Finfish Breeding and Hatchery Management

Unit 5 - Fish seed rearing techniques

5.1.3. Aquatic insects and their control

• Nursery ponds are invariably populated with a large number of aquatic insects which either in their larval and/or adult stages, in addition to preying directly on carp spawn and fry, also compete with the latter for food.
• Of the eleven orders of class Insecta, the members of the Orders Coleoptera, Hemipitera and Odonata are more important.
• Among members of the Order Coleoptera, those belonging to the families Dytiscidae, Hydrophilidae and Gyrinidae prey upon fish spawn.
• Dytiscicids, known as predacious diving beetles, are carnivorous both in adult and larval stages.
• They commonly attack small fish, tadpoles and even dragonfly nymphs.
• The larvae of Cybister were found to devour 20 advanced fry (24-40mm) within 24 hours and can kill even 10-13 mm carp fingerlings.
• Among members of the order Hemiptera, the backswimmers, giant water bugs and water scorpions cause a heavy mortality of carp spawn.
• The back swimmers are highly dangerous, attacking carp spawn, small fish, tadpoles and even small frogs.
• Backswimmers are reported to kill even 10-13mm long carp fry, in addition to preying upon plankton, mosquito and chironomid larvae and mayfly nymphs.
• Water bugs are highly predaceous and feed on frogs and fishes.
• Water scorpion and water stick insect attack carp fry and feed on them.
• Dragonfly (Order: Odonata) nymphs are predatory and feed on carp spawn. They (15 mm long) can swallow 7 carp spawn (6-7 mm) within 3 hours.
• The short and long bodied drangonfly nymphs consumed 133 and 125 carp spawn, respectively, in 24 hours.
• The source of predatory aquatic insects is : they multiply rapidly in a nursery pond and fly from pond to pond.
• To ensure better survival of carp spawn, it is important to effectively clear the pond of its insect population, prior to stocking.
• Repeated dragging of pond with a fine-meshed net can remove most of the insects.
  For effective control of insects, insecticides are recommended.
• Since the common insecticides affect adversely the zooplankton and fish spawn, it is recommended to use only selective poisons which are capable of killing only the insects, but not the spawn and fish food organisms.
• Spraying diesel or kerosene on the surface of pond is a well-known and a routine practice in malaria control.
• The CIFRI’s Sub-station, Orissa, developed a method for control of predatory aquatic insects, which consists of spraying, on a non-windy day, an emulsion of oil and soap in the ratio of 56:18 kg/ha.
• The recommended dose of teepol B-300 which is readily soluble in water and easy to mix with oil, is 560 ml, emulsified with 56 kg of mustard oil.
• The Department of Fisheries of Maharashtra Government prescribed an emulsion prepared by mixing light speed diesel (1 l), the emulsifier hyoxide 1011 (0.75 ml) and water (40 ml) for every 200 m² water surface.