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Abstract.-We evaluated habitat use, spawning, and species associations of <br />a little known population of the endangered humpback chub Gila cypha in <br />the Yampa and Green rivers, Dinosaur National Monument, from 1986 to <br />1989. Adult and jvenile humpback chubs (N=133, 8% of all chub captures) <br />were captured in high-gradient, canyon reaches that were dominated by <br />rocky runs and rapids. The fish was most prevalent in shoreline eddy <br />habitats in spring, but moved to other locations as water levels declined. <br />Fish in breeding condition (N=39) were captured during declining spring <br />runoff at river temperatures of about 200C. High turbidity precluded <br />observation of spawning behavior and spawning microhabitat, but ripe males <br />and females were captured, and presumably spawned, in shoreline eddy <br />habitats. Humpback chubs were captured in association with 18 other fish <br />species, including 7 native and 11 introduced forms. Of these, roundtail <br />chub G. robusta and introduced channel catfish Ictalurus punctatus <br />dominated the catch in habitats yielding humpback chub. Co-occurrence of <br />ripe humpback and roundtail chubs indicated some temporal and spatial <br />overlap in spawning, but few morphologically-intermediate fish (N=3) were <br />captured. Similarity in feeding habits and habitat use by humpback chub <br />and channel catfish during spring runoff suggests a potential for negative <br />interactions between these fishes. <br />The humpback chub Gila c ha, a large-river cyprinid fish endemic to <br />the Colorado River basin, is federally protected under provisions of the <br />Endangered Species Act of 1913, as amended. It is one of a complex of <br />forms collectively known as Colorado River chubs which also includes the <br />bonytail G. elegans and roundtail chub G. robusta. <br />2