Exotic Aquatic Organisms - International Development Research ...

Grass carp
The grass carp, Ctenopharyngodon idella, was introduced mainly for purposes of biological
control of submerged aquatic vegetation in fishery waters, in particular Hydrilla, Vallisneria and
smaller floating weeds like Lemna sp. However, because of its fast growth rate, it was very soon
included as a component in composite fish culture. It does not usually breed naturally even after
hormone injection and therefore has to be stripped. It feeds on aquatic weeds and more
voraciously on soft aquatic vegetation like Hydrilla, but does not feed on Eichhornia, Pistia and
Salvinia (Singh et al. 1967) and grasses and leaves of leguniinious plants. The grass carp has
been widely used in the country for weed control in canals (Kulshreshthra and Sharma 1976;
Singit, pers. comm.), water cooling tanks (Chaudhuri et al. 1976) and ponds (Keshavanath and
Basavaraju 1980). In composite culture systems, the grass carp lends an added advantage in that
it consumes vegetation more than its own body weight and voids most of it in semi-digested
form, which serves as food for omnivorous fishes and also as pond fertilizer. The introduction of
grass carp, therefore, has proved beneficial.
Tawes
The Tawes, Puntius javanicus, is known to be macrophagous, and is cultured in only some
parts of the country. It breeds naturally and attains a weight of over one kg in a year. However,
the species is not popular in culture practices in India.
Tench
The Tench (Tinca tinca) introduced into Nilgiris in 1874 is established well in the Nilgiris
region and individuals weighing over 1 kg can be caught from Ooty Lake. It matures in the 2nd
or 3rd year and breeds under pond conditions. However, recent reports indicate the
disappearance of the tench from Nilgiris waters (Sreenivasan et al. 1988).
Bighead carp
Bighead carp (Aristichthys nobilis) were accidentally introduced into India (Das 1988),
where they remained confined to the fish farm of Central Inland Fisheries Research Institute at
Cuttack and later disappeared altogether. They have reportedly reappeared in the country in
1987, presumably through private trade from Bangladesh. Jhingran (1985) has discussed in detail
the likely consequence of the introduction of bighead to Indian waters. Its feeding habits are
similar to that of catla and is likely to pose a threat to catla. The suitability of bighead is yet to be
assessed in terms of consumer preference, growth rates under Indian conditions and disease
resistance.
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