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I?% 0"P&LW 6u >* o-1 Transactions of the American Fisheries Society 127:943-956, 1998 © Copyright by the American Fisheries Society 1998 Dispersal Patterns of Subadult and Adult Colorado Squawfish in the Upper Colorado River D. B. OSMUNDSON* U.S. Fish and Wildlife Service, Colorado River Fishery Project 764 Horizon Drive, South Annex A, Grand Junction, Colorado 81506-3946, USA R. J. RYEL Utah State University, Department of Rangeland Resources, Logan, Utah 84322-5230, USA M. E. TUCKER' AND B. D. BURDICK U.S. Fish and Wildlife Service, Colorado River Fishery Project 49 l tom. W. R. ELMBLAD Colorado Division of Wildlife, 711 Independent Avenue, Grand Junction, Colorado 81505, USA T. E. CHART Utah Division of Wildlife Resources, 1165 South Highway 191, Suite 4, Moab, Utah 84532, USA Abstract.-We investigated distribution and dispersal patterns of subadult and adult Colorado squawfish Ptychocheilus lucius (recently renamed the Colorado pikeminnow) throughout their range in the upper Colorado River. Annual, river-wide, capture-recapture data were used to document movements during a 5-year period (1991-1995). Average total length of Colorado squawfish progressively increased upstream: juveniles and subadults occurred almost exclusively in the low- ermost 105 km of the 298-km study area, whereas most adults were concentrated in the uppermost 98 km. This was most pronounced early in the study and less so later due to the effect of two or three strong year-classes that dispersed through the system. Only 16% of subadult and adult fish initially captured and tagged in the upper reach were later located more than 10 km from the previous capture site; of those tagged in the lower reach, 58% were later located more than 10 km from the previous site. Most movements greater than 10 km were directed upstream, and many fish tagged in the lower reach moved to the upper reach; the smallest of these fish was between 421 and 449 mm in total length (TL) when it moved. No movement was detected from the upper reach to the lower. Distance moved was inversely related to fish size: displacement of fish shorter than 550 mm TL averaged 33.6 km; for those longer than 550 mm, average displacement was only 7.5 km. Movement of young adults may have been a response to changing food needs. Upstream movements placed fish into areas with greater availability of larger prey, and body condition of large adults during spring was significantly higher in the upper reach than in the lower reach. Water temperatures, however, were inversely related to adult distribution despite a preference for warmer water. We suggest that portions of the upper reach offer adults the best balance between food and water temperature. Introduction Understanding the life history of the endangered Colorado squawfish Ptychocheilus lucius (recently renamed the Colorado pikeminnow; Nelson et al. 1998) is prerequisite to developing strategies for its recovery. This warmwater piscivore, endemic to the Colorado River system and the largest cyp- rinid native to North America, has integrated * Corresponding author: doug-osmundson@fws.gov 1 Present address: U.S. Fish and Wildlife Service, Northern Central Valley Fish and Wildlife Office, 10950 Tyler Road, Red Bluff, California 96080, USA. movements into its life strategies. Annual, pota- modromous, spawning migrations (e.g., Tyus and McAda 1984; Tyus 1985, 1990; McAda and Kaed- ing 1991; Ryden and Ahlm 1996), and downstream drift of emerging larvae (e.g., Haynes et al. 1984; Nesler et al. 1988; Tyus 1990; Tyus and Haines 1991) have been well documented. Longitudinal differences in fish size within the Colorado (Valdez et al. 1982; Osmundson and Burnham 1998, this issue) and Green (Tyus 1986) rivers suggest that movements may serve purposes or functions in addition to those associated with reproduction. Early life stages are found primarily in lower reaches, whereas adults are found pri- 943