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Key words: Fish predation, backwater invertebrates, JUchocheilus Lucius, Green River, <br />benthos, plankton <br />INTRODUCTION <br />The role of biotic interactions in structuring natural communities has been well studied <br />(Connell 1983, Schoener 1983, Sih et at. 1985). Investigations of freshwater lentic habitats, <br />especially pelagic, have revealed that predator induced trophic cascades are often a driving <br />force in the communities (Carpenter et at. 1987, Johannes et at. 1989, McQueen et at. 1989, <br />McQueen 1990, Shapiro and Wright 1984). However, studies of freshwater lotic communities <br />have had mixed results. Lotic communities with coarse substrates have failed to show <br />significant invertebrate density effects from vertebrate predation (Allan 1982, Reice 1983, <br />Flecker and Allan 1984, Culp 1986, Reice and Edwards 1986), while invertebrates in pools <br />and soft substrates of small streams have responded to fish predation (Wilzbach et at. 1986, <br />Schlosser and Ebel 1989, Angermeier 1985, Gilliam et at. 1989). However, few <br />investigations have tested predatory effects of fishes on invertebrate communities of large <br />rivers. In this study we examined the role of fish predation on the structure of invertebrate <br />communities of the Green River, a large river in eastern Utah. <br />Backwaters are defined as shallow embayments adjacent to the main river channel with no <br />measurable water velocity (Tyus and Haines 1991). At least eight native, and fifteen <br />non-native fish species occupy backwaters of the Green River from mid-summer through fall <br />(Haines and Tyus 1990). One of the native fish species, the Colorado squawfish <br />(Ptvchocheilus luciu , uses backwaters as a nursery area (Haines and Tyus 1990). The most <br />1