Freshwater aquaculture

Unit 3 - Nursery, Rearing and grow-out ponds

3.2.4. Eradication of Predatory and Weed fish

• Predatory fish severely affect survival of fish primarily in nursery and rearing ponds.
• Weed fish compete with stocked fish for food, space and oxygen and result in reduction in production of desirable fish.
• Common predatory fish are murrels (Snakeheads), Catfishes such as Wallago attu, Clarias batrachus, Heteropneustis fossilis, Ompak sp. Etc.,

• Weed fish include Puntius, Barbas Danio, Aplocheilus, Anabas etc.,

• Most predatory and weed fish breed prior to the onset of carp breeding
• They infest the ponds before carp fry and fingerlings are stocked
• Hence their eradication prior to stocking of carps is necessary
• Dewatering followed by sun drying is most effective way to control weed and predatory fish.
• In ponds which cannot be dewatered, piscicides are used.
  

A suitable piscicides should have the following characteristics

• Effective at low dose
• Not injurious to people and animals
• Doesn’t make fish unsuitable for human consumption
• Gets detoxified quickly
• Easily available and economical

Types of piscicides
Following are the three types of piscicides that can be used to eradicate weed fish and predatory fish

• Plant origin
• Chemicals
• Pesticides (Chlorinate hydro carbons and organophosphates)

Piscicides of plant origin

Derris root powder

• Rotenone is the active ingredient
• It is a contact poison
• Lethal to other organisms also such as zooplankton, Benthos and insects
• Dosage is 4-20 ppm (mg/l)
• The powder is mixed thoroughly with water and sprayed all over the pond
• It is effective only on sunny days when the temperature is above 250C
• It is less effective in cold waters.

Mahua oilcake

• The active ingredient is saponin
• It causes lysis of the RBC and kills fish, frogs, snakes and turtles
• Dosage is 250 ppm
• The cake is soaked in water for 2-3 hrs and applied all over the pond.
• Detoxification takes about 25 days
• The toxicity can be reduced to 10 days through aeration and application of oxidizing agents.

The other less widely use fish toxicants of plant origin are

• Tea seed cake – 60 ppm
• Tamarind seed husk – 50-100 ppm
• Jaggery – 1%