In India ponds are relatively small and shallow bodies of impounded water with limited wind action. They may be called perennial if they retain water the year round or temporary/seasonal, if they do so seasonally. They may be further classified as drainable ponds and undrainable ponds, depending upon the drainage facility by gravity. Drainability imparts a very desirable feature to a pond and some authors prefer to call only the drainable type of ponds as fish ponds. However, Indian experience has shown that experimental fish production to the tune of over 10 tonnes/ha/yr can be achieved even from such undrainable ponds through a proper understanding of the biotic and abiotic components of the ecosystem and adoption of suitable culture technologies. Hence, before adopting any culture technology, it is imperative to have an idea about the basic biology of the pond types in terms of environmental factors, community structure and community metabolism.
The periods of ‘plenty rain’ and ‘no rain’ usually prevail in regions having undrainable ponds. With the onset of monsoon, torrential downpours sweep across the land and the amount and frequency of rain decrease towards the end of the monsoon. Severe floods may occur, whereas a late monsoon or early monsoon of short duration may result in serious drought. Both flood/ rain and drought influence the ecosystem of the ponds on such lands. Small, shallow and seasonal ponds get filled or dry, whereas deeper perennial ponds exhibit considerable fluctuations in water levels accordingly. Though these ponds are basically constructed for storing water in such areas for multiple uses, ranging from supplying drinking water for human population, live stock, etc., to supplying water for agriculture, recent trend is to utilize them for fish culture. The description of the undrainable ponds is based upon the studies conducted at the Central Institute of Freshwater Aquaculture, Dhauli, Bhubaneswar, India, under an extensive environmental monitoring programme of rural undrainable ponds.
Undrainable ponds in general are relatively small, perennial or seasonal water bodies constructed or excavated for multiple uses. Some of these ponds have proper embankments. They greatly vary in their dimensions ranging from 0.02 ha to over 2.5 ha in water surface area and 50 cm to 250 cm in depth. Larger ponds are relatively deeper while smaller and seasonal ponds are shallower. Unlike shallow seasonal ponds, the bottom of the perennial ponds is never exposed to sunlight and therefore the whole ecosystem of such ponds is quite different from those of shallow and seasonal ponds. Use of these ponds by villagers for multipurpose provides the source of organic enrichment. Usually the only source of water for these undrainable ponds is the heavy rainfall during the monsoons. However, in some cases, the pond bottom is cut below the water table so that ground water enters the ponds. As soon as the monsoon ceases, the water level starts decreasing gradually and shortage of water is quite common during the pre-monsoon season. Water is lost from the pond through evaporation, seepage and transpiration by aquatic macrophytes and the trees and shrubs planted along the pond sides. Macrophytes tend to appear in both perennial and seasonal types of ponds but with increased intensity in shallower ponds.
Presence of thick sediment layers in the bottom is the most characteristic feature of these ponds which gradually get accumulated during the course of time and vary between a few centimeters to over a meter and half in thickness. The quality and quantity of sediment deposition depend mainly upon the original soil, method of construction, nature of embankments, macrophyte cover, pond productivity, organic and inorganic additions, species cultured, etc. Although sedimentation is relatively faster in smaller ponds, there is a positive correlation between the age of a pond and its sediment thickness.
The water depth and total volume of water available for individual fish are crucial in fish culture systems. Adequate water depth is needed not only for optimum growth, but also to provide enough space and oxygen for fish life. Water levels in these ponds are mainly dependent upon monsoon rains. After the monsoon season, the water level starts decreasing gradually and shortage of water is quite common during the summer season which is the most crucial time for fish culture since the fish growth rate is faster in this period. In fact, during the time of lowest water level the ponds contain the maximum biomass. In shallow and seasonal ponds, sufficient phytoplankton population fails to appear and the soft sediment layer is vigorously stirred up by fish, making the water more turbid, thereby reducing the photosynthetic process by limiting light penetration. Eventually, the total amount of available dissolved oxygen may not be, at times, sufficient to meet the demand for total community respiration and the chemical oxygen demand of the sediment, resulting sometimes in mass fish kill and planktonic collapse (Radheyshyam et al., 1986). On the other hand, in deeper perennial ponds where the water column is more than 3 m, fish life is again adversely affected. In such ponds the photosynthetic or oxygen producing zone is less in comparison with the oxygen consuming layer.
In addition, the sediment proper and the sediment community also consume a considerable amount of oxygen. All such conditions lead to a negative oxygen balance.
The water in most of these ponds remains slightly alkaline (pH 7.0–9.0). The NH4-N (ammonia-nitrogen) content of the waters remain below 0.02 mg/l with even lesser quantities of No3-N (nitrate-nitrogen). The Po4-P (phosphate) concentrations remain low and these chemical features of such ponds suggest that these waters are highly nutrient-deficient, particularly in nitrogen.
On the contrary, the pond sediment is rich in organic and inorganic nutrients. The organic carbon ranges between 3 (in newly excavated ponds) and 50 mg/g dry sediment weight (in older ponds). The nutrient status of the sediment differs completely from that found in the overlaying water column (Olah, 1983). In general, all the basic nutrients in the pond sediment are about thousand times higher than in their respective water column.
Carbon dioxide and oxygen are the most important gases affecting the pond community including fish. During the photosynthetic activity, carbon dioxide is usually at zero level while during the darker period its concentration increases. At higher concentrations it may be toxic to fish life. Carbon dioxide toxicity increases with decreasing level of dissolved oxygen. Carbon dioxide concentration can be tolerated upto 20–30 ppm in these ponds provided oxygen is near saturation.
Bacterioplankton and phytoplankton constitute the basic food for the fine filter feeder fish species and also for the zooplankton which form the main food of the rough filter-feeder species. The bacterioplankton population is always higher in those ponds which are associated with the activities of larger human and livestock populations. Most of the relatively older ponds with frequent appearance of Microcystis bloom have higher levels of bacterioplankton population (3–10 million/ml). On the contrary, the newly constructed and recently desilted ponds have less dense bacterioplanktonic community, around 1–2 million/ml. Macrophytic infestation also significantly limits the bacterioplankton production.
The planktonic detritus originates mainly from decomposing fragments of the phytoplankton and zooplankton and has generally a concentration range of 2 000 – 20 000 number/l. The main groups of phytoplanktonic population are Myxophyceae, Chlorophyceae, Euglenophyceae and Bacillariophyceae; whereas copepods, cladocerans and rotifers constitute the majority of zooplanktonic population. Some of the very old ponds having excessively thick sediment layer face Microcystis blooming. In such ponds the benthic animal fauna is represented by a very small number. In the majority of the ponds the benthic animal communities are dominated by red chironomids and oligochaetes indicating the general oxygen deficiency in the sediment layer.
The bacterial decomposition and nutrient recycling in ponds are greatly influenced by the anaerobic nature of the sediment. At the initial stage of Microcystis bloom in older ponds, the oxygen production has been found to be the highest (1) (over 15 g O2/m2), whereas the total community respiration remains considerably low (2) (below 10 g O2/m2). However, during the active decomposition stage of Microcystis (plantonic collapse stage) the total oxygen production level goes lower (3) (5–6 g O2/m2) than that of the community oxygen consumption (4) (6–7 g O2/m2). In older ponds, especially those having thick anaerobic sediment, the biochemical oxygen demand ranged between 70% and 90% of the total oxygen production, ultimately causing anoxic condition leading to fish kills.
The majority of rural undrainable ponds are characterized by anaerobic benthic sediments. The dead and decaying organic matter settles down to the pond bottom (sedimentation) where it is subjected to further decomposition and mineralisation. The upper layer of the sediment remains aerobic while the deeper layers are deficient in oxygen and thus anaerobic. Some of the distinguishing features of drainable and undrainable ponds are summarised in Table 3.
These perennial undrainable ponds in tropical monsoon lands with yearround warm water under plenty of light offer an excellent possibility for fish culture. Most of the species cultured greatly depend upon natural fish food resources and with a limited dependence upon artificial supplementary feed. However, without proper environmental management, the water remains infertile due to the overall nutrient deficiency with a very pronounced nitrogen limitation, although they possess a very high production potential. On the contrary, the pond sediments have extremely high level of organic nutrients in almost locked-up conditions which remain unutilized due to the anaerobic nature of the pond bottom (Fig. 7). However, though regular raking up of the pond sediment, either by manual or biological means, the organic nutrients could be released for making the pond water more productive.
Proper management methods can optimise fish production in perennial ponds at most economical rates while seasonal ponds can suitably be utilized for fish seed rearing and also for short-term fish production depending upon the duration of water retention. The general feature of the properly managed and ill-managed undrainable fish ponds are shown in Figures 8a and 8b.
Ponds which can be supplied with water and drained of its water according to the requirements of the fish farming operation are known as drainable ponds. These ponds require suitable ground with proper embankments, inlet and outlet structures and adequate supply of water on regular basis. Studies conducted on the filling up of ponds from various sources of water supply showed the following cost figures (Table 2).
Figure 7. Microbial Decomposition Process at the Sediment - Water Interface
Figure 8a. Well Managed Pond
Figure 8b. Badly Managed Pond
| Parameters | Undrainable ponds (natural condition) | Drainable ponds (cowdung treated) |
|---|---|---|
| Water pH | 7.0 – 9.0 | 7.7 – 8.2 |
| Total alkalinity (mg/l) | 50 – 250 | 88 – 200 |
| Ammonia-nitrogen (mg/l) | .005 – 0.300 | .005 – 0.25 |
| Nitrate nitrogen (mg/l) | .005 – 0.020 | .005 – 0.20 |
| Phosphorus (PO4-P) (mg/l) | 0.001 – 0.050 | 0.040 – 0.160 |
| Plankton | ||
| Phytoplankton (number/l) | 59 – 3 911 | 3 – 860 |
| Zooplankton (number/l) | 124 – 2 770 | 11 – 209 |
| Bacterioplankton (million/ml) | 1.2 – 12.9 | 1.385 – 2.312 |
| Benthos (number/m2.) | 0 – 2 660 | 1 415 – 19 099 |
| Decomposition rate of | ||
| Eichornia leaves: | ||
| Surface (% dry wt. loss/day) | 2.88 – 3.86 | 2.37 – 2.86 |
| Bottom (% dry wt. loss/day) | 4.02 – 10.99 | 1.54 – 2.84 |
| Gross production (g carbon/m2/day) | 1.76 – 10.99 | 1.85 – 8.11 |
| Net production (g carbon/m2/day) | (-)1.29 – 1.35 | (-)2.4 – 1.875 |
| Community respiration (g carbon/m2/day) | 1.66 – 11.34 | 2.261 – 6.071 |
| Sediment oxygen consumption | 4.887 – 7.943 | 0.1766 – 3.514 |
| (g oxygen/m2/day) | ||
