The biological control of nuisance macrophytes and algae is potentially
the most effective method in terms of costs and long-term control.
our knowledge of how to use biological controls is not well developed.
Biological controls are employed to obtain an acceptable level of plant
biomass through the introduction of species that graze or parasitize specific
plants, or by the manipulation or elimination of endemic animal species that
The use of biological controls
directly or indirectly control plant
can produce a slow, gradual response that may be long-lasting, in contrast to
This part describes biological control techniques that are currently
available or are undergoing testing and provides general guidelines for their
Emphasis is placed on phytophagous fish and insects and upon the control
of nuisance algae through manipulation 'of fish populations.
Portions of these
topics have been reviewed by Schuytema (1977) and Cooke et al.
The grass carp (white amur, Ctenopharyngodon
Val.) was introduced
to the United States in 1963.
Earlier, it was exported to Europe, Japan, and
Mexico, primarily as a food fish and later as a controller of macrophytes.
The fish's native habitat is in eastern China.
animal has some character-
istics which make it desirable as a biological control agent for aquatic
plants, including tolerance to low dissolved oxygen and to a wide range of
temperatures (Opuszynski 1972).
In other ways, this fish is far from ideal,
since its tolerances are so high that it cannot be restricted in range and it
is not easily controlled or eliminated if it escapes to a nontarget lake or
The following paragraphs discuss the biology of grass carp.
on their reproduction (including the development of sterile triploids), feed-
ing preferences, stocking rates, effectiveness, costs, and associated problems
Because grass carp have the potential to drastically alter the
of a reservoir, and in some states they cannot be used at all, managers