are urged to consult with the appropriate state agency before stocking this
fish.
Reproduction
A significant question about the use of grass carp in reservoirs, where
escape to downstream ecosystems could easily occur, is whether they will
infest or reproduce in sites where vegetation is desirable.
A marsh or lake
that is important for nesting or migrating waterfowl would be an example
It was originally believed by early importers of grass
this kind of habitat.
carp that the stringent conditions for reproduction would not be found in the
United States.
Spawning occurs in the deep channels of large rivers following
a sharp rise in water level, temperature above 17" C, and a current velocity
-1
above 0.6 to 0.8
Survival of fry depends upon a downstream quiescent
area where plankton are abundant and predation low (Smith and
1983,
Pauley and Thomas 1987). However, direct evidence of reproduction in Arkansas
and Louisiana has been reported
Gallagher, and Chatry
leading
investigators to search for a means of obtaining sterile fish which are as
effective in consuming vegetation as the fertile, diploid (both members of
each pair of chromosomes in each cell) fish originally introduced to the
Nation's waters.
Early attempts to eliminate the possibility of reproduction involved the
use of
populations and surgically sterilized animals.
Shortcomings of
this approach included the possibility of an unwanted introduction of animals
of the opposite sex and the regeneration of gonads,
A second approach
involved the intergeneric cross of female grass carp and male
carp
to
sterile, hybrid offspring. Two major
problems with the
approach were the production of some
but fertile diploids and a comparatively low feeding efficiency (Allen and
Wattendorf 1987; Bonar, Thomas, and Pauley 1987).
A solution to the problems of the sterile hybrid involved the production
of pure (unhybridized) triploid (three members of each chromosome in cells)
grass carp.
Hydrostatic pressure or high temperature techniques are used to
produce nearly loo-percent triploids (Cassani and
1986).
Since no
procedure can produce loo-percent triploidy consistently, and because the
diploids and triploids cannot be accurately separated by looking at them, fish
producers needed a technique to verify that every fish sold is triploid.
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