The perennial undrainable ponds in tropical monsoon lands with year-long warm water and plenty of sunshine offer an excellent possibility for fish culture, mostly natural fish food resources with a very limited amount of supplementary food. Earlier work on natural food resources mostly included lists of species but showed no quantification of time. In the present investigation, in addition to the taxonomic approach, detailed quantification of community structure both in the sediment and the water have been attempted.
The daily production of fish food organisms in the water column is far more important than the daily production in the sediment or even in the macrophyte stand. This is because, compared with the sediment, the water column is characterized by a very short turnover period for most of the processes in the energy flow and mineral cycling. The total bacterioplankton counted on membrane filter and the phytoplankton constitute the basic food for the zooplankton which form the main food of the rough filter-feeder fish species. Moreover, the bacterioplankton and phytoplankton themselves form food for the fine filter-feeder fish species.
The quantity of bacterioplankton depends on the primary production inside the pond and on the organic load supplied from outside. Undrainable ponds receive a considerable organic load from the human population and also from livestock nearby. This organic load manures the bacteria present and becomes the dominant factor governing the fish production process. In the present survey, the total bacterioplankton on membrane filter has been measured after erythrosine staining. A comparison with the total count on agar media as practised earlier shows that the number of bacteria was maximum - some few thousands in 1 cm3 of water - whereas their quantity on membrane filter was as high as 0.2 and 12.9 million in 1 cm3. The number of bacterioplankton was highest in the ponds with the largest associated human and animal livestock populations (Table 5). The bacterioplankton was also very significant in all those ponds in which the Microcystis bloom was observed during the survey. In pond 3 where a large diverse phytoplankton community was present, with Oscillatoria dominating, the number of bacterioplankton was also very high. Ponds 31 and 32, operating for over 20 years at the Cuttack Fish Culture Unit, were characterized by a heavy Microcystis bloom with total bacterioplankton at 9.7 million cm-3. In all the newly constructed or de-silted ponds (2, 4, 25, 26, 27, and 28) their numbers were much lower (around 1–2 million cm-3). Ponds with floating macrophyte had less bacterial population than those ponds with emergent or submerged macrophytes. It appears that the floating communities of Eichhornia and Pistia species do not give as much opportunity for adequate nutrients for the bacterioplankton as the submerged or even the emergent macrophytic communities. The daily release of the freshly assimilated organic matter by Eichhornia and Pistia is not known. However, in phytoplankton-dominated fish ponds, the daily release of organic nutrient during the photosynthetic activity of algal species is considerably more important, and the decomposition of decaying algal populations also becomes an important autochthonous nutrient source for the bacterial populations.
In pond 1, the number of bacterioplankton exhibited a detectable diel rhythm. Their number reached the highest value during the afternoon when the organic matter released by the phytoplankton is known to be maximum.
During the routine survey in January the number of bacterioplankton was 3 million cm-3 and in March, when a much larger Microcystis population developed and started to decay, their number reached the value of 10 million cm-3. Such a situation can be aggravated by the application of cowdung, and mass fish mortality can occur. Thus, the organic matter released by plankton, aquatic weeds, decaying organisms and the addition of organic matter, support and maintain the bacterioplankton populations in the fish pond.
The survey showed that 16 ponds had less than 3 million bacteria in 1 cm3: this indicates the low fish productivity of the pond and may imply that the pond is nutrient deficient.
Planktonic detritus above the size of 10 μm have been counted in all the ponds surveyed. The detritus particles freely suspended in the water column show the same pattern of distribution as those of bacterioplankton (Table 3). Their numbers were low in macrophyte-infested ponds and high in ponds with algal blooms. The planktonic detritus originates mainly from the decomposing fragments of the phytoplankton and zooplankton species. Their numbers were surprisingly high but most of them were in an extremely decomposed, stabilized state which is more resistant to further decomposition, and they comprised mainly of cellulose, polysaccharides and chitin. This form of detritus has less nutritive value than those less decomposed and full of bacteria. The ratio of this latter detritus was much higher in ponds with flourishing algal populations.
A brief comparison of the dominant groups of organisms in the phytoplankton shows that in most of the ponds the Microcystis species were by far the most important followed by Oscillatoria and Euglena species (Table 6). Ponds 1, 3, 4, 5, 16, 19 and 31, where there was significant Microcystis bloom, had a relatively higher weight of this size fraction of the seston which also contained the larger zooplankton species.
The next size fraction of seston collection taken with a bolting net with mesh size of about 150 μm, represents the larger species of zooplankton. Their quantity was also small in most of the ponds surveyed except for ponds 1, 3, 4, 5, 14, 16, 17, 19, 20, 21 and 31 (Table 5). The zooplankton communities comprised the Ceriodaphnia, Diaptomus, Cyclops, Keratella and Brachimus (Table 6). It is obvious from the table that in January at least both phytoplankton and zooplankton were not in abundance.
The highest zooplankton standing stock was present in Pond 3 with a very diverse phytoplankton community. Ponds 4 and 5, which also had mixed populations of phytoplankton, had abundant populations of zooplankton. In some of the old rural fish ponds which have a thick sediment rich in organic matter and a permanent Microcystis bloom (e.g., Pond 1), the quantity of both seston size fractions was rather high. But in many of the ponds, e.g., 7, 23, 32, where Microcystis dominated the phytoplankton community, the quantity of zooplankton was extremely low especially compared with the large biomass of the Microcystis. The large Microcystis colonies with their indigestable gelatinous envelope are not available as food for most of the zooplankton species. They become consumable only after they start to decay during the process of bacterial decomposition. An overall low phytoplankton biomass or the large Microcystis biomass with low direct nutritive value seems to be a general phenomenon in these undrainable rural fish ponds in January.
The large biomass of Microcystis produces significant amounts of organic matter as primary producers. Using the diel oxygen curve method and the McConnel calculations procedure, the primary production has been determined to be three times as high in pond 1. The organic matter produced by Microcystis, and to a less extent by Anabaena species, was 5.2 g expressed as organic carbon/m2/day on 18 February 1983, 8.1 g on 26 February 1983 and 5.7 g on 15 March 1983.
The sediment detritus constitute the food resources of benthophagous fish species which utilize it directly. The detritus weight of all those particles, which are bigger than 400 μm and are easily available for detritus feeder fish species, have been measured. Most of these particles originate from the decomposing macrophyte remains. There is a close relationship between the quantity of sediment detritus and the size of macrophyte cover. Ponds 3, 7, and 11 with a small macrophyte cover accumulated their significant amount of detritus during a previous period when they were choked with macrophytes (Table 7).
Though the animal populations living in the sediment or among the macrophytes have much longer generation time than the plankton ones (and hence their production is much lower), they occupy an important place in the natural fish food resources. Many of the surveyed ponds characterized by thick and organic rich sediment and by inadequate oxygen supply at the sediment water interface, contain very small numbers of benthic animals if any. Small benthic animal populations were also found in most of the newly constructed or excavated ponds. In ponds 25 and 26 large amounts of oligochaetes were found in the sediment. In the overwhelming majority of ponds the benthic animal communities were dominated by red coloured chironomids and oligochaetes indicating the general oxygen deficiency in these undrainable ponds (Table 6). Some ponds had significant gastropod populations in the sediment.
Ponds with significant macrophyte cover have diverse animal communities (Table 6); nearly all groups of insects are present, and also rich populations of gastropods and shrimps. The number of these large animals reached the value of several thousand/m2 (Table 7). However, it should be emphasized that this community is almost the only food resource for fish species due to the lack of any significant populations of bacteria, algae, zooplankton or zoobenthos. At times, the large animal feeder fish populations overpower them and gradually destroy their populations. In these ponds, there is no food at all for the important filter-feeder fish species.
