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<br />Reprinted from THE SOUTHWESTERN NATURALIST
<br />Vol. 38, No.4, December 1993
<br />Made in United States of America
<br />
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<br />PREDATION ON FISH LARVAE BY ADULT RED SHINER,
<br />YAMPA AND GREEN RIVERS, COLORADO
<br />
<br />....
<br />
<br />JACK B. RUPPERT, ROBERT T. MUTH, AND THOMAS P. NESLER
<br />
<br />Larval Fish Laboratory, Department oj Fishery and Wildlife Biology,
<br />Colorado State University,
<br />Fort Collins, CO 80523 (fBR, RTM)
<br />Fish Research Section, Colorado Division oj Wildlife, Fort Collins, CO 80526 (TPN)
<br />
<br />Minckley (1991) hypothesized that introduced
<br />nonnative fishes, enhanced by river alterations,
<br />are primarily responsible for the demise of native
<br />fish species in the Colorado River system. Neg-
<br />ative interactions with nonnative competitors and
<br />predators may impact all life stages of virtually
<br />all native fishes of the system. Predation by non-
<br />native fishes on young Colorado squawfish
<br />(Ptychocheilus lucius) and razorback sucker (Xy-
<br />rauchen texanus) is hypothesized as one reason
<br />for the decline of these two endangered species
<br />endemic to larger rivers of the Colorado River
<br />Basin (Kaeding and Osmundson, 1988; Marsh
<br />and Langhorst, 1988; Lanigan and Tyus, 1989;
<br />Marsh and Minckley, 1989). Early life stages of
<br />Colorado squawfish, razorback sucker, and other
<br />sympatric native fishes use low-velocity habitats
<br />(e.g., backwaters and shoreline embayments) as
<br />nursery areas. Fish assemblages in these habitats
<br />are now usually dominated by nonnative cypri-
<br />nids, especially red shiner (Cyprinella lutrensis),
<br />and often include introduced centrarchids and ic-
<br />talurids (Tyus et al., 1982; Haines and Tyus,
<br />1990; Karp and Tyus, 1990). Previous studies on
<br />diets of fishes collected during daylight in summer
<br />and fall from natural backwaters of the Colorado,
<br />Green, and Yampa rivers in Colorado or Utah
<br />(areas where wild populations of Colorado
<br />squaw fish and razorback sucker occur) revealed
<br />little evidence of predation on fish larvae by non-
<br />natives and failed to demonstrate that nonnatives
<br />ate larvae of native fish species (McAda and Tyus,
<br />1984; R. T. Muth and T. P. Nesler, pers. obser.).
<br />The objective of this study was to document pre-
<br />dation on fish larvae, especially larvae of native
<br />fishes, by adult nonnative cyprinids or juvenile
<br />centrarchids or ictalurids collected from ephem-
<br />eral shoreline embayments near the confluence of
<br />the Green and Yampa rivers in early summer.
<br />Unlike the previous diet studies, we sampled on
<br />a diel basis and took specific measures to mini-
<br />mize regurgitation when collected fish were killed
<br />and preserved.
<br />
<br />Sampling was conducted in the lower 0.2 km
<br />of the Yampa River and 0.5 km of the Green
<br />River immediately below their confluence in Di-
<br />nosaur National Monument, Colorado. This lo-
<br />cation is downstream of known Colorado squaw-
<br />fish and razorback sucker spawning sites in the
<br />lower Yampa River (Tyus and Karp, 1989). Oth-
<br />er fishes commonly occurring in this area include
<br />native round tail chub (Gila robusta), speckled dace
<br />(Rhinichthys osculus), blue head sucker (Catosto-
<br />mus discobolus), and flannelmouth sucker (c. la-
<br />tiPinnis), and nonnative red shiner, common carp
<br />(Cyprinus carpio), sand shiner (Notropis stram i-
<br />neus), fathead minnow (Pimephales promelas),
<br />redside shiner (Richardson ius balteatus), and
<br />channel catfish (/ctalurus punctatus). Embay-
<br />ments sampled were 10-30 m2 in surface area,
<br />0.2-1.0 m deep, had negligible water velocities"
<br />and had primarily sand, silt-sand, or silt-mud
<br />substrates.
<br />Collections with fine-mesh seines (1.6-mm-
<br />square mesh) were made on 30 June and 2, 14.,
<br />19, and 25 July 1991. Sampling on 30 June was
<br />at 1600-1800 h, and on other dates at 0000-0200,
<br />0400-0600, 1200-1400, 1600-1800, and 2000-
<br />2200 h. Three to six embayments in both rivers
<br />were sampled. Adults or juveniles of nonnative
<br />fishes were killed immediately after capture with
<br />an overdose of tricaine methane sulfonate (MS-
<br />222) to minimize regurgitation, sorted by species,
<br />and fixed in 10% formalin for 24-36 h. Fish were
<br />preserved in 3% phosphate-buffered formalin af-
<br />ter fixation. Presence of fish larvae in each em-
<br />bayment was noted at time of sampling, and those
<br />collected incidentally were fixed, preserved, and
<br />identified.
<br />Nonnatives collected and included in our anal-
<br />ysis were yearling channel catfish and adult red
<br />shiner, sand shiner, fathead minnow, and redside
<br />shiner. All channel catfish and selected larger
<br />cyprinid specimens from each collection were ex-
<br />amined. Each fish was measured (total length,
<br />TL), and its entire digestive tract was removed,
<br />
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