Aquatic Weeds

Aquatic weeds are unwanted and undesirable vegetation that reproduce and grow in water. If left unchecked may choke the water body posing a serious manace to pisciculture.

• They provide breeding grounds and harbour predatory insects. 
• Provide shelter to predatory and weed fishes and molluscs.
• They restrict free movement of fry.
• They cause obstruction during netting.
• Limit living space for fish  
• Limit plankton production
• Reduce sunlight penetration and nutrients.
• Upsets the equilibrium of physico-chemical properties of water.
• Cause imbalance in dissolved oxygen budget.
• Promote accumulation of deposits leading to siltation
• Reduce water movement, thereby limits oxygen circulation in water.
• Some weeds release toxic gases that cause fish death and add foul smell to water.

Classification of Weeds

1. Floating weeds : Eichornia, Pistia, Azolla, Lenpa, etc.

2. Marginal weeds : Colecasia, Typha, Cyperus, Marsilia, etc.

3. Emergent weeds : Nymphae, Myriophyllum, Nelumbo, etc.

4. Submerged weeds: Hydrilla, Valisnaria, Chara,, Ceretophyllum, etc.

5. Algal weeds : Spirogyra, Microcystis, Oscillatoria, Dinoflagellates, etc.,

Removal of aquatic weeds

All these weeds have to be eradicated using one or more of the following methods.

1. Manual method

Manual removal of weeds involves physical removal of the weeds by hand. This may be practical if the pond is small and labour is cheap.

2. Mechanical method

Some machines or implements are used for removing aquatic weeds. This method is normally applicable for larger water bodies. It is capital intensive and beyond the means of average fish farmer.

3. Chemical method

• Weeds are eradicated using chemicals.
• Different weedicide are used for removal of different weeds present in aquaculture pond. 

Common weeds and the weedicides for their control

#	Type of weeds	Herbicide	Dosage	Method of application
1	Water hyacinth	2,4-D	8 - 10 kg/ha	Foliar spraying
2	Ipomoea spp.	2,4-D	2 - 4 kg/ha	Foliar spraying
3	Sedges and rushes	2,4-D	5 -10 kg/ha	Foliar spraying/ root zone treatment
4	Lotuses and lilies	2,4-D	5 - 10 kg/ha	Root zone treatment
5	Ottelia, Vallisneria	2,4-D	10 -20 kg/ha	Root zone treatment
6	Aquatic grasses (in young stages)	Dalaphon	5-10 kg/ha	Foliar spraying
7	Aquatic grasses	Paraquat	2  kg/ha	Foliar spraying
8	Aquatic grasses	Diuron	4  kg/ha	Root zone treatment
9	Microcystis, other planktonic and filamentous algae	Diuron	0.1-0.3 ppm	Root zone treatment. Dispersal in water column
10	All submerged weeds	Ammonia	10-15 ppm	Root zone treatment . dispersal in water column
11	Pistia	Ammonia	1% aqueous solution with 0.25% wetting agent	Foliar spraying
12	Pistia	Paraquat	0.2 kg/ha	Foliar spraying
13	Salvinia	Ammonia	2% aqueous solution with 0.25% wetting agent	Foliar spraying
14	Salvinia	Paraquat	0.4 kg/ha	Foliar spraying

4. Biological method

• The method is more advantageous since the undesirable weeds are converted into fish flesh.
• It is cheap as no labour is involved and most suitable from the social and environmental point of view.
• The method employs certain organisms which feed on the weeds. 
• The grass carp which can eat up much more aquatic vegetation is itself an excellent example of biological weed control.
• The common carp helps in uprooting of certain plants.
• Tawes, Puntius gonionotus is also a good feeder of aquatic weeds.
• The Yamuna turtle consumes water hyacinth in the ponds. 

Common weed eating fish and the weeds of their preference

Fishes	Names	Feed upon
Grass carp	Ctenopharyngodon idella	Submerged weeds e.g: Hydrilla, Najas , Ceratophyllum, Potamogeton, Ottelia and duck weeds
Common carp	Cyprinus carpio	Tender shoots
Gaurami	Osphronemus goramy	Tender shoots of submerged weeds and filamentous algae
Pearl spot	Etroplus suratensis	Filamentous algae
Silver carp	Hypophthalmichthys molitrix	Algal bloom