Aquatic vegetation
Weeds are the unwanted plants that interferes the agricultural practices or production. They grow in the pond, river, stream, lakes and any other water body restricting the sunlight, movement of fishes and compete for nutrient, food and space with the cultured species. They also compete for fish eating organisms affecting fish production. Their propagation is very fast through shoot, fragmentation, runners, stolons, root, tubers, corm, bulbs, rhizomes and other diverse form. Knowledge of aquatic weeds their various forms and management is essential for optimum fish production.
15.1. Aquatic plant descriptions:-One of the major problems in fish ponds is to control the excessive growth of aquatic weeds. The presence of some aquatic plant in pond to a limited extent is desirable but their excessive growth is very harmful from both pond management and fish production point of view. It is necessary to identify the harmful species of aquatic plants so that they can be controlled effectively. When considering aquatic plants, the two major categories are algae and vascular plants (macrophytes). Invasive vascular plants, or water weeds are non-native plants that exhibit aggressive growth habits and can outcompete and displace native plant species, contributing to a loss of biological diversity and overall aquatic habitat degradation.
Aquatic weeds in fish ponds and their control
Aquatic vegetation can be generally classified as
(i) Algae (ii) Floating (iii) Submerged (iv) Emergent
Algae can be unicellular and filamentous
(a) Unicellular Algae:-Unicellular algae are species of microscopic single celled plants, or colonies of single celled plants, that remain suspended in the water. They form the basis of life as synthesize oxygen and other complex organic matter. When they become over abundant, they can give the water a soupy green or brown color. This condition is known as an algal bloom. Algae do not bear flower or seeds unlike most other aquatic plants. Their multiplication can be by asexual division, cyst formation or sexual reproductions that result in spore formation.
In pond aquaculture, some of the most problematic algae are single –celled and small multi-cellular species that proliferate in vast numbers eg. Anabaena, Anacystis, Cosmarium, Coelastrum. The dense algal populations are referred as bloom. Though they produce dissolve oxygen they also consume it during night hours causing rapid fluctuation. They also cause wide fluctuation in carbon dioxide, nitrogen and phosphorous levels in pond.Toxic species of algae are Anacystis, Anabaena, Gonyaulax, Gymnodinium and Pfiesteria.
(b) Filamentous Algae:- Filamentous algae are species of plants that consist of visible hair-like strands. These strands may be straight, branched, or even arranged in net-like structures. They may feel slimy, woolly or cottony. These algae begin their growth on the pond bottom, but may float to the surface due to entrapped bubbles produced during photosynthesis. There are no roots and no recognizable plant structures such as stems or leaves. Spirogyra, Lyngbya, Pithophora, Oedogonium is often problematic in pond, cage, pen and race way culture systems. They start growing often at the pond bottom and attach themselves to the hard substrate forming thick mats. They trap the gases in filaments that later on help them to float on the surface. These floating mats of filamentous algae are known as pond scum or pond moss. This kind of algae trap small larval fishes, feed making it unavailable to the cultured fishes. They reduce space and restrict movement of organism, compete for nutrients and restrict netting. The largest multi-cellular or macro-algae species are marine kelps. Some of the algae are promoted in pond as they form the basis of the food chain and therefore have important role in plankton growth.
15.2. Floating plants:- These are aquatic vegetations with roots, free leaf floating on the surface of the water such as water hyacinth (Eicchornia cressipes) Water lettuce (Pistia stratiotes) Bladder wort (Utricularia spp.),Water ferns (Azolla and Salvinia spp.), Duck weeds like Watermeal ( Wolffia columbiana) are the smallest flowering angiosperm. Some of the duckweeds are also used as feed for tilapia and grass carp.
Marsilia
15.3. Submerged plants:- It refers to the plants that grow underwater and in some species up to the water surface. They are usually flaccid and depend on the water column for support. Some of these plants may bear flower as well as seed heads extending above the water surface. Hydrilla (Hydrilla verticillata, Egeria (Egaria densa), Eurasian watermilfoil (Myriophyllum spicatum), Pond weed (Potamogeton), Vellisneria, Naiads (Najas)
Coon tail (Ceratophylum demersum ), Naiads (Najas) are the common examples. They also reduce the effective culture volume of production unit, may entangle larvae, trap feed and restrict light dispersal of feed and netting
Vellisneria beside compete with fishes for nutrients and space. The fluctuation in dissolve oxygen is also other critical parameter affected.
15.4. Emergent weeds:-These plants are rooted at the bottom and have flexible long stem and long petiole, leaves, flowers etc. the petiole , leaves and flowers are present on the water surface example- Typha, Syperus, Eurale.
Typha
15.5. Marginal weeds:- Several rooted plants and grasses occurs in the margina;l region of the water body. These vegetations spread either on the surface of water or on the adjoining land. Example- Ipomea, Penicum, Typha, Marsilia, Cyperus etc.
15.6. Duckweed:-Duckweed is the common name for a family of small aquatic herbs known as Lemnaceae that grows in fresh water. They have the unique distinction of being the smallest flowering plants on earth. These plants are also unique in that they do not have any stem or leaf structures. The plant is simply a fleshy ovoid or flattened structure that may or may not bear simple roots. Duckweed is ubiquitous to most temperate and tropical regions of the world, making it readily available to most farmers. They are typically found floating in thick mats of homogeneous populations in quiet streams or ponds containing high levels of organic matter. Another amazing feature of these plants is that they can double their mass in less than two days under ideal conditions of nutrient availability, sunlight and temperature.
16.2. Harmful effects of aquatic weeds
1. The weeds compete for essential nutrients necessary for the production of food of fishes.
2. They also check the penetration of sun light into the water essential for photosynthesis.
3. Dense weed also interfere with fish movement, netting operation and provide shelter to fish enemies (insects, frog, snakes, birds, tortoise etc).
4. Putrefaction of these weeds pollutes the water body.
5. These weeds also convert ponds into swamps by trapping silt and debris.
6. Aquatic weeds require oxygen for respiration at night which causes oxygen deficiency in water, causing difficulties in respiration.
Table: 4. Types of aquatic plants
|
Groups |
Scientific name |
Common name |
|
Floating
|
Eichhornia crassipes |
Water hyacinth |
|
Pistia stratiotes |
Water lettuce |
|
|
Salvinia cucullata |
Water fern |
|
|
Lemna minor |
Duck weed |
|
|
Emergent |
Nymphea Mexicana |
Banana water lily |
|
Nymphea tuberose |
Fragrant water lily |
|
|
Nymphoides spp. |
Floating heart |
|
|
Submerged |
Hydrilla verticillata |
Hydrilla |
|
Najas marina/minor |
Najas |
|
|
Potamogeton crispus |
Curly leaf pondweed |
|
|
Vallisneria spiralis |
Eel grass |
|
|
Marginal |
Ipomea aquatic |
Ipomea |
|
Jussiaea spp. |
Water primrose |
|
|
Typha anqustata |
Cat-tails |
|
|
Cyperus spp. |
Cyperus |
|
|
Algal blooms |
Microcystis aeruginosa |
Microcystis |
|
Anabaena |
Blue green algae |
|
|
Algal mats |
Pithophora |
Horse hair clump |
|
Spirogyra |
Filamentous algae |
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