Common aquatic weeds
Aquatic weeds belong to various families of dicots, monocots and single-celled and filamentous algae. From the point of view of aquaculture and weed control, the macrophytic and algal weeds can be best classified according to their habits and habitat. According to Philipose (1968), they can be divided into
1) floating weeds, which are unattached and float with their leaves above the water surface and roots under water (e.g. Eichhornia, Pistia, Azolla);
2) emergent weeds, which 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, Myriophyllum);
3) submerged weeds, which are completely submersed under water, but may be rooted in the bottom soil (e.g. Hydrilla, Najas) or freefloating (e.g. Ceratophyllum, Utricularia);
4) marginal weeds, which fringe the shore line of the water body and are mostly rooted in waterlogged soil (e.g. Typha, Phragmites);
5) filamentous algae, which form ‘mats’ in the marginal area or ‘scums’ in the main body of water (e.g. Spirogyra, Pithophora);
6) andalgal blooms, occurring dispersed in the water body (e.g. Microcystis, Anabaena).
Water hyacinth (Eichhornia) is probably the most widely known floating weed, not only infesting aquaculture farms but also all other types of water bodies in Asia and many other parts of the world. The spread of the plant is truly phenomenal, as it can increase in volume by about 700 per cent within 50 days (Parija,
1934) or from a pair of plants to 1200 in four months. Two other floating weeds that deserve special mention are Pistia (water lettuce) and Salvinia, which grow very fast and cover largeareas of aquaculture farms and open-water bodies. Lemna (duck weed), although easier to
control, can grow rapidly and cover a pond or enclosure in a short period of time.
Submerged weeds are generally more difficult to control and are therefore considered more noxious than all the others. Hydrilla,
Najas, Nitella, Vallisneria, Potamogeton, Cerato-phyllum, Urticularia and Chara are some of thepersistent submerged weeds which it requires considerable efforts to eradicate.
Common emergent weeds are Nymphaea, Nelumbium, Trapa and Myriophyllum, whichcan be more easily controlled; some of them, like Nelumbium and Trapa, are in fact some-times grown in association with fish.
Although marginal weeds are considered undesirable in fish ponds in many parts of the world, in some East European countries a reed belt is maintained in large fish ponds to control wave action. They are actually planted on the berm of pond dikes, by seeding as a soil root mixture, or as root or shoot cuttings. A density of at least 70 reeds/m2 is considered necessary. Phragmites and Typha are the two commonmarginal weeds used in this manner.
While many algae are the food of fish and other aquaculture species, it is the excessive growth of filamentous algae like Spirogyra and Pithophora and persistent blooms of planktonicalgae such as Microcystis and Anabaena that account for their sometimes being considered as weeds in aquaculture farms.
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