- Some earth ponds (fishfarms) have partially been abandoned or have been unproductive as the ponds have bottoms of permeable (sandy) soil. The pond bottoms of the earth ponds of Narakkal in Kerala State (Type V) and in Kakinada, Andhra Pradesh State (Type IV) have permeable bottoms, a soil characteristic obtaining in both site types.
- In the same centres, there was reported a reduction in production during the dry season due to lack of freshwater to control conditions of increase in salinity, another condition typical of both types. Conversely, there was the problem of decrease in salinity during the monsoon periods. Measures to maintain favourable water salinities will be needed.
- Tidal ranges in the northern areas of the east and the west coasts at Types I and II necessitated construction of high earth bunds for protection against the wave and wind surges; the high perimeter dikes constructed without berms on the pond side posed problems of pond inspection, and the soil erosion of the side slopes during the monsoon rains results to silting of the ponds at the sides.
- The high tidal amplitudes in Types I and IV have not been fully or partially utilized in supplying water to the ponds by tidal energy. Instead the ponds were designed to be fully pump-fed. Why the pond designs did not fully or partially utilize tidal energy may have to be looked into, particularly on economics of design and of workability.
- The pump-fed farms have water reservoirs for supply integrated into the farm layout; the reservoirs have elevated bottoms in order to supply water to the ponds through supply-cum-drainage canals by gravity flow. To flood one pond compartment alone from among the many compartments served by the reservoir as in the Polekkuru farms in Kakinada and Matwad, Types IV and I respectively, the reservoir must be full and the canal water surface be higher than the pond water surface.
- In all the existing fishfarms visited, the pond compartments are single gated, square-shaped and are served by supply-cum-drainage canals. It is not possible to drain a pond compartment while at the same time flood another in the group served by the same canal. Production ponds should have filling and drainage system permitting them to fill and drain independent of other ponds.
- Seepage problems were encountered at most fishfarms visited - apparently the flow of pond water through the base of the dike built on permeable layer and, conversely, the flow of water from a peripheral canal into the pond compartment. The desired pond water depth could hardly be attained, but to attain this depth, the ponds were built with the pond bottoms well below canal bottom; drainage of the ponds by gravity flow and/or by pumping have not been possible.
- Streaming of water through the sides of a wooden gate in the fishfarms of Narakkal, Type V, was observed and investigations revealed the absence of anti-seep walls at the gate sides. Streaming of water through the monkculvert gate sides from the supply canal to the ponds was seen in a farm in Ratnagiri, Type III as the culverts may have been laid without anti-seep collars
- In the same fishfarm, the monk was placed at the supply canal side and the culvert outfall at the pond side. Where water exchange has not been done, the cultured fry would tend to seek refuge inside the culvert where water may not be of good quality.
- Selection of suitable sites involves the determination of what harmful substances are released or used upstream, the geographic pollutant dumping areas and the evaluation of water pollution. The farms of Cochin Bay are fed by the back waters of the Cochin fishing and industrial harbours.
- The pond compartments of fishfarms now in the construction phase have ponds which are square in shape and are single gated. Effective water exchange is not possible in a situation where a gate serves as the inlet and also the outlet structure.
- The ponds of the Narakkal original block can never be drained dry by gravity flow into supply-cum-drain canals serving the ponds. The peripheral canals also serve as drainage basins of the villages.
- Shrimp ponds should preferably, have separate inlet and outlet gates served by independent supply and drain canals respectively. The supply offtakes of the Polekkuru farms could only possibly handle the requirement of a 10% water exchange and as such, the pond compartment can be filled to a desired depth in not less than 10 days. Filling and draining of the ponds shall be accomplished in a short duration of time.
- In the Digha farms of the West Bengal State Fisheries Development Corporation, inspite of the high tidal amplitude and its 4-km influence inland, less than a meter of the 4m-spring tides and none of the neap tide ranges are available for supplying water to the ponds. The massive sand drifts and the dunes that occur due to the wave and wind action and of the littoral currents, prevent the entrance of tidal waters into a small creek at all times.
- Concreting of a secondary sluice gate of the farms of Assangao under construction, Type II, was in progress during the visit. The fine and coarse aggregates have uniform particle sizes and did not have the right grading required for a good quality concrete. Porous baskets of weaved bamboo were used in transporting concrete; The gate foundation did not have the cut-off wall necessary for preventing percolation and seepage under the gate foundation; there was absence of anti-seep walls necessary for prevention of streaming of water at the gate sides.
- There seems to be a lack or absence of plans and details and of material and work specifications that go with any job.
- The dikes of the Polekkuru and of the Assangao farms under construction, have been built from soil excavated in the dry condition and the soil cubes were piled loosely and improperly compacted. Rocks were seen to have been used in building the dikes. The path of the dikes was not cleared of vegetation and tree trunks were seen within the dike fill.
- The concrete, prefabricated offtakes for supply were cast, in the same farm, without anti-seep walls and were improperly located and installed.
- The soil of the Assangao farm looked suitable by field tests, but in some areas it lacks the proper amount of clay. Soil amendments for these areas must have to be done.
- The Matwad farms have very small (650 m2) pond compartments and have been designed to be a fully tide-fed farm. The soil quality is good and some physical and chemical parameters for water were reported within suitable ranges. Inquiries made revealed some 200 ha. of available land at the North and South of the Daudi-Nava-san road at the farm vicinity which is suitable for brackishwater aquaculture development.
- The Ratnagiri farms, Type III, have tidal amplitude of 2.2 m at the main feeding canal and was designed to utilize 0.70 m of this tidal energy. The ponds can be drained dry - condition which allows treatment and reconditioning of the acidic pond soil by liming. Acidity was said to be due to a type of soil which is laterite. The main supply canal has a lower bed bottom and water in the canal has to be pumped to drain. The tops of the secondary gates were built at elevation below pond water level. Supply water from the basin has massive algal growth especially of filamentous algae and measures have to be developed in order that algae do not enter the gates to the supply canal.
- The Dubha estuarine areas seem to have potentials for the establishment of a commercial fishfarm workable for the Type II area. The area shall be investigated further for site suitability. The plans which have already been prepared (layout) will need further scrutiny.
- There exists a need to modify the present layout of the farms existing and those under construction. The layout of the Polekkuru farms may have to be altered such that half the area shall have rectangular ponds so that comparisons on management and on production could be reached.
- Gate wingwalls provide the transition from the concrete gate to the earth canal. These wingwalls allow enlargement of the canal cross-section necessary for a reduction of canal flow velocity. Where a gate feeds a supply canal, this transition must be provided.
