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~,. <br />EXECUTIVE SUNIIVIARY <br />This five yeaz study to detenmine the effect of Colorado River flows on humRback•chub (Gila <br />cypher) reproduction and recruitment is a component of the Recovery Implementation Program <br />(RIP) Aspinall Studies. The goal of this project was to develop relationships between observed <br />flows and humpback chub life i orv responses, while concentrating on the eazlier life stages. <br />To meet this goal, the following study objectives were identified: determine spawning and nursery <br />requirements, descn~e the relationship between geom~ rocesses of sediment transport and <br />nursery habitat formation, identify and describe reproductive isolating mechanisms, and to assess <br />recruitment. Similar reseazch is being conducted on the Green River in Desolation Canyon as <br />part of the Flaming Gorge Studies. . <br />Three components of the Westwater Canyon fish community were studied: the young of the year <br />(YOY) ,juveniles (AGE 1+ and 2+) and late juvenile /adults. Both humpback chub (Gila <br />cypher) and roundtail chub (G robusta) are common in Westwater Canyon. Unlike some Upper <br />. Basin chub populations, the adults of these two Gila spp. are morphologically distinct in <br />` Westwater, Canyon. However, young chubs are difficult to identify to species wherever more than <br />one of these species is found. YOY sampling was therefore broken into sub-reaches: above <br />Westwater Canyon (where only roundtail chubs are common), within Westwater Canyon <br />(roundtail and humpback aze equally represented in the adult fish community) and below the <br />• canyon (where roundtail chubs are present in low numbers and humpback chub are absent). <br />Juvenile chubs were monitored with shoreline electrofishing in the canyon, and late juvenile and <br />adults were same with hoop and trammel nets as well as with electrofishinQ. The design of <br />. the study focused on YOY densities (fish / mZ) and habitat use to make the relationships with <br />observed flows. Juvenile chubs (still not readily identified to species) were monitored in the <br />canyon to take the flow /reproduction relationship to the next step. Only three of the five cohorts <br />produced during this study~(1992, 1993, and 1994) could be tracked past their first summer (Age <br />0); the 1995 cohort was virtually non-existent and further monitoring of the 1996 cohort was <br />beyond the scope of this study. -Late juvenile and adult chubs were monitored to determine <br />population trend and stability in light of the recent recnutment. Pre-project data were <br />incorporated into these analyses. <br />Hatching time and growth of young Gila spp. were correlated with flow parameters (peak flow <br />• at State Line gage and pre-peak flow in excess of 6000 cfs (a cumulative flow metric similar to <br />. degree days) and water temperature (degree days > 10°C prior to June 1 S). In general, YOY <br />chubs hatched earlier above the canyon indicating roundtails likely spawned earlier than <br />humpbacks. Catch rates of YOY Gila spp. were greatest in the above-canyon sub-reach with a <br />project high density, .679 ,recorded there during July and August of 1993. The greatest density <br />.:: DRAFT , . .. <br />