Population Estimate for Humpback Chub (Gila cypha) and Roundtail Chub (Gila robusta) in Westwater Canyon, Colorado River, Utah 2003-2005. Final Report.

摘要

Mark-recapture sampling was conducted in Westwater Canyon on the Colorado River to estimate the population size of humpback chub (Gila cypha) and roundtail chub (Gila robusta). Three sampling passes were conducted during the fall months from 2003 to 2005. Based on closed population estimate models, a decline in humpback chub populations was determined, while roundtail chub populations remained stable. Confidence limits during the study period did overlap for humpback chub, yet when added to the trends from earlier population estimates (1998-2000), confidence intervals between the first and last year did not overlap, indicating a significant decline. Estimates of the humpback chub population in Westwater Canyon coupled with the most recent estimates in Black Rocks (making up a core population) hover near and perhaps below the minimum viable population estimated in the 2002 humpback chub recovery goals. Population estimates for humpback chub in 2003 were 2,973 (CI, 1,710 to 6,042), declined in 2004 to 1,729 (CI, 1,121 to 2,967), and declined further in 2005 to 1,210 (CI, 880-1,769). Roundtail chub population estimates remained stable and increased slightly from 3,288 (CI, 2,963 to 65,760) in 2003, to 3,867 (CI, 3,124 to 4,912) in 2004, to 4,273 (CI, 3,356 to 5,613) in 2005.