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y DISTRIBUTION AND ABUNDANCE The Colorado River below Glen Canyon Dam has changed from a system of endemic and warmwater species prior to and immediately after closure of the dam to a system dominated by coldwater and eurythermal fishes. Although some native species have persisted through the changing environment, many have been extirpated including Colorado squawfish, bonytail chub, roundtail chub, and, probably, razorback sucker. Most warmwater species present in 1963 have either disappeared or decreased in abundance. Rainbow and brown trout have increased due to stocking and a more favorable environment. Water temperature, clarity, food abundance, fish stocking, tributary locations, and backwater abundance seem to be important factors to fish distribution in the river. Decreased water temperature has been considered one of the primary reasons for the extirpation of native fishes from regulated portions of the Colorado River. Rainbow trout were collected throughout the study area, though catch rates were highest in the tailwater. Stocking of fingerling rainbow trout and the relative abundance of food items may contribute to the abundance of trout in Reach 10. Brown trout abundance in the river seems to have increased since the examination of fish distribution by Carothers et al. (1981). Most of the increase has occurred in reaches 30 and 40 above and below the confluence of Bright Angel Creek. Brook trout distribution declined with distance downriver. Their distribution appears to be controlled more by stocking and dispersal downriver than by natural reproduction. Other introduced species such as common carp and fathead minnow were most abundant in the lower reaches of the river. This may be related to higher water temperature. Adult and larval bluehead sucker and speckled dace were most abundant in the lower reaches. Most adult flannelmouth were collected in Reach 10, with juvenile and larval fish in the lower reaches. The lower portion of the river serves. as an important nursery and rearing area, and as these fish grow they distribute throughout the river. Adult humpback chub, though found in reaches 20-50, were generally captured in the proximity of the Little Colorado -3-