Exotic Aquatic Organisms - International Development Research ...

Salvinia molesta, a floating fern of South American origin, is another obnoxious weed
infesting the Indian fishery waters. It was earlier noticed only in the backwaters of Kerala, but is
now slowly spreading to other areas. Though it has been demonstrated that this fern could be
controlled easily by spraying aqueous (1-2%) ammonia, the technique is not used widely because
of certain inherent limitations. However, recent investigations have indicated the possibility of
complete elimination of this weed using the insect Cyrtobagous salviniae (Jayanth 1987; Joy et
al. 1985). Pistia stratiodes and Ipomea carnia, both of south American origin, are the two other
obnoxious exotic weeds. Even though the present extent of infestation is much less compared to
water hyacinth and Saivinia, there is need to contain it before they too pose a serious threat to
fishery waters. As it is, 2-4 D has been found to be effective in controlling these weeds
(Ramachandran and Prabhu 1988).
Conclusions and Recommendations
The deliberations of the Indian Branch of the Asian Fisheries Society at its Workshop on
Exotics, April 1988, form the main basis for the conclusions and recommendations listed below.
The Chinese carps have helped in increasing fish production to 10 t/halyear from
composite fish culture, involving three Indian major carps and three exotic carps. However, as
compared to the Indian carps, the consumer preference is rather low for the exotic carps,
particularly the silver carp, because of its poor keeping quality. It is now known that silver carp
competes with catla for food to some extent.
The Chinese grass carp has proved most useful and efficient in the biological control of
submerged weeds. It has also proved to be a good table fish. Under Indian conditions, it grows
fast and can easily be maintained on any of the submerged aquatic weeds, terrestrial grasses and
leaves of leguminous plants.
The accidental introduction of silver carp into a few reservoirs has led to a sharp decline
in the catla population, as well as total fish production.
As the Prussian strain of common carp is the only species of exotic carp that can do well
in upland waters, more emphasis is required to be given for its propagation in upland waters.
In water bodies where tilapia has already gained a firm foothold, it should be possible to
reduce its numerical dominance by continuous stocking of these water bodies with major carps.
Some other species of tilapia are known to have better cultivable qualities than Oreochromis
mossambicus. It may be desirable to consider introduction of some species into India purely for
experimental purposes, to assess their suitability for culture under Indian conditions either by
themselves or through the production of hybrids.
There does not appear to be a need to introduce any new salmonids. Instead, good
hatchery facilities should be developed for the existing species and more attention given to their
genetic improvement, and prevent degradation of the ecology of trout streams because of
pollution.
Instead of introducing a number of species for different purposes, improvement of the
genetic quality of the indigenous major carps should be given more attention.
It is also necessary to regulate the introduction of exotic species into neighbouring
countries, such as Nepal, Bhutan, Pakistan and Bangladesh through existing regional
organizations, since such introduced species could easily cross over into Indian territory.
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