Freshwater Aquaculture Research and Training Centre
In recent years there has been a growing awareness among fish culturists regarding aquatic weed problems. Presence of small amount of certain aquatic plants in fish culture waters may be useful at times as they have a definite role in the development and maintenance of a balanced community. But dense growth of weeds assimilate a large proportion of nutrients in the pond and thus compete with fish population, limiting available fish food. They restrict fish movement and interfere with fishing operations, besides serving as ideal hunts for predatory fishes and insects etc. The surface cover of floating weeds prevent light penetration. Excessive development of algae have also been causing nuisance in cultivated fish ponds and small reservoirs. They even cause fish mortality due to oxygen depletion or release of extra-cellular metabolites which are toxic.
Carp culture is best done in three stages, where the spawn are reared to fry in nursery ponds, the fry are reared to fingerlings in rearing ponds and fingerlings raised to table size fish in stocking ponds. To get desired results proper management of these three category of ponds before introducing the stocking material and their subsequent maintenance is essential.
The higher aquatic plants that occur in fish ponds belong to various families and genera, but from the point of view of weed control, major plants have been grouped on the basis of their growth. The groups are : i) floating weeds, which remain free floating with their leaves above surface and roots within water (e.g. Eichhornia, Pistia, Salvinia, Spirodella, Lemna, Wolffia and Azolla) ; ii) emergent weeds, which have roots in the bottom soil but the leaves and flowers emerge to water surface (e.g. Nymphaea, Nelumbo, Euryale, Nymphoides, Myriophyllum and Phragmites) ; iii) submerged weeds are those that remain below water surface and may be rooted (e.g. Hydrilla, Najas, Potamogeton, Vallisneria, Ottelia and Nechamandra) or rootless (e.g. Ceratyphyllum and Utricularia) ; iv) marginal weeds, which grow on the shore but very often spread all over the water body (e.g. Ipomoea, Jussiaea, Typha, Cyperus, Paspalidium and Eleocharis); v) algal weeds, which in fish ponds are either planktonic or filamentous forms. Very often a particular phytoplankton form multiply rapidly to farm dense masses when environmental conditions and availability of nutrients are most favourable. Such dense growths referred to as ‘water bloom’ are responsible for imparting to the water, colours like green, yellow green, reddish brown and blue-green depending on the type of bloom forming algae. Filamentous algae very often grow densely covering the pond surface or pond bottom.
In fish ponds the blooms observed could be temporary which do not usually remain for more than a few days or permanent when it persists throughout the year. The algae responsible for temporary blooms mostly belong to the Chlorophyceae (Chalamydomonas spp., Pandorina morum, Volvox aureus, Chlorella vulgaris), Bacillariophyceae (Melosira granulata, Synedra ulna), Dinophyceae (Peridinium inconspicuum) and Euglenineae (Euglena spp., Trachelomonas spp.). The permanent blooms are constituted mostly by Myxophyceae (Microcystis spp., Anabaena spp., Raphidiopsis spp., Oscillatoria chlorina). The common filamentous algae are Spirogyra, Pithophora and Oedogonium.
The control measures practised fall into four major categories, viz; preventive, manual and mechanical, chemical and biological. However a combination of these methods at times may be necessary for satisfactory results.
Considering the high cost of clearance of weed infestations, certain preventive measures are suggested which will reduce the chance of weed incidence.
The preventive measures have to be thought of in advance and have to be integrated in the general management of water bodies. Some of the common measures considered are deepening of pond margins, disilting of old silted ponds, ploughing or burning of marginal weeds and providing barriers to entry of floating weeds.
This involves physical removal of aquatic weeds manually or by power operated devices. Manual removal is the age old practice. The free floating weeds are hand picked or draged ashore by wire or coir rope nets. The submerged weeds are either pulled by hand or hand drawn bottom rakes or uprooted with bamboo poles having a cross piece at their lower ends (Philipose, 1963). Repeated cutting of the leaves of water lilies as reported by Swingle and Smith (1950) is also useful at times. Hickling (1962) reports use of manual labour for repeated cutting of grasses and sedges of pond margins in Malaysia. Singh et al. (1976) observed control of Typha angustata by cutting the aerial shoots and flooding the stumps for four weeks.
Use of mechanical devices for removal of aquatic weeds have drawn considerable attention in recent years. The use of steel cables, cutting chains and diesel operated winches (Mitra, 1956) for clearance of rooted submerged weeds are prevalent. Cutting boats of different type having petrol operated underwater weed cutters are used in some regions. According to Blackburn (1968) mechanical weed control is like mowing a lawn, the operation must be repeated frequently. Floating weed harvesters have been used which consist of a barge with a conveyor system which lift the plants from water and carry them to the shore for disposal. Velu (1976) has developed a device which can clear both floating and submerged weeds at the rate of 1–1.5 ha/day depending on the intensity of infestation. Mechanical method have certain draw backs such as initial high cost, skilled operator for handling, difficulty in transporting and installing heavy machinary in remote areas.
This method of control is available in varying degree of efficiency for most of the aquatic vegetation. Total kill and disintegration of weeds can be achieved which ensure return of the nutrients back to soil and water and divert it to production of fish food organisms. No one chemical has been developed so far which would control all aquatic weeds. So it is essential to know the weed species, appropriate herbicide and their rate and time of treatment. The chemicals which kill the plants by direct contact resulting in the destruction of protoplasm are termed as contact herbicides. The other chemicals which pass from the treated part to other parts of the plant by translocation are termed as translocated herbicides. Some of the herbicides show selective action against specific plants or group of plants, while others affect almost all plants and are non-selective.
Floating weeds: Among floating weeds water hyacinth (Eichhornia crassipes) a native of South America is causing serious problems in the tropical water bodies. A number of chemicals have been tried against water hyacinth with varying degree of success. Some notable contributions in the field are that of Mitra (1948), Vass (1957), Ramachandran and Ramaprabhu (1968), Misra and Das (1969), Ramachandran et al. (1973) and Patnaik (1983). As seen from Table the herbicide 2,4-D (2,4-dichlorophen oxyacetic acid) is most effective for control of water hyacinth. The chemical is to be sprayed over the infestation by a foot pump sprayer. The dilution for proper coverage has been estimated at 400 l/ha. For approaching the interior of thick infestation in bigger water areas, a pair of stout hollow bamboo poles are to be laid on top of infestations on which an operator could walk with the help of a prop while spraying on both sides. He carries the sprayer lance served with the spray solution through a long polythene tubing by a foot pump sprayer kept on the sore. The bamboo poles are alternately pushed further ahead to continue spraying. Complete kill of plants after spraying takes about 25 days but total collapse resulting in sinking of the plants takes 2 to 3 months. However if required to utilize the water area early the dead plants which have softended considerably and reduced in bulk can be netted out easily. The stray patches of weed which might have escaped spray should be picked up manually or sprayed again to prevent regeneration.
TABLE I: Important floating weeds and the effective dose of chemical for their control
|Eichhornia crassipes||2, 4-D sodium salt||detergent||Ramachandran (1969)|
Small (13 kg/sqm)
big (35 kg/sqm)
|E. crassipes (Medium)||-do-||6 kg/ha||0.2%||Patnaik (1983)|
|-do-||2, 4-D amine salt||2 kg a.i./ha||-||Joshi (1976)|
|Salvinia molesta||Kerosene + Urea||91/ha + 4 kg/ha||0.5 kg/ha||George (1976)|
|Salvinia cucullata||Paraquat||1 kg/ha||-||Patnaik (1976)|
|-do-||Aqueous ammonia 10–15% solution||75–100 kg/ ha||-||Ramachandran & Ramaprabhu (1976)|
|Pistia stratiotes||Paraquat||0.1–0.2 kg/ha||detergent 0.1%||Patnaik(1972)|
|-do-||Aqueous ammonia (1%)||50–75 kg/ha||Wetting agent 0.25%||Ramachandran et al. (1976)|
The other floating weeds like the valvet weed (Salvinia) and water lettuce (Pistia) can also be effectively cleared by chemicals as shown in the Table.
The smaller floating weeds like Spirodela, Lemna and Azolla can be fully cleared with 0.1 kg a.i./ha of ‘Gramoxone’ as reported by Patnaik (1976).
Emergent weeds: The primary emergent plants which create problems in ponds are water lilies (Nymphaea), lotus (Nelumbo) floating heart (Nymphoides) and water chestnut (Trapa). Encouraging results have been reported by Srinivasan and Chacko (1952) in the control of Nymphaea with Dicotox (2,4-D ethyl ester) and by Singh (1962) in the control of Nelumbo and Euryale with Taficide-80 (2,4-D sodium salt). Mitra and Banerjee (1966) attained considerable success in eradicating Nymphaea and Nymphoides by applying copper sulphate pelleted with mud in the root zone in four to five instalments, at the rate of 35 kg/ha at an interval of one week. Large scale clearance of lotus, water lily and water chestnut was obtained by spraying 2,4-D(sodium salt) at the rate of 8–10 kg/ha with detergent (0.25%) added to herbicide solution. The chemical is to be diluted at the rate of 300 1/ha and sprayed by a Knapsac sprayer from a small boat.
Submerged weeds: The primary submerged weeds which infest fish ponds are tap grass (Vallisneria), water plantain (Ottelia), bushy pond weed (Najas), coon tail (Ceratophyllum), bladder wort (Utricularia), Hydrilla and Nechamandra. Philipose (1963) found sodium arsentte at 4–6 ppm effective against Hydrilla and Najas without killing fish. Inspite of its proven effectiveness it has the inherent drawback of toxicity to mammals for which it is being discouraged. Ramachandran (1960) found anhydrous ammonia at 15–18 ppm N effective in clearing Hydrilla, Najas and Ceratophyllum in fish ponds. The ammonia from a gas cylinder is injected into the weed infested area below the water surface. Rooted submerged weeds like Hydrilla, Vallisneria, Najas and Nechamandra are eradicated by localised application of copper sulphate pelleted with mud at the rate of 35 kg/ha as advocated by Mitra (1977). Three to four applications at one week interval are required for total clearance. The affected plants get uprooted and float on the surface which are to be dragged ashore by employing manual labour. Patnaik and Das (1981) obtained clearance of submerged weeds, Hydrilla, Najas, Vallisneria and Ceratophyllum by paraquat at 3 to 4 mg/l in two weeks but the clearance works out expensive.
Marginal weeds: A variety of weeds like water primrose (Jussiaea), water ipomoea (Ipomoea aquatica), grasses (Paspalidium), rushes (Eleocharis) and sedges (Cyperus) are common in pond margins. Spraying with 2,4-D amines and esters ranging from 3.4 to 13.5 kg a.i./ha proved effective against a number of grasses, sedges and rushes as reported by Philipose (1968). Ramachandran and Ramaprabhu (1968) obtained effective kill of young Cyperus plants with 2,4-D sodium salt and 2,2-dicholoropropionic acid (Dalapon) at the dose of 28 kg/ha. Panchal and Sastry (1976) obtained clearance of Typha angustata at the pre-flowering stage by application of diuron at the rate of 4 kg/ha along with 1 l/ha paraquat. Both water ipomoea and water primrose have been cleared by spraying 2,4-D (sodium salt) at the rate of 8 kg/ha. A second application may be needed.
Algal blooms: For control of algal blooms, copper sulphate (blue vitriol) traditionally served as an algicide (0.2 to 0.6 mg/l) is not effective in most fishery waters. The reason being most of the bloom ponds are highly alkaline (water pH above 8) and thus when copper sulphate is added it is largely precipitated as floculent hydroxide and thus reduces its algicidal properties. To counter act that, higher concentrations of copper sulphate are to be added which is likely to be toxic. A prior treatment with sulphuric acid will make low concentration of copper sulphate effective (Banerjee and Mitra, 1954) but this is costly and not easy for fish farmers to follow. Localized treatment with high concentrations of copper sulphate in alkaline waters as suggested by Kessler (1960) is not very effective. Patnaik and Ramachandran (1976) recorded full clearance of Microcystis bloom by application of 0.3 mg/l simazine (2-Chloro-4, 6-bis (ethylamino)-s-triazine) in 16–20 days. The dose is non-toxic to animal life of the pond. Another chemical, diuron (3-(3,4-dichlorophenyl)-1, 1-dimothylurea) at 0.2 to 0.4 mg/l can also clear algal blooms in 15 days without affecting fish of the pond. An organic copper complex ‘Cutrine’ was observed to be effective with dose 2 mg/l against Anabaena, Microcystis and Peridinium as reported by Patnaik (1980). Full clearance of Peridinium bloom by ammonia at 4 mg/l, produced by the action of slaked lime on ammonium sulphate was obtained by Ramachandran et al. (personal communication).
This method which involves use of biotic agents for aquatic weed control is gaining importance in recent years. A number of animals feed on aquatic plants or cause damage resulting in their eradication in fish ponds. The grass carp Ctenopharyngodon idella is now being tested throughout the world for aquatic weed control. The fish feeds primarily upon submerged plants but also take small floating plants like Spirodella, Lemna, Wolffia and Azolla. Judicious manipulation of stock density and size depending on the nature of water body, type and quantum of weed infestation are important factors for successful control of weeds. Singh et al. (1967) found grass carp of 600 g size to be effective against most submerged weeds under stocking density of 250 to 500/ha. The fish can utilize upto 286 % of their initial body weight as observed by them. To avoid any chance of grass carp interfering with native fish stock in natural waters only triploid are being permitted for large scale use in USA. The other fishes which are considered useful in controlling some aquatic weeds are Puntius javanicus, Pulchellus pulchellus, Tilapia mossambica, T. melanopleura and Ophronemus gorami. However their large scale use needs further trials. The common carp Cyprinus carpio though does not consume aquatic weeds helps in uprooting the plants in its search for bottom living food.
The other animals employed for weed control work are the ducks and geese. Hickling (1962) states that geese will keep grass on pond banks trimmed. The duck, ‘Kakki Cambell’ reported to be feeding on Salvinia need to be assessed for their ability to eradicate weed infestation.
The utilization of insects or their larvae as control agents for eradication of aquatic weeds has drawn considerable interests in recent years. Sankaran (1976), has investigated the possibilities of biological control of Eichhornia crassipes and Salvinia molesta two serious alien weeds in India. The host-specific enemies are two species of weevils (Neochetina spp), one lepidopterous borer (Acigona) and a mite (Orthogalumna) all attacking water hyacinth and a semi-aquatic grasshopper (Paulinia acuminata), a lepidopterous borer and a weevil attacking water fern. Neochetina has been successful in clearance of large areas in USA. However thorough and careful screening is necessary before introduction in the tropics.
The control of submerged weeds by shading with easily removable floating weeds like water hyacinth and water lettuce is very often tried in small ponds. But the weeds reappear after the cover weed is removed. Fertilization of ponds with inorganic fertilizers to encourage growth of phytoplankton which form a dense shade and prevents the establishment of submerged weeds by eliminating light penetration as suggested by Swingle and Smith (1947) is not always useful. In South East Asia fish ponds are fertilized by organic manures and even by introducing sewage (Prowse, 1962) which help in high turbidity due to dense phytoplankton growth and prevent growth of weeds.
For control of algal blooms very often easily removable floating weeds like water lettuce or duck weeds are introduced in the pond sufficient to cover the whole water area. It was observed that the bloom reappears after the cover plants are removed. The predominantly phytophagous silver carp (Hypophthalmichthys molitrix) when introduced in sufficient numbers in stocking ponds may exercise some check on the development of bloom forming algae. Since isolation of virus active against some genera of algae by Safferman and Morris (1963), new viruses on diverse blue-green algae have been reported by Padan et al. (1967) and Singh and Singh (1967). The role of these viruses in the control of natural population of algal blooms as emphasized may play a great role in future programme of biological control of bloom formation.
Aquatic weed composts can be used for fertilizing fish ponds in aquaculture. Nutrients locked up in the weeds become available for primary production and increasing fish food organisms by applying the composts. The organic components are recycled slowly or quickly depending on environmental conditions as aquatic weeds can release minerals much faster than terrestrial plants since they have a shorter mineral regeneration time (Gaudet, see Mitchell, 1974). Plankton production was found to be promoted by water hyacinth compost (Mitra and Banerjee, 1976).
Composts of water hyacinth are reported to increase the yield of Tilapia Oreochromes niloticus (Edwards, 1983). A simpler and easier way of aquatic green manuring in ponds is by killing weeds in situ by spraying suitable herbicides like 2,4-D, paraquat, ammonia, diuron etc. The dead weeds and algae decompose and form a suitable detritus as well as a compost and release the nutrients for increasing fish production through plankton/fish food organism development and also by direct consumption of the dead organic matter. In experiments and field trials conducted with water hyacinth, Pistia and other weeds fish production was doubled after killing and decompositing the weeds (Ramaprabhu et al., 1983). In experiments of 10–20 weeks duration, 400 to 600 kg/ha of fish production was achieved with common carp, rohu and mrigal by converting 10 to 35 kg of weeds while about 200 to 300 kg/ha were obtained under weed infested conditions. In field experiments productions of 1100–2300 kg/ha/yr were obtained after killing and converting the biomass into manure for the fish pond stocked with Indian major carps and common carp as compared to about 700 to 900 kg/ha production by manually removing the weeds.
Nutrient enrichment in water after killing the weeds was also significant.
In fish culture the use of aquatic weeds as feed for fish could be the cheapest and natural method of using the weeds as well as controlling them. Among the herbivorous fish different aquatic weeds are consumed by them selectively and the weeds which are not eaten by them become difficult to control. The utilization of such weeds into processed feeds is more economical and can be adopted as a management practice.
Grass carp (Ctenopharyngodon idella) is a well known fish which can utilize some of the common submerged weeds like Hydrilla, Najas, Ceratophyllum, Ottelia, Nechamandra and vallisneria in that order of preference (Singh, et al., 1967).
A recent survey of the feeding of grass carp on weeds in water at temperature of 20 to 30°C is shown below:
|Feeding reported||Feeding and non feeding reported||Non feeding reported|
|Submerged species (including mosles)||70||9||0|
|Floating species (including feru)||22||31||52|
1. Includes Eichhorina crassipes feeding on which is often restricted to roots and young leaves.
2. Two Nymphaea species two Nymphoides species and Ranunculus flutians.
3. One Scirpus species and Cicuta virosa (From International Agriculture Development May/June 1983)
It is evident that the grass carp is a voracious user of aquatic weeds and grows to weigh over 30 kg converting the wasteful weeds into valuable fish flesh. In composite culture the fish production is greatly enhanced by grass carp by its fast growth and attracts much commercial value both for consumption as well as for control/ utilization of aquatic weeds in fish culture. The duckweeds Lemna, Spirodela and Wolffia and Azolla are preferred by the young grass carp until they grow to larger sizes and are able to feed upon larger macrophytes (Murthy et al., 1976).
Diets containing different amounts of water hyacinth meal as food for some fish (Matrincha-Brycon sp.) showed that fish fed on diet containing 9.5 percent water hyacinth (34.9 per cent protein) grow from 87.9 to 208 g(1.2 g/day) in 90 days. The food conversion factor was 1.71 (Saint Paul and Teixeria, 1981). Fish fed on diet containing 18.9 per cent water hyacinth (34 per cent protein) grew during the same period from 84.5 to 190.3 g(1.1 g/day) with a conversion factor of 1.8. The experiment indicated that fish are not adversely affected by the addition of water hyacinth in the diets. Water hyacinth is chopped into small pieces and ground with 5 kg ricebran to produce a soft-pellet like mixture (Hiranwat, 1983). Feeding fish (carps-rohu, bighead, grass carp) and Tilapia with at least 2.5 per cent body weight (water hyacinth) gave encouraging results in pen culture in reservoirs.
In integrated farming systems having poultry or duck rearing, animal husbandry (cattle or pig rearing), fish culture and rice cultivation, the animal wastes are used in biogas plants and the sludge from biogas plants provides rich organic manure for fish ponds. The nutrient rich pond water can also serve for fertilizing paddy fields in which rice-fish cultivation is carried out by rotation or simultaneously by careful management. In other types of bioenergy aquaculture systems, aquatic biomass is produced from diluted waste water from animal or other sources and the biomass is utilized for biogas production or preparing suitable animal or other feeds. In this method both microscopic algae rich in proteins and larger aquatic weeds like water hyacinth are cultured in manifold pond systems. Similarly, in sewage fed fisheries, biomass production can be integrated for energy production and other purposes, besides fish culture.
Aquatic weeds especially water hyacinth play useful role in biological treatment facilities. They absorb inorganic and some organic compounds also from the water to build up their tissues and remove pollutants present in sewage so that the clean water is reusable in irrigation and industry. The biomass of plants can be harvested and used for suitable purposes.
The Nitrates and ammonium compounds present in sewage after removal by aquatic weeds become a rich source of fortilizers.
The fertilizer recovery method consists of leading waste water effluents through shallow ponds planted with weeds. The plants are harvested periodically to keep a continuous production of biomass and water purification. This method is easily applicable in small farm, village or municipalities to treat raw sewage/farm wastes after lagooning and diluting them. Waste water from house holds, food processing factories also can be purified in the above manner. Potentially harmful or odorous agents from drinking water and other heavy metals like cadmium, nickel, mercury, phenols which are carcinogenic can be removed by aquatic plants as they can concentrate these elements 4000–20,000 times more than in the water.
Most of the plants which seem to grow best and suitable for use in water treatment include common feed Phragmites communis, bulrush (Scirpus lacustris) water hyacinth, duck weeds (Lemna, Spirodela spp.) submerged weeds Hydrilla, Elodea, Egeria and Ceratophyllum.
The various methods dealing with the control of aquatic weed growth in fish culture waters are discussed in the foregoing account. These methods should be viewed as useful tools in an integrated system of weed management in which different techniques complement each other to produce the desired result. There is no permanent ‘cure-all’ for aquatic weed problems at present. Their control is a continuing process starting with a initial massive clearance operation followed by a persistent follow-up programme and subsequent management measures. For successful, control of weeds in fishery waters it is necessary to consider the type, age and density of infestation, cost of labour the feasibility of the method under local conditions, possible after-effects on the existing fishery.
It is also necessary that the initially cleared water body should be inspected at regular interval and remove whatever stray plants that might have appeared. The cleared water areas must be brought under fish culture so that it draws regular attention.
Since algae are primary producers in aquatic ecosystem an algistatic agent preventing growth is preferable to an algicide. Blue-green algae do not grow in habitat where pH is less than 6 or 7. Thus by maintaining the water at low pH the occurrence of bloom can be prevented.
The economics of clearance is an important consideration in selecting method of clearance. The cost of clearance by any of the methods varies from place to place and time to time depending on several factors. Hence for a fair assessment of the economics of weed control measures, the average cost of operations during a block period must be considered, keeping in mind the potential loss due to non-utilization of the weed chocked water body for fish culture.
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