population estimates for humpback chub - Upper Colorado River ...

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LIST OF KEY WORDS humpback chub, Gila cypha, roundtail chub, Gila robusta, Westwater Canyon, Colorado River, population estimate, growth, movement viii
EXECUTIVE SUMMARY Humpback chub (Gila cypha) are listed under the Endangered Species Act of 1973, as amended. In accordance with recovery goals finalized in 2002, a population estimate was completed for adult humpback chub in Westwater Canyon on the Colorado River. This population estimate was conducted from 1998 to 2000 with the objectives of obtaining population estimates for humpback chub and roundtail chub in Westwater Canyon. Sampling occurred during September and October throughout the three years of the study. Three passes were conducted annually with approximately one week between passes. The primary method of capture was via trammel netting with supplemental electrofishing on one pass per year. Population estimates were generated from recapture data using the null estimator (M o ) within Program CAPTURE. Separate estimates were generated for each year of the study. Results indicated a decline in the adult humpback chub population between 1998-1999 with a retention in abundance between 1999-2000: 4,744 in 1998; 2,215 in 1999; and 2,201 in 2000. This overall declining trend was not statistically significant, but may be of concern. The adult roundtail chub population in Westwater Canyon during this time period was relatively stable: 5,005 in 1998; 4,234 in 1999; and 4,971 in 2000. Length-frequency analysis for both species indicated shifts in the size class structure through the three years. Growth of humpback chub was slower than roundtail chub based on recapture data from this and previous studies. Analysis of recaptures indicated more net movement from humpback chub than from roundtail chub. However, both species exhibited movement between Black Rocks and Westwater Canyon. This movement was frequent enough to consider Black Rocks and Westwater Canyon a single population for humpback chub and roundtail chub. Analysis of catch per unit effort (CPUE) data from this study and historic interagency standardized monitoring indicated a continued declining trend in mean CPUE for humpback chub that was significant. Mean CPUE for roundtail chub also indicated a continued declining trend, but it was not statistically significant. The results of this study will provide valuable information for conducting future population estimates of chub in the upper Colorado River basin in addition to providing three point estimates that will be used to determine if humpback chub have met the recovery goals. ix
Page 1 and 2: State of Utah DEPARTMENT OF NATURAL
Page 3 and 4: ACKNOWLEDGEMENTS We would like to a
Page 5 and 6: TABLE OF CONTENTS (continued) DISCU
Page 7: LIST OF FIGURES Figure 1. Figure 2.
Page 11 and 12: to a much larger ongoing project en
Page 13 and 14: etween years using the same analyse
Page 15 and 16: Growth Mean annual growth rates of
Page 17 and 18: Length-Frequency The length-frequen
Page 19 and 20: population trends and population dy
Page 21 and 22: Comparison with ISMP The decreasing
Page 23 and 24: RECOMMENDATIONS • Increase tramme
Page 25 and 26: Nesler, T.P. 2000. Recovery of the
Page 27 and 28: Table 3. Population estimate (N) fo
Page 29 and 30: Westwater Canyon Humpback Chub Leng
Page 31 and 32: Humpback Chub ISMP Length-Frequency
Page 33 and 34: Westwater Canyon Adult Roundtail Ch
Page 35 and 36: Westwater Canyon Roundtail Chub Ann
Page 37 and 38: Roundtail Chub ISMP Length-Frequenc
Page 39 and 40: Appendix II. Summary of population
Page 41 and 42: Appendix III Table 2. Daily instant
Page 43 and 44: Appendix III Table 5. Mean daily te
Page 45: Appendix III Table 8. Mean daily te

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