (Anderson et al., 1987). The pattern of change in ¹³C over time was similar in ponds that received the pig finisher diet (Figure 1a). In this case the isotope values of the free-swimming tilapia were consistently more positive than those of the caged tilapia, indicating a higher rate of assimilation of the pig finisher compared to the caged tilapia. The ¹³C value of the plankton in the ponds receiving the pig finisher steadily increased (became more positive), possibly reflecting assimilation of the diet as well. The pattern of ¹³C values in ponds that received the rice bran was distinct from that of ponds that received the other two diets (Figure 1c). The rice bran was isotopically distinct from the plankton in this case also, but the diet value was much more negative than all other components in the pond, whereas the other two diets were much more positive than the other pond components (Figures 1a and 1b). The ¹³C of both free-swimming and caged tilapia gradually became more negative during the study. However, in this case the ¹³C of both the diet and the plankton were more negative than the fish (free-swimming and caged) initially. Therefore, it is not possible to determine the relative contributions of the plankton and the diet to the final ¹³C. The ¹³C of the plankton in this treatment first increased then decreased during the study. The reasons for this pattern were not clear.

Figure 2 shows the change in ¹⁵N values in ponds receiving each of the diets over time. The isotope values for the test diet and plankton overlapped and were both lower than the values for free-swimming and caged tilapia (Figure 2a). The ¹⁵N value of the free-swimming tilapia fluctuated more than that of the caged tilapia, indicating a more varied diet. The free-swimming tilapia appeared to be tracking the plankton and/or diet in the middle of the study, but then the final value became more positive, indicating a greater emphasis on the prepared diet at the end of the study, or possibly the assimilation of other more isotopically positive nutrient sources.

The ¹⁵N of the free-swimming and caged tilapia in ponds that received pig finisher were very similar throughout the study (Figure 2b). They became slightly lower by the end of the study. The diet and plankton values were not distinct at any point in the study, although both were much lower than those of the fish. It was not possible to determine the relative contributions of the plankton and pig finisher to the nutrition of the tilapia using ¹⁵N in this case. The ¹⁵N of the plankton and rice bran were distinct in ponds that received rice bran (Figure 2c). However, the ¹⁵N of the free-swimming and caged tilapia changed less in this treatment over time than in any of the others. The ¹⁵N of the plankton declined markedly during the study, but this decline was not reflected in the ¹⁵N values of the fish (free-swimming or caged). Again, the lower growth rate of fish fed rice bran compared to those fed the test diet or pig finisher limited the ability to discriminate between the food sources of the fish using stable isotopes.

The isotope technique is more effective in pinpointing nutritional inputs of an animal when the inputs are isotopically distinct from each other and from the animal itself (Anderson et al., 1987). The discriminating power of the isotope technique may be improved in future studies by fractionating the plankton into different categories consistently before isotope analysis, as fish gut content analysis in the Eighth Work Plan indicated that there were major differences in the relative use of zooplankton and phytoplankton for Oreochromis and Clarias in the different treatments (Lochmann and Perschbacher, 2000). The nitrogen isotope data clearly provide a distinct data set from the carbon isotope data. The point-in-time data for Clarias indicated that the dietary habits of all of the fishes had considerable overlap. However, in this study there were not sufficient data collected over time on Clarias to elucidate dietary habits in relation to trophic level of the fishes that were fed different diets.

Anticipated Benefits

Production efficiency of Oreochromis and Clarias can be improved once the quantitative importance of different nutrients under defined experimental conditions is established using the isotope technique in conjunction with comprehensive production data. Furthermore, the procedures used to define the importance of various components in this aquaculture production system may be modified and applied to other systems in other regions.

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