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I I I I I I I I I I I I I I I I I I I INTRODUCTION This Interim Report presents preliminary data from field work conducted to monitor the distnbution and relative abundance of humpback chub, (Gila cypha) within Cataract Canyon. Field work was conducted September 27 - October 2, 1990. The results of this trip will be integrated with findings from the first Cataract Canyon field trip conducted June 26 - July 2, 1990, and presented in an Annual Report in January 1991. Information in this report will generally be presented in a tabular format to provide a synopsis of pertinent trip details and preliminary data analysis. Recent investigations, funded by the Bureau of Reclamation, in the Cataract Canyon Region of the Colorado River documented the existence of a small population of humpback chub that occupies the 16-mile reach between the confluence of the Green and Colorado Rivers and the inflow to Lake Powell (Valdez 1985, 1987, 1988, 1990). During the Cataract Canyon Studies, from 1985 to 1988, a total of 108 humpback chub were captured in this region, including 11 larvae, 19 YOY, 56 juvenile and 22 adults. Additional work in 1989, resulted in the capture of 3 more juvenile humpback chubs, (Valdez and Masslich 1990). The taxonomic and distributional analysis presented by Valdez (1990) support the existence of a self-sustaining population of humpback chub in Cataract Canyon. This population is believed to consist of a geomorph unique to the Cataract Canyon Region. The chubs classified as G. cyplul in Cataract Canyon were generally small (most less than 250 mm TL), with a blunt head, shallow nuchal hump, overhung snout, scaleless nape and breast, and a relatively deep body. Dorsal and anal fin ray counts were variable, ranging from 919 to 10/10, with 919 and 9/10 being most common. The extreme morphological features such as a pronounced hump were reduced. It is believed that humpback chubs inhabiting Cataract Canyon may be remnants of a larger population of humpback chub that occupied the canyon before the closure of Glen Canyon Dam in 1963. This is supported by collections of chubs in the Lake Powell area from 1962 to 1967 (Holden and Stalnaker 1970; Holden and Stalnaker 1975). These collections revealed that a large number of intergrades were present in the area, that the authors felt reflected hybridization as a result of habitat changes created by. the closure of Glen Canyon Dam. This together with the idea that Cataract.. humpback chub represent a remnant population utilizing possibly marginal or peripheral habitat may account for the morphology of the humpback chub currently occupying Cataract Canyon. Although specific spawning sites were not located and adults with expressible gametes were not found during the Cataract Canyon Studies, the presence of larval and YOY humpback chub indicated that spawning by this species occurred within the study area. Spawning was suspected in at least one location within Cataract Canyon, between river miles 201.5 and 212.4. Larval, YOY, juvenile and adult fish were captured in this reach, including two adults in spawning condition. Fourteen fish tentatively identified as bonytail (Gila elegans) were also captured during the Cataract Canyon studies, suggesting the existence of a possible enclave of bonytail inhabiting Cataract Canyon (Valdez 1990). Although these species did not exhtbit all of the definitive characteristics of G. elegans, they exhibited a compliment of morphological and meristic features that more closely resembled this species rather than the other two congeneric Colorado River chubs, humpback chub (G. cypha) and roundtail chub (G. robusta). A more detailed taxonomic study of the Gila complex within Cataract Canyon is required to confirm the identification of these fish as well as answer other questions pertaining to the Gila complex in Cataract Canyon. 1