Various types of Aquatic Weeds in A Village Fish Pond and Their Control

Aquatic weeds become menacing to fish cultivation when their growth is excessive. Excessive vegetation of all categories is harmful, be it emergent, floating or submerged type of weed. The disadvantages caused are many. Control of weeds is an important part of pond management. In this topic different types of aquatic weeds with example and their advantages, disadvantages and their control measures are described.


Introduction
Weeds are unwanted aquatic plants of uncontrolled growth. Aquatic plants are the primary producers of the aquatic ecosystem. They help the fishes in providing food, oxygen, shelter etc. The presence of these unwanted and undesirable aquatic plants certainly pose threat to the productive potential of the aquatic body. In fact, the presence of certain aquatic plants is frequently desirable in fish ponds. Any slackness in controlling their excessive growth diminishes productivity of the water body by utilizing the nutrients or check the penetration of sunlight by shading may lead to either super saturation or depletion of O 2 . Aquatic weeds provide harboring place to predatory and weed fishes are profuse feeders and prolific breeders and over populate themselves. These factors limit the availability of foods. Gases like H 2 S and CH 4 are formed which are harmful to fishes. On the other hand, presence of aquatic weeds in small numbers is necessary because they form natural food for many fishes. They form good fertilizers in the pond when decayed.
Over 140 species of plants have been reported to act as aquatic weeds both within and around various kinds of aquatic bodies in India. The states comprising West Bengal, Odisha, Bihar, Assam, Tripura and Manipur have the maximum incidence of weed infestation ranging from 40-70%, whereas in other states it may range between 20-50% (Philipose, 1968) [1][2][3][4][5].

Types of Aquatic Weeds
From fisheries standpoint the most useful classification of weeds based on habitat and habit have been enlisted as follow:

Planktonic algae
The algae form the phytoplankton. They may be blue green algae, green algae etc. their rapid proliferation results in algal blooms. e.g. Oscillatoria, Microcystis, Anabaena, Volvox etc ( Figure  1

Emergent weeds
These weeds are rooted in the bottom soil but have all or some of their leaves, leaf, lamina or shoots above the water surface. e.g.

Marginal weeds
These types of weeds grow on the margins or on the shore line of the water body. They are mostly rooted in water logged area. e.g. Typha, Nymphaea, Marsilia, Ipomoea etc (Figure 17-19).

a)
They form natural food of many species of fish.

b)
Fertilize the pond when decayed.

c)
They provide shade and shelter to many fish.

d)
Oxygenate the water.

e)
They reduce turbidity.

f)
Provide spawning beds for fishes.

a)
Aquatic weeds occupy more space and they restrict space for fish life.

b)
They retard the free movement of fish.

c)
Plankton production is reduced.

d)
They prevent the penetration of light.

e)
They provide shelter for predators and parasites.

f)
Aquatic weeds produce Hydrogen Sulphide (H 2 S) and Methane (CH 4 ) which are harmful to fishes.

g)
They consume more nutrients.

h)
They obstruct fishing operations.

Control of Aquatic Weeds
Weeds are controlled by the following methods:

Manual control:
Weeds can be removed by employing man power with simple devices. They can be removed by hand picking. The floating weeds can be removed by bamboo poles, ropes or nets.

Management control:
a) Floating weeds can be controlled by constructing barriers. b) Periodic draining and drying of the ponds.

c)
Drying and burning. d) Submerged weeds can be controlled by increasing turbidity.

Mechanical control
Aquatic weeds can be removed by machines. Water hyacinth can be removed by JCB.

Chemical control
Weeds can be control by chemicals called weedicides. The weedicides should have the following qualities: a) They should be cheap and easily available. b) They should kill the weeds at low concentration.

c)
They should be non-toxic to human and fish. d) They should not pollute the aquatic ecosystem.

Chemical control of emergent weeds:
i.
Some birds have been found in controlling aquatic weeds. For example, Swans, Ducks feed on algae like Wolffia, Lemna, Marginal grasses etc.

Control through utilization
Aquatic weeds have economic value. They are used for various purposes. The removal cost of aquatic weeds by manual, mechanical and chemical methods is high and this cost can be compensated by utilizing the weeds for the following purposes i. Fertilizer

ii.
Feed for animals/fishes/birds.

iv.
Manufacture of paper.

Conclusion
Native aquatic plants are an important part of the ecosystem, but invasive aquatic weeds degrade the pond habitat and limit the use of affected waters. Most aquatic weeds have been intentionally introduced, so exclusion or prevention is the first line of defense against them. Once a pond has been invaded, several strategies can be used to manage aquatic weeds, including cultural, mechanical, biological and chemical control. Pond owners should first identify the invasive species, then select a product labeled for that weed and for the situation, making sure to apply the product according to label instructions to avoid damage to native plants and fish.