Currently, the best technique is to use a
Counter to examine a
Triploid red blood cells are larger than
blood taken from each fish.
and the Counter easily verifies cell size.
Three workers can examine
Triploids are apparently functionally sterile,
2,000 to 3,000 fish per day.
and there is an extremely low probability that triploids could be a source of
a large population of reproducing diploids (Allen and Wattendorf 1987; Allen,
The production and verification of loo-percent sterile fish has prompted
several more states to allow their introduction.
As of September 1987,
18 states prohibit grass carp, 12 have no constraints on the use of fertile
diploids, 16 allow only triploids, and'4 are studying the triploid prior to
release (Allen and Wattendorf 1987).
Grass carp (diploid and triploid) are voracious consumers of aquatic
plants but exhibit distinct feeding preferences that seem to vary from region
Table 14 is a summary of these preferences.
to region in the United States.
The data for Florida are extensive, reflecting the longer history of the spe-
cies' introduction and the number of investigators.
The regional differences in grass carp food preferences could have
is a preferred species in
important management implications.
Florida but is not preferred or may not be eaten when plants grown in
is a preferred plant
Washington are offered to fish.
in Florida, variably eaten in Oregon-Washington trials, but not eaten by
Feeding trials in Illinois were with diploid fish.
Illinois grass carp.
question remains whether palatability of plants varies from region to region,
whether there is some genetic basis to carp feeding behavior or whether fur-
ther studies will demonstrate that these apparent geographical differences are
It appears that when preferred plants
produced by the design of experiments.
many species of Potamogeton,
or native Elodea are
present, and stocking rates are moderate to low, nonpreferred or
nuisance species that are also present could predominate after several years
of grass carp feeding.
It should be noted that major nuisance exotic species, including
hyacinth, alligatorweed, and Eurasian watermilfoil, are not eaten or may be
Much additional research is needed with regard to grass
feeding preferences and their management implications.