Aquatic weeds/vegetation

Aquatic weeds are the defined as the unwanted and undesirable vegetation which reproduce and grow in water and, if left unchecked may choke the water body posing a serious menace to pisciculture.

Classification of aquatic weeds: On the basis of their habit and habitats, aquatic weeds have been variously classified. According to Philipose (1968), they can be divided into:

1. Floating weeds: Floating weeds are unattached and float with their leaves above the water surface and roots under water. E.g., Lemna, Wolffia, Azolla.

2. Emergent weeds: Emergent weeds are rooted in the bottom soil but have all or some of their leaves, leaf laminae or shoots above the water surface. E.g., Nymphaea, Trapa, Nelumbo.

3. Submerged weeds: Submerged weeds are completely submersed under water, but may be rooted in the bottom soil or free floating. E.g., Hydrilla, Najas, Utricularia.

4. Marginal weeds: Marginal weeds are fringe the shore line of the water body and are mostly rooted in water logged soil. E.g., Typha, Phragmites, Cyperus.

5. Filamentous algae: Filamentous algae are form ‘mats’ in the marginal area or scums in the main body of water. E.g., Spirogyra, Pithophora.

6. Algal blooms: Algal blooms occurring dispersed in the water body. E.g., Microcystis, Anabaena.

References: Pillay, T.V.R., (1991), Aquaculture principles and practices.

Control of aquatic weeds

The common methods adopted for control of aquatic weeds are: manual, mechanical, chemical and biological.

1. Manual methods: In Bangladesh, where labour is cheap, manual methods are often employed to remove weeds. In small water bodies, the traditional methods of hand picking, uprooting emergent and marginal weeds and cutting them with scythes, wire mesh etc. are considered suitable. Floating weeds can be controlled by netting, e.g. water hyacinth water lettuce, salvinia, duck weed, Azolla, spirodella.

2. Mechanical methods: Several types of equipment have been devised but most of them can be used in very expensive pond farms, e.g., the farms with large individual ponds (up to 100 ha).

The mechanical methods in Bangladesh as well Indian sub continent have been tried on a limited scale.

Ø    Aquatic weeds controlled mechanically by-

Ø    ‘Weed cutter’ used for cutting submerged and emergent weeds.

Ø    Floating weed cutters are generally driven by paddle wheels.

Ø    Control of marginal weeds and grasses by grazing and deepening of marginal shelves, draining, desilting and erecting barriers to prevent floating weeds from spreading.

Ø    Artificial creation of turbidity with silt and liquid raw cow dung to control algal blooms in nursery pond by shading.

Ø    Mechanical devices used for clearance of rooted submerged weeds are steel cables, cutting chains and diesel operated winches.  

3. Chemical methods: Certain commercially available chemicals (herbicides) can provide an efficient means of eradication of undesirable aquatic weeds. Total kill and disintegration of weeds can be achieved by this method ensuring full return of the nutrients back to pond soil and water for production of natural fish food. The common herbicides used to eradicate different ecological types of aquatic weeds are as follows:

a. Floating weeds: Water hyacinth is one of the most important weeds of this group. Depending on its degree of infestation they are categorized in three groups, viz., small, medium and big based on their wet weight per unit area. The recommended doses of the herbicide 2, 4-D (2, 4-Dichlorophenozy acetate) are-

Addition of a detergent (0.2% concentration) at 400 l/ha to the aqueous solution gives better results.

Water lettuce which often causes a serious problem in fish ponds can be controlled with 0.1-0.2 kg of paraquat/ha.

Small floating weeds e.g., Spirodela, Lemna and Azolla can also be cleared with 0.1kg/ha of paraquat.

b. Emergent weeds: Water lily, lotus, and floating heart can be cleared by spraying the herbicide 2, 4-D at the rate of 8-10kg/ha with detergent (0.25%). The chemical is diluted at the rate of 300 l/ha and sprayed through a foot pump sprayer.

c. Submerged weeds: Ottelia, Vallisneria, Hydrilla, Najas, Potamogeton and Ceratophyllum can be controlled by paraquat at the rate of 3-4ppm within two weeks. It can also be controlled by application of anhydrous ammonia at the rate of 15-20 ppm.  

d. Marginal weeds: Ipomea, Jussiaea, etc., could be controlled by spraying the herbicide 2, 4-D at the rare of 8kg/ha.

e. Algal blooms and mats: The control of algal blooms is best with copper sulphate (CuSO₄).

The recommended dose 0.2-1.0 ppm at normal pH (7). Microcystis bloom is cleared with 0.3-0.5 ppm of Diuron. Simazine [2-chloro-4, 6 bis (ethylamine)- triazine] also cleans the bloom in 16-20kg days and the rate of application is 0.3-0.5 ppm.

The common mat forming algae which occur in fish ponds are Spirogyra, Pithophora, Oedogonium and Cladophora are cleared with 0.3-0.5ppm of Diuron.

4. Biological method: Another important controlling method is by introduction of weed-eating fishes. Certain varieties of herbivorous fishes viz., common carp, grass carp, tilapia, a species of Puntius are commonly used to control aquatic weeds in many parts of the world.

Grass carp is the most effective biological control agent against most of the submerged and floating weeds. Grass carp normally consumes choiced aquatic weeds, at least 50% of their body weight in a day. About 300-400 fish each of about 0.5kg weight, are enough to cleat 1 ha of Hydrilla  infested water body in about a month.

Common carp can indirectly control vegetation growth by digging the bottom mud for food and increasing the water turbidity, which caused the growth of submerged weeds, is suppressed. The common carp stocked at a rate of 400/ha or more could prevent the growth of submerged weeds.

Tilapia sp. have also been found to be useful in controlling the growth of soft submerged vegetation (Najas) and filamentous algae.

Certain species of Puntius feeds on a variety of weeds such as Hydrilla, Azolla, and Chara etc.

Ducks can also help to control submerged vegetation in the shallower parts of ponds while searching for food. They also feed on some floating plants such as duckweed, Lemna sp.

References:

Gupta, S.K., and Gupta, P.C., (2006), General and applied ichthyology (fish and fisheries).

Kumar, D., (1992), Fish culture in undrainable ponds (A manual for extension).