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<br />C) <br />(:-) <br />...... <br />(.O <br />.- <br />C) <br /> <br />NONNATIVE FISH INTERACTIONS <br /> <br />Introduction <br />Potential interactions among native and nonnative fishes include predation, competition, and hybridization, <br />Interactions with nonnative fishes were implicated in the decline of razorback sucker and Colorado <br />pikeminnow for many years (Holden and Stalnaker 1975, Minckley et al. 1991, Tyus 1991), and these <br />interactions were identified as the primary cause for lack of recruitment of razorback sucker in the lower <br />Colorado River (Minckley et al. 1991). Nonnative fish studies were part of the core studies conducted <br />during the 7 -year research period, and all aspects of potential interactions were studied (Brooks et al, <br />2000). Additional information was collected by several other studies, including nursery habitat studies <br />(Archer and Crowl 2000a) and secondary channel studies (Propst and Hobbs 2000), <br /> <br />Predation <br />Numerous authors hypothesized that predation by introduced nonnative fish species is one of the factors <br />responsible for the decline of native fishes in the Colorado River Basin (Kaeding and Osmundson 1988, <br />Marsh and Langhorst 1988, Minckley et al. 199 I), Various nonnative species were documented as <br />predators on native fishes, including channel catfish (Marsh and Brooks 1989), mosquitofish (Gambusia <br />affinis) (Meffe 1985), and red shiner (Ruppert et al. 1993). Although nonnative fish species frequently <br />dominate fish assemblages in many Colorado River Basin rivers and streams, including the San Juan River <br />(Propst and Hobbes 2000, Brandenburg and Gido 1999), the degree of predation exhibited by a particular <br />species can vary dramatically between different locales. Marsh and Brooks (1989) reported high predation <br />by two species of catfish on hatchery-reared razorback sucker stocked in the Gila River, Arizona, with up <br />to 55% of the channel catfish stomachs examined containing razorback sucker and up to 90% offlathead <br />catfish (Pylodictus olivaris) stomachs containing razorback sucker. However, Tyus and Nikirk (1990) <br />found fish remains in only 7% of the channel catfish stomachs examined from the Green and Yampa rivers, <br />Colorado and Utah, A number of factors can impact predator/prey interactions, including the relative <br />density of predators and prey (Wooten 1990), the availability of alternative food items, and feeding habits <br />of the predator. Sampling time can also affect prey determinations, since digestion can be rapid for many <br />predators. Hence, if predators feed primarily at night, and sampling occurs late in the day, predator <br />stomachs may be nearly empty even though they fed the night before, <br /> <br />Native fishes (primarily flanneImouth sucker) comprised 75.4% of all fishes collected during <br />adult monitoring efforts in primary channels from 199 I to 1997, However, nonnative fishes dominated <br />the species assemblage: 19 of the 26 species collected were nonnative (Ryden 2000a), The most- <br />abundant nonnative fish known to exhibit piscivory, the channel catfish, comprised 13.1% of all <br />fish collected, Channel catfish are abundant throughout the river below RM 166.6, where the PNM <br />Weir appears to inhibit upstream movement (Ryden 2000a). Common carp eat eggs of other fishes, <br />and they made up 9.2% of total collections. Red shiner comprised 1.7% of total fishes collected, <br />but because of their small body size, they are not as susceptible to electrofishing and were probably <br />under represented in the primary channel collections (Ryden 2000a). Red shiner were often the most- <br />abundant fish in secondary channels in late sununer and fall (Propst and Hobbes 2000), and <br /> <br />September 2000 <br /> <br />3-44 <br /> <br />Program Evaluation Report <br />