Control of aquatic weeds

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  • 1. AQUATIC WEEDS CONTROL For fresh Water Fish Culture & Inland Fisheries Students By R.P.Kshirsagar
  • 2. Control of aquatic weeds • Aquatic weeds when present in a water body compete with fish seeds or fish for oxygen. competition for oxygen results in fish dieses in fish or mortality of fish is called as biological indicator for any slight unfavorable change in the water body is immediately reflected by fish on its body or results in mass mortality of fish. A dense cover of aquatic weeds in a water body blocks sunlight and we know that sunlight is very important for any ecology. • In case of blocked sunlight the plankton production becomes poor in a water body resulting in competition among fish for food in the absence of artificial feeding and result in mortality of fish or fish seeds. A dense cover of aquatic weeds also provides safe havens for predatory organisms like snake, frog and aquatic insects. These predator feeds on fish or fish seeds and because of weeds are difficult to remove hence it is always advisable to eradicate aquatic weeds in time. Aquatic weeds when present in excess in a water body also pose threat to fish life by blocking its respiration by settling on its gills.
  • 3. • Presence of excessive aquatic weeds also interferes during the trial netting or netting operations masking the correct reading of growth and health of fish species cultured. It is for the reasons mentioned above the eradication of aquatic weeds are must before undertaking any fish culture. • There are many methods for the eradication of weeds. Normally for the eradication of aquatic weeds effective and cheap method is to be selected. As far as floating weeds are concerned they are easy to eradicate and also requires less money. Since free floating weeds are on the water surface and also there roots are not fixed to any substratum i.e. they are free and therefore they are easy to eradicate. The common method for the free floating aquatic weeds is by hand picking or by repeated netting operations.
  • 4. In this method the free floating aquatic weeds are either hand-picked from a water body or removed completely by repeated netting operations in the pond infested with it. These weeds thus collected by hand picking and or repeated netting are then gathered at one place are then dried and destroyed by burning them. Burning of aquatic weeds is necessary since weeds are nothing but plants in water bearing flowers and seeds and if not burnt the seeds of an aquatic weeds may enter in another pond/ water body germinate in it and again weed infestation in that pond starts and weeds may invade another pond being used for fish culture. Thus burning of aquatic weeds may completely eliminate it from a water body/ pond.
  • 5. • Second method used for eradicating aquatic weeds is by erecting barriers. Concept of barriers is simple and not complicated one. Barrier may be a rope as far as free floating aquatic weeds are concerned. If free floating aquatic weeds are excessive then we can use barriers. Free floating weeds are collected at one end of a water body and a barrier of rope is erected at that point so that weeds cannot moves at random in a water body. They are then are removed dried and burnt for complete eradication.
  • 6. • Emergent and Submerged weeds- Emergent and submerged aquatic weeds are deeply rooted in the soil or substratum and therefore are not easy to eradicate manually. Eradication by handpicking is impossible or very difficult as far as these weeds are concerned. If the water body is excessively infested by the emergent and or submerged weeds then before stocking of fish or fish seeds, during pre-stocking management when the water body i.e. pond is drained and subjected to sun drying the emergent and or submerged weeds are uprooted, however one cannot guarantee the complete eradication of weeds by this method of uprooting, for roots of weeds in the substratum sometimes cannot be removed completely resulting in reappearance of an emergent and or submerged weeds. Uprooting stems of an emergent and or submerged weds is not enough but the weed has to be uprooted completely for its total eradication. Weed harvesters are employed for the eradication of these weeds.
  • 7. • Use of different chemical can bring about total eradication of an emergent and or submerged aquatic weed. Different chemicals are prescribed for the eradication of emergent and or submerged aquatic weeds. The chemical are used in the form of paste, liquid or in the injection form is administrated for the eradication of aquatic weeds. For example paraquate is prescribed for the eradication of marginal weed like typha. Liquid ammonia is administered/ injected at the top levels of hydrilla to eradicate hydrilla completely. Amitrol is the chemical in prescribe dosages used to eradicate emergent aquatic weeds like lotus. 2-4D is the commonest weedicide which is used to eradicate an emergent and or submerged aquatic weed. The use of a chemical should be done cautiously for it may affect the fish or a person working in a water body or pond after drying of a pond and after application of weedicide, which is to be is done prior to atleast 7 to 10 days of stocking fish seeds or fish or else the poisonous effect of chemical will result in mortality of fish or fish seeds resulting in diminished fish production or losses in fish business. Some chemicals are harmful not only to fish but also to the other animals like cow, hen, pig and human also it is therefore advisable to apply weedicide prior to 7-10 days before stocking of fish seeds. A much care has to be taken while using chemicals for the eradication of aquatic weeds when the culture is integrated fish culture.
  • 8. • The best method for eradication of aquatic weeds is biological control of aquatic weeds. Biological control of any living thing is nothing but to control one of the unwanted organisms by other which feeds on it. It is very effective method as far as fish cultured is concerned and also cost effective for the fish farmers. In this method commercially important varieties which feed on weeds are introduced in a pond/ water body. The fish like Ctenophryngodon idella commonly called as grass carp is introduce in a pond infested with weeds. Grass carp is known to eat aquatic weeds 8 times of its body weight bringing about eradication of aquatic weeds effectively. Other fish varieties like common carp, tilapia which feeds on aquatic weeds can be introduce in a pond infested with aquatic weeds and weeds can be effectively controlled. Many fish varieties are herbivorous and can be used successfully for the biological control of aquatic weeds. In this method on one hand the farmer get rid of aquatic weeds while on other hand he gets the production of commercially important fish like grass carp, common carp etc. Not much money id required for the artificial feeding of fish introduce in a pond infested with aquatic weeds for aquatic weeds itself is the food of the fish introduced in such a water body.
  • 9. • Mats and Scums- Algal blooms are found in stagnant waters. In case of ponds not properly maintained or neglected water bodies it is a common phenomenon. Algal blooms are very dangerous for fish culture especially when fish seeds are stocked in a pond. In these cases algae can settle on the gills of a fish seeds and can cause mortality of fish seeds so also it may cause oxygen depletion of water body and also it create competition for space among the fish seeds resulting in mortality or stunted growth of fish. Filamentous algae like spirogyra, nostoc, chara,nitella etc. causes mats in the water body thick mats of these algae may also block sunlight. It is therefore essential to look after and or proper care of ponds are to be taken during the phase manuaring or during the artificial feeding of fish and or fish seeds, if brownish layer is appeared on the pond during the fertilization of pond or during the artificial feeding the operation of phased fertilization or of feeding should be stopped for the brownish or greenish layer may be start of algal blooms.