Status and Trends of the Grand Canyon Population of Humpback Chub

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H 1 Abundance of Hmpback Chub c T IWO 10 � T ' 4 T = 1 T co T -p T T a :E .; TTT_L i 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Year Estimated adult humpback chub abundance (age 4+) from Age - Structured Mark - Recapture (ASMR) model incorporating uncertainty in assignment of age. Error bars represent mini- mum 95- percent confidence intervals (from Coggins and Walters, 2009) and do not consider uncertainty in growth or mortality rates. population and recruitment trends. This method provides a relatively strong indica- tion of the population size trend; however, it offers somewhat less confidence for the single population estimate. The data show a steadily declining trend of adult hump- back chub from 1989 to 2001 (Coggin and Walters, 2009). From 2001 until 2008, the population reversed, increasing to an esti- mated 7,650 adult fish in 2008, an increase of approximately 50 percent (Coggin and Walters, 2009). The ASMR model has been verified as an appropriate approach for estimat- ing trends in humpback chub population size by a series of independent reviewers. Although the current adult population size is estimated at approximately 7,650 indi- viduals, this estimate is very sensitive to assumptions about humpback chub growth and mortality rates. Considering a range of probable values for these parameters, it is likely that there are between 6,000 and 10,000 adult chub in the Grand Canyon population. This level of uncertainty sug- gests that managers should focus on the temporal trend in humpback chub abun- dance rather than estimates of absolute abundance. Causes of the Population Increase The factors contributing to the estimated increases of adult humpback chub numbers in Grand Canyon are not easy to determine. Between 2000 and 2008, both human- caused and natural events have taken place that may have improved conditions for humpback chub, including experimental water releases from Glen Canyon Dam, removal of nonnative fish, and drought - induced warming of the Colorado River. Experimental Water Releases A series of experimental releases from Glen Canyon Dam took place between 2000 and 2008 that may have benefited humpback chub and other native fish. For example, during the summer of 2000, releases from Glen Canyon Dam were con- strained during a low steady flow experi- ment, which caused peak downstream water temperatures in Grand Canyon to exceed 20°C (68.5 °F). Nearshore water temperatures during the summer of 2000 exceeded the minimum temperature that humpback chub need to reproduce and may have created conditions that allowed fish produced in 1999 to grow more rapidly than in normal, colder temperatures. Addi- tionally, a series of experimental releases from the dam, including three high -flow experiments, may have disadvantaged non- native fish and improved humpback chub habitat. Removal of Nonnative Fish The removal of large numbers of nonnative fish, particularly rainbow trout (Oncorhynchus mylass) and brown trout (Salmo trutta), from the area near the confluence of the Colorado and Little Colorado Rivers may also have benefited the humpback chub population in that area. Not only can trout prey on juvenile humpback chub, but also they rely on the same food sources— aquatic and terres- trial invertebrates, algae, and small fish. Between 2003 and 2006 the rainbow trout population in the Colorado River near the Little Colorado River was reduced by more than 80 percent. Drought- Induced Warming Drought conditions are also thought to play a role in the recent rebound in hump- back chub numbers. Water temperatures below Glen Canyon Dam increased starting in 2003, as drought caused the level of Lake Powell reservoir to drop, allowing warmer surface water of the reservoir to be released downstream. In 2005, water tem- peratures in the mainstem Colorado River near the Little Colorado River exceeded 17°C (60.8°F), the warmest temperatures recorded there since the reservoir filled in 1980 and approximately the minimum temperature needed by humpback chub to successfully reproduce. Although native fish likely benefit from warmer water releases, there is great concern that warmer water temperatures may also benefit nonna- tive warmwater fish such as channel catfish (Ictalurus punctatus) and other predators. In the future, USGS, U.S. Fish and Wildlife Service, and Arizona Game and Fish Department scientists will continue to monitor the Grand Canyon population of humpback chub, trying to isolate the condi- tions responsible for recent improvements in humpback chub population and recruit- ment trends. Research activities will exam- ine, among other questions, how the March 2008 high -flow experiment affected habitat used by native fish and whether high flows disadvantage nonnative species. Matthew E. Andersen Edited by Peter H. Stauffer Layout by David R. Jones Reference Coggin, L.G., Jr., and Walters, C., 2009, Abun- dance trends and status of the Little Colorado River population of humpback chub —an update considering data fiorn 1989 -2008: U.S. Geologi- cal Survey Open -File Report 2009 -1075, 18 p. For more information contact: U.S. Geological Survey Southwest Biological Science Center Grand Canyon Monitoring and Research Center Flagstaff, AZ 86001 928 -556 -7094 or mandersen @usgs.gov This Fact Sheet and any updates to it are available online at http : / /pubs.usgs.gov /fs/2009/3035/