Population Estimate for Humpback Chub (Gila cypha) in Desolation and Gray Canyons, Green River, Utah 2001-2003

摘要

Humpback chub population estimates were determined for Desolation and Gray canyons (Deso/Gray), on the Green River, by conducting multiple pass capture-recapture sampling. Three sampling passes were conducted in 2001 and 2003, and two passes were conducted in 2002 due to low water. In 2003, sampling was moved to the fall to avoid capturing Colorado pikeminnow that use Desolation Canyon for spawning. Twelve sites were sampled throughout the two canyons. Included within these sites, were four long-term sites sampled annually by Utah Division of Wildlife Resources since 1989. Analysis of point estimates generated by capture-recapture data demonstrated that the adult humpback chub population was composed of 1,254 individuals in 2001, 2,612 individuals in 2002, and 937 individuals in 2003. Many factors were at play during the three-year period that may have contributed in the variation in point estimates among years. Non-biological factors complicating the validity of the estimates include moving sampling from summer to fall and low water conditions at the time of sampling. Factors that may have directly affected the humpback chub population biologically include, the Rattle Complex fire and subsequent fish kill in late summer 2002, and the apparent increase in smallmouth bass occupying Deso/Gray. The authenticity of the point estimates will be more clearly analyzed when population estimates are conducted consistently during the fall months of 2006-2008. This study represents the first sampling period since finalization of the amended recovery goals for humpback chub in 2002 and is valuable in the initial evaluation of population size of humpback chub in Deso/Gray. Furthermore, it serves to illuminate any necessary adjustments to the sampling design before population estimate sampling for humpback chub in Deso/Gray resumes in 2006.