Observation on Feeding Habits of Fish in Ponds Receiving Green and Animal Manures in Wuxi, China

by
Shan Jian, Chang Laifa, Gua Xianzhen, Fang Yingxue,
Shu Yun, Zhou Xiaoxing, Zhou Enhua, and G.L. Schroeder

Regional Lead Center in China
Asian-Pacific Regional Research and Training Centre
for Integrated Fish Farming
Wuxi, China

* This paper was accepted for publication in the
Aquaculture journal (Ref. Aquaculture, 46(1985) 111–117)

NETWORK OF AQUACULTURE CENTRES IN ASIA
Bangkok, Thailand
December 1984

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OBSERVATIONS ON FEEDING HABITS OF FISH IN PONDS
RECEIVING GREEN AND ANIMAL MANURES IN WUXI, CHINA

BY

SHAN Jian, CHANG Laifa, GUA Xianzhen, FANG Yingxue,
SHU Yun, ZHOU Xiaoxing, ZHOU Enhua, and G.L. SCHROEDER

Asian-Pacific Regional Research and Training Centre for
Integrated Fish Farming, Wuxi, China

ABSTRACT

Fish yields and ratios of naturally occurring stable carbon isotopes, ¹³C: ¹²C (reported as δ C) from 1 mu (670 m²) and 5 mu (3330 m²) ponds were measured. The ponds received daily inputs of green and animal manures as the sole organic inputs. Fish yields exceeding 40 kg/ha per day were attained in both the small and large ponds. δ C data indicated a strong contribution of the fresh fluid pig manure to growth of the two filter-feeding species, silver and bighead carp. Two grass-eating species, grass carp and Wuchang fish (Megalobrama amblyscephola), had significantly different δ C values, indicating partially separate food sources for each of these two species.

INTRODUCTION

Green manures (grasses, aquatic plants) and animal manures are common crude-matter inputs to fish ponds in China and Southeast Asia. Fish yields from these ponds are often very high (exceeding 40 kg/ha per day). To enable transfer of the technology essential for successful management of such manure-loaded ponds to other pond environments, it is useful to understand the processes that lead to these fish yields. To help achieve that understanding, a study was conducted of the feeding habits of fish in two earthen ponds receiving green and animal manures. The study was based on measured patterns of the ratio of the two stable isotopes of carbon, ¹³C: ¹²C, found in the environments of these ponds and in the fish grown therein. The ¹³C : ¹²C data are reported using the conventional δ C notation:

The standard is Pee Dee Belemnite (PDB carbonate (Craig, 1957).

The usefulness of δ C as a tracer of food webs has been discussed in detail elsewhere (DeNiro and Epstein, 1978 Fry and Parker, 1979; Schroeder, 1983a, b). In brief, the δ C of plants is determined by the photosynthetic pathway of the plant. Two major pathways exist, C₃ and C₄, which produce significantly different δ C values in the plants (Smith and Estein, 1971). The δ C of an animal's body is close to that of the food it used to build its body. Hence, comparison of body δ C with the δ C values of the available foods permits inference as to which of these foods were assimilated during growth by the animal.

METHOD

Two ponds in Wuxi, Jiangsu Province, People's Republic of China were selected for this study. A 1-mu (670 m²) and a 5-mu (3330 m²) earthen pond, 1.5 and 3 m deep, respectively, were stocked on 15 April 1983 with silver carp (Hypophthalmichthys molitrix Val.), bighead carp (Aristichthys nobilis Rich, grass carp (Ctenopharyngodon idella Val.), Wuchang fish (Megalobrama amblyscephola, Crucian carp (Carassius carassius) and common carp (Cyprinus carpio L. . All were in their second year of growth. Stocking densities and sizes of individual fish are listed in Table 1. During their first year of growth these fish had been kept in similar ponds. During both the 1982 and 1983 seasons the only organic inputs to the ponds were daily supplies of fresh, fluid pug manure and green manure. The pig manure was added at a nominal daily rate of 2% (dry weight basis of the fish biomass. Green manure consisted of aquatic plants plus local grasses (Table II added at rates such that at the end of the day-some residual plant matter remained floating on the pond surface. After 102 days approximately 5% of the fish were netted and weighed. One-gram portions of flesh cut from two or more individuals of each species were taken for δ C analysis. Specimens of supplied grasses, aquatic plants and animal manure were also taken for δ C analyses. In the 1-mu pond the following natural foods were sampled: sestion centrifuged from 300 ml of unfiltered pond water sestion retained by passing 30 liters of pond water through a 37- um (400 mesh) net, sediments at the pond bottom-water interface and slimes on submerged surfaces.

TABLE I Fish stocking and sampling data for 1-mu and 5-mu ponds

Note: Stocking date for this season was 15 April 1983. Sampling date was 102 days later, 26 July, 1983. Approximately 5% of the fish of each species were captured.

TABLE II Summary of δ C values in 1-mu and 5-mu ponds receiving green manure and pig manure

Fish δ C values are for samples taken on 26 July 1983 (Table 1). Rye grass (a C₃-type grass) was cultivated and supplied in April and May; wild local grasses (all C₄ type) were supplied only in mid-July; the aquatic plant, Vallisneria spiralis, was supplied June through October. Total seston, present in very low concentrations consisted primarily of green algae > 37 μm. The slime coating submerged surfaces was predominantly the blue green algae, Oscillatoria, plus protozoans. Organic content of sediments was 6%, dry weight basis.

TABLE III Daily diets of pigs producing manure used in this study

The quantity of seston retained on the 37 μm net was small (less than 1 mg dry weight/20 liters) and was not analysed for δC. The sediments were acidified with phosphoric acid to remove inorganic carbonate, leaving the organic carbon for δC analysis. Sediments were also analysed for percent volatile (organic) matter as weight loss in a 90°C dried sample when combusted at 500°C for 5 h.

Aquatic plants for the ponds were taken from nearby Lake Taihu. The dominant plant was Vallisneria spiralis. Some Hydrilla verticillata and Potamogeton matainus were occasionally used.

The composition of the diets of the pigs producing the manure is listed in Table III. From April through June and August through October fattening pigs were present. In July piglets supplied the manure.

Upon collection, all samples were dried at 90°C. Procedures for δC analyses have been described in detail elsewhere (Schroeder, 1983a,b). Combustion temperature was either 850°C or 500°C. The δC values for replicate samples combusted at 500°C and 850°C agreed to within 0.4‰. Flesh taken from three fish of the same species and same pond had δC values ± 0.5‰ of their average δC. This δC agreement has been observed in other polyculture ponds (Schroeder, 1983b) and indicates a consistent feeding pattern within each species for the given pond environment.

RESULTS AND DISCUSSION

Fish weight gain data for the two ponds are listed in Table I. The daily total fish yields, averaged over the entire growth season, exceeded 39 and 42 kg/ha per day for the 1 mu and 5 mu ponds, respectively. This production is impressive, especially considering that the only additions to the ponds were high fiber plants and pig manure. For the 5-fold range in pond area and 2-fold range in pond depth, the yield were essentially equal. In both ponds, ¾ of the production was associated with silver carp (SC) plus grass carp (GC) growth although these fish accounted for only half of the total number of fish stocked.

Comparisons of δC values found in the pond environment give an indication of the feeding habits of the fish. δC analyses deal with the total carbon of an organism. It is essential that this carbon be related to the system of interest. The δC of a fish that gains only a few percent of its total weight during a short time in a certain pond will not provide insight into the dynamics of that pond since its carbon will have been mostly accumulated prior to entering the pond. In the ponds considered in this study all the fish more than doubled their weight during the 102 days of the 1983 season (Table I). In 1982, they were grown in simular ponds with the same organic inputs (green manure plus pig manure). Based on these conditions, we consider that it is valid to relate the δC of these fish to the δC of the organic matter added to, or found within, these ponds.

SC and bighead (BH) are filter feeders with gill rakers spaced at approximately 30 and 80 μm, respectively (Spataru et al., 1983). Normally they feed on plankton: SC usually on microalgae and BH, with its more widely spaced gill rakers, on zooplankton. In these ponds the standing crop of all plankton species larger than 37 μm was less than one mg dry matter/20 liters pond water. This low concentration probable resulted from intense overgrazing by the 6000 filter feeders (SC plus BH) stocked per ha. The low standing crop of plankton could not provide a base for plankton production suitable to sustain the measured SC plus BH growth. The δC data confirm this. Both the SC and BH δC values are far from the seston δC (Table II). The extreme similarity of the δC of the folter feeders, both between species and between ponds, is at once apparent. For the four cases, all δC values fall between -25.9 and -27.4. (The two grass-eating species have twice this spread in δC values). The similarity of δC for the filter-feeding fish implies that both species feed on the same food web. Others (Buck et al., 1978) have suggested that SC and BH may obtain food from the same sources. Filter feeders are probably the most passive of fish in their feed selection. Size discrimination, based on gill raker spacing and mucus secretion may be the only factor distinguishing between the foods used by the two species.

The main input to the pond having a δC similar to these filtering species was the fresh, flocculated pig manure. Observations (Schroeder, 1983b,c) in polyculture ponds stocked with about 1 fish/ m² and receiving dry chicken manure show the δC of SC to match the δC of the mic oalgae but not the δC of the dry manure. However, in similarly stocked ponds manured with fresh, fluid manure (goose manure), the δC of SC shifted strongly (by 7‰) toward the δC of the manure (Schroeder, 1983c). The flocculant nature of the fluid goose manure may have permitted its suspension in the water column for sufficient time for it to have been filtered by the SC. The δC data of the Wuxi ponds indicate that the fresh pig manure has an effect similar to that of the fresh goose manure. Its fluid nature makes it available, either directly or after microbial recycling, to the filter feeding species, SC and BH.

The two grass-feeding fish species, grass carp (GC) and Wuchang fish (WF), are interesting in themselves. In both the 1-mu and 5-mu ponds, the δC of WF was significantly more negative than that of GC. The intestines of both fish were packed with chewed grass. However, δC values indicated that each species was getting part of its nutrition from a source not available to, or less exploited by, the other grass-eating species. WF appears to derive more of its growth from foods originating from a food web based on a C₃ source than does GC. The δC of G C, less negative than typical C₃ δC values, indicated food sources based on both C₄ and C₃ species. Several combinations of the added organic matter could account for this (Table II).

The Crucian carp is considered to be an omnivorous, particulate feeder. Its δC indicates a strong dependence on C₃ type foods and is more negative than that of fish species known to eat grass. This implies a food web in part associated with the pig manure, the only year-round input with a sufficiently negative δC, plus some growth coming from less negative sources, perhaps the sediments and seston δC - 22.3 and - 23.5, respectively). The δC data show that if the slime (δC - 14.4 the blue green alga Oscillatoria dominant, plus protozoans) makes any contribution to Crucian carp growth in these ponds, it is very small.

CONCLUSION

In polyculture ponds stocked with 18 000 fish/ha, production exceeding 40 kg/ha per day was achieved with grass, aquatic plants and fresh pig manure as the only organic inputs to the ponds. For the conditions in these ponds, which had very low standing crops of plankton, δC data indicate that: silver carp and bighead carp take nutrition from the same source, grass carp and Wuchang fish, though both considered to be grass eaters, take part of their nutrition from different source; and the Crucian carp is strongly influenced by the pig manure, with some input from other organics in the sediments or seston.

Comparison of δC data with growth data for individual fish species indicates that approximately half of the total pond production is based on an animal manure food web and half on a green manure food web. The experience of centuries of pond management has apparently balanced organic inputs with their utilization by target animals.

ACKNOWLEDGEMENTS

The encouragement in this work by Dr. Chen Foo Yan of the United Nations Food and Agriculture Organization (FAO) is deeply apprecited. The help of Dr. T. E. Chua of the Network of Aquaculture Centres in Asia, who generously reviewed the manuscript and made his facilities available for its preparation, is acknowledged. This work was supported in part by the FAO and by the Binational Agricultural Research and Development Fund, BARD, grant no. 1. 385-81.

REFERENCES

Buck, H., Baur, R, and Rose, C., 1978. Polyculture of chinese carps in ponds with swine wastes. Symposium on Culture of Exotic Fishes. Am. Fish. Soc., Atlanta, GA, pp. 90–106.

Craig. H, 1957. Isotopic standards for carbon and oxygen and correction² factors for mass-spectrometric analysis of CO₂. Geochim. Cosmochim. Acta, 12: 133–149. DeNiro, M. and Epstein, S., 1978 Influence of diet on the distribution of carbon isotopes in animals. Geochim. Cosmochim. Acta, 42: 494–506.

Fry, B. and Parker, P., 1979, Animal diet in Texas seagrass meadows: δC evidence for the importance of benthic plants. Estuarine Coastal Mar. Sci., 8. 499–509.

Schroeder, G. L., 1983a. Stable isotopes as naturally occurring tracers in the aquaculture food web. Aquaculture, 30: 203–210.

Schroeder, G.L., 1983b. Sources of fish and prawn growth in polyculture ponds as indicated by δC analyses, Aquaculture, 35: 29–42.

Schroeder, G.L., 1983c. Natural food web contributions to fish growth in manured ponds as indicated by stable carbon isotopes. J. World Maricult. Soc., 14: 505–509.

Smith, B. and Epstein, S., 1971. Two categories of ¹³C/¹²C for higher plants. Plant Physiol., 47: 380–384.

Spataru, P., Wohlfarth, G. W. and Hulata, G., 1983. Studies on the natural food of different fish species in intensively manured polyculture ponds. Aquaculture, 35: 283–298.