4. STRATEGIES FOR WEED CONTROL

4.1 LAKE EDKU

Three types of weeds present a problem on Lake Edku:

a) submersed aquatics, mostly Potamogeton, which cover large areas of the open water in the NE, SE and SW quarters of the lake; b) floating plants, mostly the water hyacinth, Eichhornia; c) rooted reeds, mostly Phragmites.

This vegetation has two main impacts on the lake and its fishery. There is a tendency to block water flows and to present an obstacle to movements of the fishermen. In addition to the physical problem of passage through the weeds, they may have an effect on water quality by stagnating water volumes and leading to extremely anoxic and foul conditions.

Among the biological effects of the weed growth is the suppression of plankton development by the uptake and locking away of the available nutrients. At the same time there is the provision of a vastly increased surface area for the establishment of epiphytic organisms and thus enhancing the available food supplies for many of the fish species.

It is difficult to judge to what extent the growth of the reeds and the water hyacinth mats affect the flow of water through the lake, as there is no evidence of backing-up in the drainage channels. There may be an effect on the circulation of water throughout the lake area, resulting in the patchy changes in water quality observed by the consultants. Physical obstruction of the fishermen's access channel does occur and can arise from the movement of relatively small amounts of water hyacinth. The blocking by reeds or submersed aquatics does not present a problem that cannot be dealt with by the fishermen themselves.

The effect of weed growth on the production of fish, and the consequent yield to the fishery, is complex. The fact that many areas of the lake are inaccessible to the fishermen, provides a safe refuge and breeding area for many of the fish species. These inaccessible areas act as a conservation measure and will prevent overfishing. Such an “automatic” control over fishing activity is therefore an advantage for the long-term sustained production of the fishery.

The large expanses of submersed aquatic weeds provide a well oxygenated, food-rich habitat, but, on the other hand, at night may provide conditions low in dissolved oxygen and therefore may have a negative effect on fish production.

If the submersed weeds are eliminated, there will be an increase in dissolved nutrients available to the phytoplankton populations. The shading effect of an increase in these populations will aid in retarding the growth of the sub-aquatic weeds and the underwater parts of the emergents. The development of such phytoplankton populations would provide an increase in the food supply for the tilapias and the mullets (especially in their smaller stages).

Any control of the weeds on the lake should be considered from the separate aspects of the biological effect on the fishery and the improvement in the work and life of the fishermen.

If grass carp can be introduced into the lake, they will be able to exploit the dense mats of Potamogeton and Ceratophyllum. Their rapid growth will create an important additional element in the fishery. There will, however, be a continual need to restock to maintain the grass carp populations and to maintain the impact on the weed growth. Detritivores (mullet and carps) would also benefit from the destructive cycle of the grass carp.

Where weed growth is seen to be a potential hazard to the fishery, consideration should be given to the controlled spraying of small areas. In addition to easing the passage of the boats, the removal of submersed weeds from the areas of the landings will reduce the bilharzia risk through the removal of the habitat of the vector snails, and will also probably reduce the fly and mosquito nuisance. Both these effects would be enhanced by the establishment of simple landing places and fish and boat handling floors to replace the poor conditions now existing.

4.2 BARSIK FISH FARM

The growth of Phragmites reeds in the ponds reduces their production potentials because the dense stands reduce the water area and lower the water quality by creating stagnant, oxygen-depleted areas. This vegetation prevents adequate harvesting and post-harvest preparation.

Much of the farm area was originally part of the Phragmites fringe of the Lake Edku and it can be assumed that there was ample opportunity for the reeds to re-establish massive stands on the prepared pond bottoms before the farm was put into production. In this context it should be noted that the growths of reeds are much fewer in the area south and east of the farm where the land surrounding the farm area is much drier and reed-free compared with other peripheral areas.

It appears that the ponds were in many cases not adequately cleared before the original stocking and entry into production. Where ponds have been completely, or partially cleared by manual or mechanical means, the long filling period of from 2 weeks to a month provides excellent conditions for the re-establishment of the reed cover.

It was noted that where clearance has been carried out and the ponds adequately stocked with a species mix (including grass carp), the regrowth of reeds has been minimal.

The growth of fringe of reeds around the pond sides should be encouraged, especially of those bunds at the ends of the long axes of the ponds. This fringe provides a valuable means of preventing or reducing the damage from wave-lap erosion.

It is anticipated that a maintenance programme of weed clearance by cutting or spraying will be needed on some of the input and drainage canals. This should be carried out at the appropriate season of the year and in a precisely targeted fashion to achieve the maximum of long-term effect. A permanent population of some 200–300 grass carp/ha should be established in these canals. It is noted that the very large size of the individual ponds (ca. 6 ha) makes good farm management very difficult. These difficulties are particularly manifest when the ponds are being filled and emptied. This large size also demands a considerable effort in manpower for total harvesting. The lack of stock control, due to the large size of the individual ponds, means that optimum stock densities cannot be maintained through-out the growth cycle of the fish. This lack of stock management reduces the yield.

The infestation of Phragmites on the fish production ponds is mainly a problem due to lack of adequate initial preparation of these ponds. With appropriate pond management the control of weeds should require very little attention.

The eradication of the weeds and rehabilitation of the ponds into production should be carried out in a step by step programme.

1. The pond should be drained and dried out. This should be achievable by the available drainage flow, but may need to be supplemented by auxiliary pumps.

2. Whenever management practices allow drainage of ponds, weeds may be removed from the dry bottom.

3. The ponds should then be filled as rapidly as possible using auxiliary pumps to reduce the filling time to about a week.

4. The ponds should be adequately stocked.

Although liable to modification in the light of practical experience under local conditions, the following approximate survival densities should be considered for a 6-month production cycle.

Following the steps (a) to (d) will produce ponds that will effectively have no emergent or floating weeds throughout the production cycle (Table 2).

Table 2

BARSIK FISH FARM: ANNUAL PRODUCTION CYCLE*

* Example based on information relating to well maintained 6 ha ponds with little weed growth. Tilapia stocked randomly from inflow waters.