Annex 21
Observations on Fish Culture Stations

The Mission visited all completed stations and two fish culture stations that were under construction. The Mission's policy recommendations for the fish culture stations are discussed in section 7.3 of the main text. This annex discusses in some more detail the management of ponds and water for those stations that were visited by the Mission's aquaculture engineer. They were: Beragala, Dambulla, Inginiyagala, Pambala, Pitipana, Polonnaruwa and the two stations at Udawalawe. In addition, the Mission's engineer had the occasion to discuss the proposed construction works at Muruthawela with the engineer in charge of construction.

Summarizing the review of this annex, the Mission recommends that the staff of the Ministry of Fisheries consider the implications of the following observations for construction and use of ponds:

1. Make sure that pond bunds be compacted.

2. Remember to prepare canals so that they can be used for harvesting and keeping of fish.

3. Sluices and monks should be constructed with three pairs of grooves so that the bottom water also be let out during pond operations.

4. The diameter of water inlets and outlets should be related not only to the volume of water which must pass through them, but more importantly to the time that filling and drainage should take.

5. Recently constructed ponds do not have the same characteristics as “old” ponds. New ponds need more water initially, to saturate the pond bottom.

6. Newly constructed ponds should be filled to a depth of no more than half a meter. That depth should be maintained for six months.

7. Drainage of ponds through pumping should be avoided.

1. The Freshwater Fish Breeding and Experimental Station at Beragala, Badulla District

During the Mission's visit to the station, some soil samples were taken.

In the two ponds having the highest seepage, four boreholes of 0.60 and 1.50 m were drilled from the surface of the bottom of the ponds. In the boreholes clay, sandy clay and heavy clay were found. In one of them a layer of sandy gravel 30 cm thick was encountered. The soil was completely dry. Seepage will not be a problem, but it is apparent that the amount of water that has been provided has not been sufficient for soaking the soil and replacing the evaporation loss. The diameter of the inlet pipes (7.5 cm) is insufficient.

If, as the Mission were told, the bunds of the ponds were built by manpower without special compaction, the leakage will be high for a few years.

During the dry season fresh run-off water cannot be used to supply the ponds. The supply must come from stored water. It seems that the reservoir constructed for storage water was not designed considering the year round water needs of the station. It is too small.

Maps on a scale of 1:63 360 were provided by the Ministry of Fisheries and the Mission calculated the water catchment area for the station to be approximately 0.19 mi² (0.5 km²). The following mean annual runoff can be stored by the reservoir and the ponds:

V = Aca × RA × Rc (m³)

Where Aca = water catchment area in m²
RA = mean annual rainfall in m
Rc = runoff coefficient

Using the above equation with

the mean annual runoff is found to be: V = 500 000 × 1.458 × 0.35 = 255 000 m³.

The yearly water requirements of the seven ponds with a total area of 0.76 ha and the concrete tanks are about 30 000 m³. It should be possible to obtain this quantity from the catchment area.

For improving the existing inadequate water supply of the Beragala Fish Culture Station, it is suggested that:

• the capacity of the reservoir be enlarged, and that the pipelines through which water is brought to the ponds be substituted by pipes having larger diameters. If this is done, all the ponds can be refilled several times during the rainy season.

2. Fish Culture station at Dambulla, Matale District

The Fish Culture Station at Dambulla is under construction. It is located between the bund of the Dambulla reservoir and its spill tail canal, about 7 km by Minor Road from Dambulla town. The total area of the station is 10 ha which will accomodate nursery and rearing ponds, a hatchery with 10 cement tanks, and staff quarters, laboratory and store. It was reported that the Station might be expanded by another 6 ha.

At the time of the Mission's visit to the Station, four of the 20 rearing ponds were almost ready; the well for the water supply to the ponds was dug, and some building were under construction. The Mission's observations and proposals for improving this Station are as follows:

• The bunds, built by bulldozers, have not been compacted. To prevent high seepage and leakage in ponds yet to be built, bunds should be compacted by the bulldozer at intervals of 0.3 m in the height of the bund.

• Medium clay, excellent for building bunds, can be found at the site of the ponds up to a depth of 1.3 to 1.5 m below the ground surface. A rocky layer can be encountered below the clay. To avoid high seepage in the ponds the clay layer on the bottom of the ponds should be a minimum of 0.45 m thick.

• The existing well has a yield (1.5 m³/day) that is not sufficient to supply water to all the ponds projected for the station. It is recommended that the reservoir be used as the main water source and that water should be led to the station by way of a siphon.

• The hatchery, as planned, is not suitable for induced breeding of common carp or Indian major carps. A multi-purpose hatchery should be built.

The Mission's proposals to modify the general plan of ponds and hatchery, as well as for construction of the siphon, were discussed with the Maheweli Development Board's engineer in charge of the construction of the Dambulla Fish Culture Station on 27 October 1980.

The modified plans (DRG. Nos. 1 to 11) are found in Annex 25. As can be seen on the plans, the Mission proposes that the station consist of 10 nursery ponds (9 × 18 m each), 10 rearing ponds (27 × 36 m each), 5 rearing ponds (27 × 61 m each), and 5 rearing ponds (29 × 61 m each) (measuring the ponds at the bottom). In addition there would be a hatchery, a 20 m³ overhead tank, a pump house, and a lined pond for water storage (6 × 48 m, at the bottom) as well as 10 cement tanks (6 × 2.6 × 0.9 m each). Nursery ponds would be constructed with water depths of 1.06–1.20 m while the rearing ponds and the water storage pond would have water depths of 1.36–1.50 m. All ponds would have a freeboard of 0.5 m. All nursery and rearing ponds would have a 1:2 slope inside. The water storage pond would have a 1:1 slope inside. All the outside slopes of ponds should be 1:1.5, as should be the slopes of the drainage canals. Ponds would have separate inlet and outlet structures as shown in DRG. Nos. 3–5. Each pair of outlets for rearing ponds would have one harvesting box located outside the ponds. All the ponds can then be harvested without netting, by means of the harvesting boxes. The boxes may also be used for breeding hapas, holding Indian major carps or fingerlings awaiting sale.

Water will be supplied to the station by gravity from the Dambulla reservoir through a siphon of 20 cm in diameter (DRG. No. 8). Water will pass into the chamber of the lined feeder canal (0.45 × 0.68 m) carrying water to the ponds. This chamber would have a filter (1.20 × 1.20 × 0.6 m) to prevent undesirable fish and matter from getting into the ponds. The hatchery (DRG. No. 9) consists of a hatching room, housing two breeding tanks, one brood stock holding tank, 16 jars each of 6.35 1, a fibre glass collecting spawnery of 1.0 m in diameter, a table for the treatment of the brood stock and a laboratory as well as a store.

Specifications of the hatchery

The disposal of waste water from the hatchery should be through a 20 cm diameter hume pipeline connected to the drainage canal. The minimum capacity of the overhead tank would be 20 m³. The tank must be insulated in order to prevent heating of the water.

To control the temperature inside the hatching room, it would be necessary to provide a sufficient number of air conditioners and exhaust fans, and the ceiling should be insulated. To increase the survival rate of the hatchlings, two air blowers of 1.5 hp each, should be installed in the pump house. The output capacity of this hatchery should be utilized by keeping fry and fingerlings in cages in the reservoir.

3. The fresh water fish culture station at Inginiyagala, Amparai District

The station is located downstream of the Senanayaka Samudra Tank's bund along the Cart Track from Monaragala to Amparai. The area covered by the station is 3.2 ha. There is an area of 4.8 ha of rice paddies which might be used to expand the station. This station is composed of 10 rearing ponds with a total water surface of 0.56 ha and 20 cement tanks with a total surface of about 300 m². The slopes inside the bunds in the ponds are lined with bricks in cement mortar. At the time of the visit, three of the ponds were out of operation because of excessive seepage. Water supplying the ponds and the cement tanks comes from the surge tank of the hydroelectric power house by gravity through a pipeline of 10 cm in diameter. All the ponds have separate inlet and outlet structures. For draining the ponds there is an underground pipeline. In addition, there are buildings for offices, store, hatchery and staff quarters.

According to a subsurface exploration consisting of four boreholes each 1.5 m in depth made by means of a hand auger, it appears that the station is located on an old river bed that has been refilled with from 1.20 to more than 1.5 m of coarse sand and sandy gravel. This was probably done before the reservoir was built. Below the topsoil and the 1.0 to 1.30 m thick sandy layer clayey organic matters were encountered. Such soil was absent in the centre of the valley where the sandy layer is more than 1.50 m thick. Construction of additional ponds on the area available for expansion cannot be recommended. The Mission suggests that facilities of the station be improved as follows:

• The bottom of the ponds having a high seepage should be excavated to a minimum depth of 0.40 m and replaced with compacted clayey soil.

• All the brick linings in the ponds must be inspected. Any holes found under them should be filled. The linings should be repaired with cement plaster.

• The valve of the water supply pipeline in the surge tank is out of order and should be changed.

• The ponds to be constructed in 1981 should be built with clayey bottom and lined slopes.

4. Brackish water fish culture station at Pambala, Puttalam District

At the time of the Mission's visit the Pambala brackish water fish culture station was under construction. After an inspection of the site on 4 September, the Mission offers the following observations.

Calculations of the water requirements of the ponds and observations of the tides suggest that the output of the water pumps (10 cm diameter) may not be enough for filling the ponds. A gauge should be set up at the site to make it possible to record variations in the tide.

It is suggested that in the future new ponds do not need costly lining of stone in concrete, since the subsoil at the site of these ponds is clay. The slope, however, should be 1:2.

The two sumps designed to receive the water from the concrete tanks are not big enough, and the water table is only about 2.5 m below the surface. It is suggested that a supplementary drainage channel be built to the ponds. It should run behind the storage tank that supplies water to the concrete tanks. This possibility for drainage of the concrete tanks should be checked on the detailed plans, which were not available at the time of the Mission's visit.

A review of the designed fresh water supply system to the hatchery and the concrete tanks suggests that the shallow well with a diameter of 4.5 m and a depth of 7.0 m, with water 30 cm deep, is not sufficient.

The hatchery is not suitable for breeding shrimps or marine fish. The tanks as designed can be utilized only for holding fish.

The low pH of the water at Pambala is more than likely due to acid sulphate in the soil, and not the presence of coir. Acid sulphate soils are a common problem in mangrove areas. The best way to stabilize pH is by repeatedly flushing the ponds with clean water. The pH should rise to acceptable levels after 2 or 3 years.

Water samples should be taken from the ponds during their operation period, at least once every two weeks for chemical and plankton analyses, so that the amounts of the supplemental feeds and fertilizers can be calculated.

5. Brackish water fish culture station at Pitipana, Negombo, Colombo District

On 18 October, the Mission's engineer undertook soil and topographic surveys at Pitipana. The borings made in the bottoms of ponds Nos. 12 and 14 indicated muddy sand and coarse sand to a depth of 1.8 m, and it appears that the area is not favourable for the construction of fish ponds. Drawings Nos. 1 and 2 in Annex 25 show how the Mission believes the present ponds can be improved.

6. The fresh water fish breeding and experimental station in Polonnaruwa, Polonnaruwa District

This station is located in the vicinity of Polonnaruwa town. It was constructed in 1951. The total land area of the station is 7.6 ha which can be expanded with an additional area of about 20 ha from the existing rice paddies. The station contains 49 ponds of varying size with a total water surface of 1.68 ha, and 53 cement tanks with a total area of 0.1 ha. There are 9 buildings for offices, laboratories, stores, staff quarters and pump house. Water is supplied to the ponds and cement tanks partly by gravity from the Left Bank Irrigation Canal through a 23 cm diameter concrete pipeline with a length of approximately 800 m, partly by pumping from a 8.2 m deep dug well having a diameter of 1.2 m through a main distributory cement tank. The Mission, during its visit, carried out a reconnaissance soil survey in the area proposed for expansion of the ponds. Soil boring in rice paddies indicated presence of appreciable amounts of rock at 0.9 to 1.2 m. Thus, construction of additional ponds on the selected area does not seem to be advisable.

The Mission's observations and proposals for improving the existing facilites of this station are as follows:

• There are some joints with noticeable leakages along the water supply concrete pipeline. These points must be marked and after emptying the pipeline they can be plastered with cement mortar.

• It appears the water supply system inside the station has been modified without first carrying out a survey. It has been criticized as being complicated and wasteful of water. A more detailed topographic survey covering both the water supply system and all the ponds and tanks including their drainage canals should be carried out as soon as possible to find the best way of improving them.

• Algal blooms could be observed in some of the ponds. They could be eliminated with a chemical treatment of better water management including aeration of the ponds.

• Generally, the maintenance works in the ponds have been very poorly carried out. Most of the ponds will have to be desilted and their bunds renovated in the near future. The drainage canals should be re-excavated.

• It appears there is no existing drainage canal system in the rice paddies adjacent to the station. It is necessary to investigate the possibility of installing a drain in the paddy field during the topographic survey.

7. The “old” fish culture station at Udawalawe, Moneragala District

With nine new ponds, the station needs more water. The requirements should be calculated; observations of the rate of evaporation must be started as soon as possible.

This station was built on poor soil. Some ponds have rocky soil and rock outcrops; in other ponds there is sandy gravel, so the seepage and leakage are higher than in other ponds having clay soil. In the lined ponds leakage can be observed around the exterior of the outlets. Possibly the outlets have no cutoffs, and as a result the water flows out below the foundation slab. It is suggested that a trench be dug measuring 40 × 60 cm in front of the outlet and filled with concrete, cast in situ. The cracks in the linings of the concrete ponds should be plastered after emptying the ponds. The eroded bunds of the old ponds should be reshaped.

Along the bunds of the new ponds leakage can be found. These ponds were built without any compaction of the soil. Until settlement and consolidation of the bunds occur some leakage is likely. The ponds will consolidate after about two years and will then hold water without significant seepage. Any repairs or reconstruction seen to be necessary should be carried out by the contractor at no cost to the Ministry of Fisheries.

The water level in the new mud ponds should be maintained at only half the design depth during the first year of their operation. In the future this procedure should be adopted generally when commissioning new ponds.

On the slope of the bunds, along the design water level, protection should be provided against wind erosion. Local materials can be used.

8. The “new” (Chinese) fish culture station at Udawalawe, Moneragala District

As the ponds (with concrete lining) have only surface outlets, they have to be drained by pumping, a system which seems unnecessarily costly. Fish disease may become a serious problem in a few years. The spawning tanks (also lined with concrete) are not suitable as there is a high risk that workers be injured.

The mud ponds built recently are suitable for intensive production as they can be drained by gravity. The feeder and drainage canals have adequate cross sections for supplying water to the ponds and for emptying them. Part of the existing drainage canal at the new ponds could be utilized for harvesting fingerlings if the flow of water could be controlled. For this to be possible grooves must be made in the sides of the canals, in order that water control structures can be fastened.

Unfortunately, the sluice gates have only two pairs of grooves. It is suggested that three pairs of grooves be provided for all outlets to be built in the future. This will permit drainage of the water at the bottom of the pond.

By increasing the water supply to the new mud ponds the seepage could be stopped in a reasonable time.