Eradication of Weed and Predatory Fishes in Rearing Ponds

Predatory Fishes

• Fishes which predate the spawn, fry and fingerling of cultured fishes.
• They get into cultured ponds through water or seeds.
• They breed easily in confined water little earlier than carps and therefore their size will be bigger than the size of the carps.
• They compete for food, space and DO and result in poor growth of carps.
• Their complete eradication using physical methods is difficult.
• They have the habit of burrowing in the mud bottom.
• Use of pesticides is inevitable.

Common predatory fishes are:

• Channa spp., Clarias batraches, Heteropneustes fossailis, Pangsius, Mystus sp. Ompok spp., Wallage attu and Glossogobius giuris etc.

Weed Fishes

• These are uneconomical, small sized, naturally occuring or introduced accidentally in ponds along with fish seeds.
• They compete for food, space and dissolved oxygen. 
• They have high fecundity and breed well before major carps breed.
• Many of them breed throughout the year.
• Therefore, fish seeds from wild contain seeds of weed fishes.

Common weed fishes are:

• Puntius sp., Oxygaster sp., Ambassis sp., Amblypharyngodon mala. Colisa sp., Rasbora sp., Aplocheilus sp., Laubuca sp., Esomus danricus, etc.

Removal of predatory and weed fishes:

1. Repeated netting:

• Repeated netting is suitable for only those ponds having no other fishes except carps.
• It is not possible catch predatory and weed fishes simply by netting.
• Remaining fishes will breed and have sizable population in the pond.

2. Dewatering the pond is the best method.

• Dewater the pond, catch all the fish and allow the pond bottom to dry till the bottom soil cracks.
• Summer is the best time for this.

 3. Poisoning the pond in case dewatering and drying is not possible.

Fish toxicants

Fish toxicants can be grouped into (a) Plant derivatives, (b) Chlorinated hydrocarbons and (c) Organophosphates. Chlorinated hydrocarbons are the most toxic and stable compounds, not metabolised and remain stored  in fish tissues. Although organophosphates are less toxic but adversely affect other aquatic biota. These are relatively less persistent in water and rarely stored in fish body. Prior to use of fish toxicants,  certain criteria should be followed.

 (1) It should be effective at minimum dosage

 (2) It should be less costly

 (3)The fish killed by the toxicant should be suitable for consumption

 (4) The toxicity should remain only for short duration

 (5) The toxicity should be non-cumulative

 (6) The toxicants should be easily available.

Since the use of chlorinated hydrocarbons and organophosphates is not normally recommended in fish culture ponds, use of plant derivative piscicides is encouraged.

Commonly used piscicides are:

a. Mahua oil cake

• It is the most commonly used fish toxicant in our country.
• It contains about 4 -6% saponin.
• It kills fish at 200-250 ppm in 6-10 hours.
• Fishes killed by this are fit for consumption.
• The toxicity lasts for 15 -20 days in water
• Mahua cake subsequently becomes the manure for the pond.
• Applied at the rate of 2000 -2500 kg/ha at one metre average depth.
• Powdered cake soaked in water and broadcast over water surface.
• After its application, the pond is netted repeatedly to enable uniform mixing and also to take out dead fishes. 

b. Tea seed cake

• Crushed and powdered tea seeds are effective for the control of the unwanted fishes in the fish ponds.
• Kills fish @ 75 -100 ppm
• Applied @ 750 to1000 kg/ha for every one metre average depth. 
• Toxicity lasts for 10 -12 days.
• Ultimately the cake acts as fertilizer in the pond.
• The treated fishes are suitable for human consumption.

c. Ammonia

• Anhydrous ammonia is quite costly
• Kills fish @ 20-25 ppm 
• Ammonia gas comes in cylinders.
• For application, the cylinder is held in position at the bottom of the pond and moved from one side to the other releasing the gas in a controlled way so that there is neither excessive cooling nor gas loss during the absorption by the bottom soil.
• Sufficient protection should be provided to persons who apply the gas.

d. Bleaching powder

• Bleaching powder or calcium hypochlorite is a effective fish toxicant.
• It is used in eradicating the unwanted fishes from the-fish pond.
• It is easily available and less costly.
• Kills fish @ 25-30 ppm within 3 - 4 hours
• Dead fish start floating to the surface of the water.
• Applied @ 350 kg/ha, commercial bleaching powder (30% chlorine).
• Combination of urea @ 100 kg/ha + 175 kg/ha can also be used. 
• The toxicity lasts for about 7-8 days in the pond.
• The additional advantage of bleaching powder is its disinfecting and oxidizing effect on the decomposing matter at the bottom of the pond.

Recommended doses of fish poisons