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.