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MAINCHANNEL REPRODUCTION Variation in water flow has been shown to affect trout spawning and may have been responsible for the temporal shift in reproductive activity observed during this study in Reach 10. Reaches below Lee's Ferry seemed less disrupted by changes in flow. Fry density in 3 of 4 reaches appeared related to proximity of tributary streams. Brown trout population levels in the mainchannel may be controlled by reproduction in and recruitment from tributaries to the Colorado River. It appears brook trout numbers are maintained only through stocking. Mainchannel reproduction by other-introduced fish, including common carp and fathead minnow, appeared to occur only in backwaters, because mainchannel temperatures inhibit spawning. Mainchannel reproduction by native fishes also appears limited; however, this habitat serves as an important nursery and rearing area for fishes spawned in both tributaries and mainchannel. Most larval bluehead sucker were collected from backwaters in lower reaches. Concentrations of flannelmouth suckers in reproductive condition were often found at tributary mouths and in connected backwaters. Larvae were collected from the mainchannel only in reaches 40 and 50. The Grand Canyon population of humpback chub spawns between March and June in the Little Colorado River. Mainchannel temperatures may not be warm enough to initiate spawning. Y-o-y chub were collected in lower reaches of the river, however, suggesting that spawning areas other than the LCR may be present. MOVEMENT Movement was examined by tag-recapture, oxytetracycline dye marks and fin clips. The majority of tagged and recaptured fish were rainbow trout. Most movement appeared to be associated with reproduction; movement to and from tributaries during spawning season was observed for rainbow trout, brown trout, flannelmouth sucker and humpback chub. Oxytetracycline dye marked fingerling rainbow trout were stocked at Lee's Ferry from October 1983 to April 1986. Rainbow trout were then sampled from all reaches to document dispersal of „I I -6-