The macrophytes in Lake Edku are both harmful and useful. Apart from their aesthetic attributes, all the plants, including water hyacinth, bring about a considerable increase in invertebrate food for fish. The plants also provide a habitat for wildlife and a refuge for fish. In fact, the dense growth of macrophytes in Lake Edku is probably the main reason for the high fish catch. Therefore, control measures directed against the plants, either by herbicides or grass carp, may have considerable impact on the fishery, particularly if these measures are taken to the extreme. At present, weed control in Lake Edku is primarily limited to the aerial application of Ametryn. The herbicide is normally applied at 2 1/ha. Approximately 1 200 ha is sprayed each year by staff of the Institute for Weed Control, Ministry of Irrigation. Small amounts of weeds are mechanically removed to provide access for fishermen, improve water flow to marshy sections and to open up the flow from the major drains into the lake. The latter work has been done by a Hitachi MA 125 U amphibious soft terrain excavator. These units cost approximately US$ 100 000 cif Alexandria and operate for about LE 200/day (ex-depreciation). 1
1 Exchange: US$ 1 = LE 0.8217 (September 1985)
To date weed cutters such as the “Wilder Warrior” have not been used in Lake Edku.
The Ametryn aerial applications have been effective. This herbicide is not as rapid in action as 2,4-D and affected plants do not sink rapidly. The application of Ametryn to Lake Edku causes an unknown level of damage to non-targets. Extensive kills of phytoplankton probably occur in Lake Edku in the month following the aerial spraying, as Ametryn is a relatively persistent herbicide (half life of several weeks in typicalfield conditions) that is an efficient photosynthetic inhibitor. It is known to be an excellent algicide at very low concentrations of less than 0.1 ppm active ingredient. The effect of Ametryn on non-targets in Lake Edku warrants investigation.
The herbicide 2,4-D, known to be the most effective and economical for the control of water hyacinth, is not used presumably because of likely undesirable effects to sensitive crops like cotton and tomatoes. Herbicide 2,4-D would be less damaging to non-targets in Lake Edku than Ametryn and under normal pricing circumstances cost significantly less. Changing the application of the Ametryn to water hyacinth to the May-June period instead of the current October application should be seriously considered.
However, the single aerial application of Ametryn or 2,4-D to the water hyacinth is not sufficient to control this plant around the access channels and major drain inflows. This can be most economically achieved by a boat fitted with a spray unit. A flat-bottomed boat, either jet or air powered, would work continuously through the summer spraying key parts of the lake. This would be more economical than mechanical methods which are slow and expensive, particularly in a situation like Lake Edku.
The principle of using a spray boat on a daily basis has been established elsewhere and there is no economic alternative. The spray unit would also be used to control the Phragmites that encroach into the fishermen's access channels. Glyphosate is the preferred herbicide for this work at 6 1/ha. This would be a more efficient method of maintaining the access channels after the initial opening-up has been completed by the Hitachi. The two methods would be complementary.
Concluding, weed control in Lake Edku can be improved by :
Spraying the water hyacinth in May/June instead of October;
Using herbicides from a boat in a maintenance programme as a follow-up to work done by the Hitachi amphibious excavator
Introduction of grass carp (Ctenopharyngodon idella) to the lake in sufficient quantities to reduce the density of Ceratophyllum demersum and Potamogeton pectinatus. Control of these species would result in improved health of fishermen on this lake.
Dense growth of the reed Phragmites australis has reduced the fish yields in the ponds. Control of this reed can be achieved by a range of techniques or their integration. The following approaches are recommended :
Cutting the culms beneath the surface of the water in the summer or autumn; this reduces food supply to the rhizomes, root aeration ceases, and bud formation is retarded;
Judicious use of herbicides such as application of glyphosate in the summer or autumn;
The use of herbivorous and bottom-feeding fish;
The complete drying of the ponds in winter where possible. Construction of narrow deep drains in the ponds will facilitate this;
The rapid filling of the ponds in the spring. Leaves of the reed are slowly killed when submerged.
The encouragement of phytoplankton blooms to reduce the light to the submerged shoots of the reeds and other macrophytes;
Levelling and excavation if necessary of the pond beds to ensure a uniform water depth of about 1.5 m. This will further reduce the reeds' ability to “break through” the water surface and photosynthesize.
By using a combination of these control measures, the reed Phragmites australis will reduce in density and become a useful plant providing stability to the already rapidly crumbling banks and roads between the ponds. In this regard the introduced tree Casuarina glauca and the native bush Tamarix sp. are being properly used to control bank erosion and the use of the latter should be expanded. Other macrophytes present in Egypt should be considered for introduction as bank-stabilizing agents.
The dominant macrophyte in the supply and drainage channels of Barsik Fish Farm is the reed Phragmites australis. Other macrophytes include small patches of Typha sp., Alternanthera sessilis, Schoenoplectus validus, Juncus acutus, Ceratophyllum demersum, and some undetermined members of the family Cyperaceae.
None of these plants are achieving maximum growth - probably because of the poor saline soil on which much of the Barsik Fish Farm is constructed. Control of the macrophytes in the drains and channels of the farm can be readily achieved by the application of the herbicide glyphosate at 4–6 1/ha. For the best results this herbicide should be applied in mid to late summer. High volume application is desirable because the dust that coats the leaves of the plant will inactivate some of the glyphosate. Control can also be achieved by cutting manually or mechanically.
Officers at the section for combating weeds in the Ministry of Irrigation apply large quantities of the herbicide Ametryn to the irrigation system each year and also to the delta lakes. Residues from these applications periodically occur in Lake Edku and Barsik Fish Farm.
In October 1984, 6 000 litres of Ametryn was applied to water hyacinth in the lake. A similar application is planned for 1985. This amount of herbicide was localized to dense stands of water hyacinth. Two litres/ha were applied in 10 litres of water. An unknown quantity of this slow-acting (photosynthetic inhibitor) herbicide entered the water. Based on experience elsewhere concentration of the chemical would probably be greater than 0.01 ppm in some shallow stagnant areas. This is a sufficient concentration to kill phytoplankton and could persist for weeks.
If it were permitted, 2,4-D would be a safer, more target-specific and a more economical herbicide in Lake Edku. However, nearby susceptible broad-leaf crops preclude its use from the air, but it would be safe to apply from a boat.
At the Barsik Fish Farm the herbicide of choice is glyphosate. It is relatively non-toxic to all animals. It is effective on both water hyacinth and Phragmites. If sprayed carefully and in small quantities the chemical will not present a hazard to fish. Concentrations of glyphosate in the water should be kept below 0.1 ppm.
Glyphosate is registered for use either as Roundup or Rodeo1 for aquatic situations in many countries where stringent environmental regulations exist (e.g., USA and Australia). However, its application should be kept to a minimum in aquatic situations and the chemical used in conjunction with other techniques. Where possible the residues should be “flushed” into the Barsik main drainage canal.
Ametryn is not recommended in the Barsik Fish Farm or the feeder canals leading to it as this trazine herbicide will kill the phytoplankton in the ponds at extremely low concentrations.
The insects Neochetina eichhorniae and Sameodes albiguttatis have proved effective in some parts of the world in reducing the vigour of water hyacinth. Best results have occurred in tropical areas where the insect populations are not reduced by winter dieback of the host plant. It is not known how these insects would behave in Lake Edku.
This matter has been the subject of continuing investigation by officers of the Ministry of Agriculture in Egypt and is being carried out in conjunction with a team of entomologists from the USA.
There are no insects sufficiently damaging to the reed Phragmites australis.
1 NOTE: Rodeo is the preferred formulation for aquatic situations as it contains no added surface acting agent.