Dispersal Patterns of Subadult and Adult Colorado Squawfish in the Upper Colorado River

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 lowermost 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