9547

• UCREFRP Catalog Number: 9547
• Author: Tyus, H. M. and J. F. S. III.
• Title: An Evaluation of Recovery Needs for Endangered Fishes in the Upper Colorado River, with Recommendations for Future Recovery Actions - Final Report.
• USFW Year: 1999.
• USFW - Doc Type: Glenwood Springs, CO.
• Copyright Material: NO

temperature regime is at least part of the reason for those movements. Finally, winter
studies show that the native fishes remain active through the coldest months, seeking
different habitat conditions of flow and temperature, and displaying a higher degree of
~ cold tolerance than is characteristic of many warmwater fishes (Wick and Hawkins
1989, Valdez and Masslich 1989).
The n~earch available on the temperature requirements of the endangered fishes does
not lead to unambiguous conclusions about the effects that lower river temperatures
~ have had on fish in the wild. Specifically, it has proven difficult to apply the results of
laboratory studies of temperature preference to fish in the riverine environment. The
natural habitat is complex and the range of temperatures actually available to wild fish
is greater than would be expected on the basis of temperatures recorded in the main
channel (cf., Valdez et al. 1982, Tyus 1991). Behavioral considerations that allow the
wild fish to select from the range of temperatures available in the different habitats in or
~ adjacent to the main channel provide a mechanism for ameliorating the adverse effects
of low temperatures in the main channel. The egg is probably the stage most
vulnerable to lower river temperatures because eggs are deposited at specific locations
in the main channel and have no capacity to seek more favorable temperatures. Thus,
with the possible exception of the egg stage, changes in main channel temperatures
~ may not have had a large effect on habitat that otherwise remains natural.
Biotic
For at least 50 years, scientists have been concerned about the role nonnatives have
~ played in the decline of native fishes. Dill (1944) was one of the first to suggest that
nonnatives were responsible for declines observed in native fish populations in the
lower Colorado River basin. He recognized that the decline began about 1930, and that
it was coincident with a large increase in the abundance of nonnative fishes, especially
channel catfish and largemouth bass. By 1960, populations of the big river fishes had
~ been reduced greatly. Miller (1961) noted "drastic changes" in the fish fauna and
observed that replacement of native fishes by introduced fishes in the lower Colorado
River offered the "most impressive documentation for changing fish fauna" ever
recorded. Schoenherr (1981) considered the evidence "overwhelming° for replacement
of native fishes by aggressive introduced fishes, and he provided examples in which
. predation resulted in extirpation. More recent studies and reviews add to the case for a
decline in the abundance of native fish species as nonnative species have increased in
abundance (Joseph et al. 1977, Osmundson and Kaeding 1989, Quarterone 1993). It
is not unusual now for nonnative fishes to comprise a significant portion (>25%) of
standing stock in most areas, and to comprise up to 90% in backwaters (McAda et al.
1994).
An increasing body of evidence characterizes the negative interactions of nonnative
fishes with the endangered big river fishes (Hawkins and Nesler 1991, Minckley et al.
1991, Maddux et al. 1993, Lentsch et al. 1996a). Evidence in many of the reports is
indirect in the sense that they lacked direct observations or absolute proof of predation
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