Most of the undrainable rural ponds surveyed are very old and were either excavated during house-building activities or constructed for multipurpose rural use. However, some are of natural origin. It is sometimes difficult to establish the actual time of their excavation or determine their age and enquiries with the rural population have proved unsatisfactory; yet care has been taken to trace the history and fix the age of the ponds. Some old ponds have been renovated and de-silted recently, and for scientific purposes the age has been determined according to the year when it was last de-silted (Tables 3 and 4).
The water surface varies in the range of 0.02–2.13 ha. In the case of the smaller ponds, with a steep and high embankment, the wind action has only limited effect and often there is no wind to induce internal current in the pond. Oxygen and nutrients within the water column and at the sediment water interface, are transported only by the limited force of convection current operating during the latter half of the night when the surface water becomes cooler and starts moving into deeper layers, and also by molecular diffusion. The inadequate oxygen and nutrient transport within the water column and at the sediment water interface may have adverse effects on the growth rate of the fish species in smaller ponds compared with larger ones using the same technology. This becomes more pronounced when a high organic or nutrient load is applied, and in this context the relationship of terrestrial organic load by surrounding human or livestock population to the pond needs to be known.
Water depth is a crucial factor for proper management practice, and in most cases also a limiting one. Normally, the only source of water for these undrainable ponds is the heavy rainfall during the monsoon, after which the water level gradually decreases and water shortage is quite common at the end of the dry season when the fish become larger and grow faster. During the present survey in January the water depth in these ponds ranged between 48 and 243 cm. However, the water level was usually 1–1.5 m in larger ponds whereas smaller ponds were very shallow. Adequate water depth is needed not only for optimum growth but also to provide enough space and oxygen for fish life. In shallow water depths an adequate amount of phytoplankton is not produced, and the soft sediment layer is stirred up by fish making the water more turbid. Eventually, the photosynthetic process becomes light-limited, and the total amount of the dissolved oxygen in a shallow water column may not at times be sufficient for fish since the sediment oxygen consumption of the total community respiration requires considerable amounts and ultimately this results in mass fish mortality. If the water column is too high, more than 3 m, this may also effect fish life adversely. In such deep ponds, the light layer (with photosynthetic and oxygen-producing activity) becomes small compared with that of the dark layer (with oxygen-consuming activity). Moreover, the sediment and its organisms also consume considerable amounts of oxygen; and all these conditions lead to a negative oxygen balance in the pond and result in fish kill.
One of the main features in these undrainable old rural ponds is the very thick sediment layer accumulated during decades. The sediment layer in the ponds surveyed ranged between 14 and 144 cm. The quality and quantity of sediment depend on several factors. The maternal soil has a decisive role, but the method of construction, nature of the embankment, the rapid re-silting, the macrophyte cover, bottom relief, pond productivity, and fish stocking all contribute to the nature of sediment and its thickness. Nevertheless, there is a close correlation between the age of the fish ponds and the sediment thickness. The age/depth relationship shows a marked difference in ponds covering a different area. The smaller ponds increase their sediment layers more rapidly. After 20 years, they have the same sediment depth as the larger ponds after 50 or even 100 years.
Macrophytes tend to dominate in many types of fish ponds. They commonly appear in shallow nursing ponds and also in ponds where young fingerlings of small or regular size are stocked in low density. Macrophytes are normally able to infest the water column or even the water surface when the filter-feeding fish species are stocked in abundance. With a correct density of bottom-feeder species, the spores/seeds of macrophytes are unable to develop since the fish stir the bottom underlying the water which reduces the light necessary for their germination. Once the macrophytes find the conditions in which to establish significant populations they determine the basic structure and functioning of the production processes. They successfully compete for space, nutrients and light with the phytoplankton whose presence is more advantageous and highly desirable. Most of the undrainable fish ponds surveyed had only a small percentage cover of the macrophyte populations; 24 ponds out of the 32 had a cover of less than 10 percent (Table 3). Two fish ponds were almost completely covered as a result of neglected culture activity. Among the macrophytes, the floating species were more abundant with an absolute dominance of Eichhornia and Pistia species.
The water colour in the majority of the ponds surveyed was greyish-brown corresponding to the nature of maternal soil and reflecting an overall low phytoplankton density. Ponds were found to be infested with Microcystis, Oscillatoria and Euglena. At times, the ponds were completely covered with an Euglena species. It is interesting to note that ponds with higher phytoplankton density were dominated with Microcystis blue-green algal species. Even ponds with very low algal densities had a thin Microcystis cover on the water surface. In pond 19, where the water surface was covered with Euglena species, Microcystis species dominated the water column. Nearly all the ponds with permanent Microcystis bloom have a very high human and animal livestock population nearby, which results in a significant organic load.
