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I I I I I I I . I I I, I I I 1\ I I I I INTRODUCTION The study had three objectives: (1) to determine if juvenile razorback sucker and Colorado pikeminnow occur in the inflow areas, and if found, PIT tag them; (2) examine the effectiveness of various collection techniques on juvenile native fish; and (3) examine the fish communities found in these temporary habitats. If reservoir PIT -tagged fish were recovered by other monitoring programs, such data it would help validate reservoir recruitment. The National Park Service's (NPS) initial research request was expanded to examine both native and nonnative species, and a reservoir component (control) was added to allow comparisons of fish communities between the reservoir and interface zone. We wanted to describe which fish migrated with and benefitted from these reservoir/river interface zones (hereafter referred to as "inflow areas"). Background Inflow habitat Storage of spring runoff in Lake Powell causes the reservoir levels to expand. Shorelines move up side canyons and spread across broader flood plains, but the most pronounced flooding occurs in the tributary arms. Here, increases in reservoir elevation of 15 m can cause the inflow area to advance upstream 45 lan, inundating thousands of hectares of flood plain. Typically these habitats remain flooded for >4 weeks, however, by late summer they drain as reservoir water is released to accommodate downstream water demands, provide hydroelectric power, and create additional flood storage for spring runoff. Some researchers have speculated these habitats may serve as nurseries for young native fishes that drift into the reservoir. If survival was occurring and fish repatriate upstream habitats, reservoir recruitment could help explain the recent capture of adult razorback sucker and Colorado pikeminnow in the lower reaches of the San Juan and Colorado rivers. It's well documented that river deltas are extremely productive habitats (Wright 1950). Nutrient- laden river waters mix with warmer reservoir waters to stimulate algal, zooplankton, and benthic production which supports fish. These shallow, warm habitats are ideal nurseries for both pelagic and lacustrine species, producing fish densities> 125,000 fish/hectare (Mueller and Horn 1999). Some of the last wild razorback suckers collected for brood stock in the upper basin came from these areas (Platania et al. 1991, USFWS 1998). About 2 dozen wild razorback suckers were reported taken from the San Juan arm of Lake Powell (Platania et al. 1991, Wayne Gustaveson, Utah Division of Wildlife Resources [UDWR] personal communique) and less than a dozen from the Colorado and Dirty Devil river confluences in the late 1980s (Persons and Bulkley 1980, Persons et al. 1982, Valdez 1990, Lashmett 1993, 1994, 1995). 1