Florida’s large number of shallow lakes, warm climate and
long growing season have contributed to the development of
excessive growths of aquatic macrophytes that have seriously
interfered with many water use activities. The introduction
of exotic aquatic macrophyte species such as hydrilla (
Hydrilla
verticillata
) have added significantly to aquatic plant problems
in Florida lakes. The use of grass carp (
Ctenopharyngodon
idella
) can be an effective and economical control for aquatic
vegetation such as hydrilla. Early stocking rates (24 to 74
grass carp per hectare of lake area) resulted in grass carp
consumption rates that vastly exceeded the growth rates of
the aquatic plants and often resulted in the total loss of all
submersed vegetation. This study looked at 38 Florida lakes
that had been stocked with grass carp for 3 to 10 years with
stocking rates ranging from < 1 to 59 grass carp per hectare
of lake and 1 to 207 grass carp per hectare of vegetation to
determine the long term effects of grass carp on aquatic macrophyte
communities. The median PAC (percent area coverage)
value of aquatic macrophytes for the study lakes after
they were stocked with grass carp was 14% and the median
PVI (percent volume infested) value of aquatic macrophytes
was 2%. Only lakes stocked with less than 25 to 30 fish per
hectare of vegetation tended to have higher than median
PAC and PVI values. When grass carp are stocked at levels of
> 25 to 30 fish per hectare of vegetation the complete control
of aquatic vegetation can be achieved, with the exception of
a few species of plants that grass carp have extreme difficulty
consuming. If the management goal for a lake is to control
some of the problem aquatic plants while maintaining a
small population of predominately unpalatable aquatic
plants, grass carp can be stocked at approximately 25 to 30
fish per hectare of vegetation