Finfish Breeding and Hatchery Management

Unit 5 - Fish seed rearing techniques

5.1.1. Control of predatory and weed fishes

• Predatory fishes are harmful to cultured carps.
• They not only compete with fish for food and space, but also directly prey on them.
• The term `weed fish’ is used to include all species of uneconomic, small-sized fish that naturally occur or are accidentally introduced in ponds along with carp spawn
• Weed fishes have high fecundity and mature in summer months and even breed in captivity, in the absence of rains
• Young weed fishes directly feed on carp hatchlings and take a heavy toll thereof as soon as they are released in nursery pond
• The common method of removing unwanted fishes from nursery pond is by repeated drag netting. However, certain bottom dwelling fishes like murrels catfishes and climbing perch burrow themselves in mud and are difficult to be caught by repeated dragging.
  
• The position worsens in ponds which cannot be drained completely. In such a situation the nursery should be poisoned.

Fish toxicants

• Several kinds of chemicals are used in nurseries for controlling undesirable fishes. They are (i) plant derivatives, (ii) chlorinated hydrocarbons and (iii) organo-phosphates.
• Of these, the chlorinated hydrocarbons are perhaps the most toxic to fish. Organophosphates are generally less toxic to fish. Plant derivatives are least toxic.
  

(i) Plant derivatives

• Derris powder, with 5% rotenone content, is probably the most commonly used fish poison in fish ponds.
  
• At doses 4 to 20 ppm, most of the fishes, tadpoles and bottom dwelling organisms are killed.
  
• Even zooplankton and aquatic insects are destroyed to a great extent. The toxicity of the chemical lasts up to 4-12 days.
  
• The seed powder of Barringtonia acutangula kills a wide variety of fish at 20 ppm, with toxic effect lasting for 48 hr.
  
• Tea seed cake (75-100 ppm) can be used both as a piscicide and a fertilizer. It can kill fish, tadpoles and insects.
  
• Mahua oil cake (cake obtained after extracting oil from the seed of the plant, Madhuka latifolia) is widely used to control predatory and weed fishes in Karnataka.
  
• Mahua oil cake (toxic component : saponin, 4-6%) is used as a fish poison at 200-250 ppm, killing fish within 8 hours and the toxicity lasts up to 96 h.
  Later, it acts as a pond fertilizer.
  
• Experiments conducted at Central Institute of Freshwater Aquaculture, Bhubaneshwar, Orissa, on the use of bleaching powder (calcium hypochlorite) as a fish toxicant in culture ponds has shown excellent results.
  
• Bleaching powder with 30% chlorine content, when applied at 25-30 ppm, kills all varieties of fishes, including catfishes, murrels, weed fishes and carps.
  
• The cost of treatment at 25-30ppm is economical as it has the advantage of ready availability at low cost.

(ii) Chlorinated hydrocarbons

• Aldrin: Aldrin at 0.2 ppm kills weed fishes and predatory fishes, while 0.3 ppm aldrin spares zooplankton
  
• Dieldrin : Dieldrin at 0.01 ppm effectively kills fishes and 0.5 ppm it kills prawns and insects.
• Endrin: It is the most poisonous chemical so far tried in India for eradication of fishes in culture ponds. Endrin at 0.001 ppm effectively kills all fishes, prawns and notonectids.

(iii) Organophosphates

• In India, three organophosphates, viz., thiometon, DDVP and phosphamidon have been found effective in killing fish.
  
• DDVP at 3-30 ppm kills most of the unwanted fishes and is recommended for control of trash fishes in nursery ponds.