8204

• UCREFRP Catalog Number: 8204
• Author: Tyus, H. M.
• Title: Razorback Sucker ‎(Xyrauchen texanus)‎ Recovery Plan.
• USFW Year: 1998.
• USFW - Doc Type: Boulder, Colorado.
• Copyright Material: NO

For more than 50 years, researchers have expressed concerns about the role that
nonnative fishes have played in causing the decline of native fishes in the Colorado
River basin. Dill (1944) was one of the first to suggest that nonnatives were responsible
for the observed declines in native fish populations in the lower basin. He traced the
decline to about 1930 and observed that it was coincident with a large increase in the
abundance of nonnative fishes, especially channel catfish and largemouth bass. By
1960, Miller (1961) noted that the "most impressive documentation for changing fish
fauna" occurred in the lower Colorado River where it was associated with a replacement
by introduced fishes. Schoenherr (1981) believes the evidence was "overwhelming"
that native fishes were being replaced by aggressive, introduced fishes. A decline in
the abundance of native fishes as nonnative species have increased in abundance has
been documented by many workers (e.g., Joseph et al. 1977, Behnke 1980,
Osmondson and Kaeding 1989, Quaterone 1993).
A substantial body of indirect evidence exists for predation by nonnatives on the
razorback sucker. Marsh and Langhorst (1988) reported that larval razorback suckers
in Lake Mojave survived longer and grew larger in the absence of predators.
Loudermilk (1985) observed that young larvae exhibited little defensive behavior in the
presence of potential predators. Johnson et al. (1993) compared predator avoidance of
razorback sucker larvae with that of northern hog sucker (Hypentelium nigricans) and
concluded that "larval razorbacks are not likely to survive in habitats that support high
numbers of nonnative fishes." Smaller nonnative species such as red shiner and
fathead minnow may attack or display agonistic behavior toward razorback sucker
larvae (Karp and Tyus 1990), and young of some of the more aggressive game fish
also are problematic because they are highly agonistic (Sabo et al. 1996).
Several nonnative fishes, including green sunfish, common carp, and flathead and
channel catfish, have been observed feeding on eggs and/or larval razorback suckers
(Medel-Ulmer 1983, Minckley 1983, Brooks 1986, Langhorst 1989, Marsh and
Langhorst 1988, Marsh and Brooks 1989). Karp and Tyus (1990) reported results of
predation experiments in which several nonnative species were offered razorback
sucker larvae in 4-minute trials: green sunfish consumed 90% of the larvae offered; red
shiner, 50%, and redside shiner, 10%. Afield experiment in Lake Mojave provided
indirect evidence of predation by monitoring larvae in habitats with and without
predators. Razorback sucker larvae up to 30 mm long occurred in the predator-free
environment, but larvae exposed to predation did not exceed 10-12 mm, implying that
predators removed the larger larvae (Brooks 1986, Langhorst 1989, Marsh and Brooks
1989). In addition, laboratory studies have also shown that razorback sucker larvae
may face predation by native invertebrate species, such as odonate nymphs, which are
common in backwater areas of Lake Mojave (Horn et al. 1994).
Direct observations, including stomach content analyses, of predation by nonnatives on
razorback suckers have been reported by many investigators (Table 1). The list is
extensive and should leave no doubt that predation by nonnatives is significant. Part of
the difficulty in documenting predation on larvae in early studies is that the rapid
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