7025

• UCREFRP Catalog Number: 7025
• Author: Hawkins, J. A. and T. P. Nesler.
• Title: Nonnative Fishes in the Upper Colorado River Basin
• USFW Year: 1991.
• USFW - Doc Type: An Issue Paper.
• Copyright Material: YES

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Populations of nonnative fishes can become established from either intentional or
accidental releases. Taylor et al. (1984) listed four reasons for intentional introductions of
nonnative fishes:
(1)
(2)
(3)
(4) to release unwanted pet [or bait] fish.
Accidental releases usually occur from private ponds, stock tanks, and culture facilities as
a result of flooding or through improperly designed drainage pipes [or ditches]. Successful
establishment of an introduced fish and the affect it will have in a novel environment
depends on the physiological, behavioral, and ecological properties of the species, and the
physical and biological properties of the ecosystem receiving the introduction. Taylor et al.
(1984) also identified that many introduced species persist because they:
(1) are very hardy, survive transport, and thrive in disturbed environments;
(2) are very aggressive and eliminate natives through competition or predation;
(3) are ecologically or behaviorally distinct from natives, and natives do not
interact with them or are unable to deal with a new style of predation or
competition;
have reproductive strategies that seem to confer on them an unusual degree
of "fitness";
are preadapted to distinctive local environmental conditions; or
possess a combination of several of these characteristics.

to add new food, game, or forage species to the native assemblage;
for biological control of unwanted plant or animal pests;
for aesthetic or ornamental reasons; or

(4)
(5)
(6)

According to Pimm (1987) there are two important questions regarding effects of
introduced species. First, what determines if an introduction will be successful, and second,
what determines if a successful introduction will have an impact on the community it invades.
Impacts of nonnative fishes on a native fish are measured by identifying effects relative to
either the individual or the population. A nonnative may create stress on an individual and
cause a change in behavior or even death, eventually affecting population health. The
impact that an introduced species will have on a native species is determined by examining
the interactions between the two. Taylor et al. (1984) stated;

The impact of an exotic fish species on a freshwater community is defined as
any effect attributable to that exotic that causes - directly or indirectly-
changes in the density, distribution, growth characteristics, condition, or
behavior of one or more native populations within that community. The
definition is independent of human judgments as to the 'value" of an
introduction; impacts are nothing more than the changes in native populations
brought about by the activities of an exotic species. A demonstration of
impact implies verification of a causal relationship between changes in a native
population and the presence of an exotic.