Status and Trends of the Grand Canyon Population of Humpback Chub

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fect unless you tell us to ring your subscription and T he�rAMW*PB Lrts oaieit n of - CtiVe fish Colnfn IME Pao • Questio nines i 'ea, ineiading the click he The humpback chub (Gila cypha) is a freshwater fish that may live as long as 40 years and is found only in the Colorado River Basin. Habitat alterations, including changes caused by dams and reservoirs and the introduction of a wide variety of non- native fish, have contributed to population declines that caused the humpback chub to be placed on the Federal list of endan- gered species in 1%7. Only six populations of humpback chub are currently known to exist, five above Lees Ferry, Arizona, and one in Grand Canyon, Arizona. The U.S. Geological Survey's (USGS) Grand Canyon Monitoring and Research Center oversees monitoring and research activities for the Grand Canyon population under the auspices of the Glen Canyon Dam Adap- tive Management Program. Reaching 20 inches in length, the humpback chub possesses features that distinguish native Colorado River fishes— large adult body size, small depressed skull, large predorsal hump, and small eyes. These features are assumed to be Population U.S. Geological Survey scientists estimate that the adult population of humpback chub (Gila cypha) in the Grand Canyon increased 50 percent between 2001 and 2008. (Photograph courtesy Arizona Game and Fish Department) adaptations to life in the historically turbu- lent and seasonally sediment -laden Colorado River. Today the species is confined to iso- fated deep canyon stretches of the Colorado River and its tributaries. In Grand Canyon, most humpback chub are found in the Little Colorado River and at its confluence with the Colorado River. The Grand Canyon population repro- duces primarily in the Little Colorado River because mainstem Colorado River temperatures are generally too cold for spawning and juvenile rearing. Humpback chub require a minimum temperature of 1600 (60.8017) to reproduce. Except during drought conditions, the water - intake struc- tures at Glen Canyon Dam are well below the surface of Lake Powell, where the water is not warmed by the sun. Colorado River temperatures in Grand Canyon generally range from 7°C to 12 °C (44.6 53.6°F). Humpback chub have been affected not only by dam- induced changes but also through predation by and competition with nonnative fish. Fish originating in many parts of the world are currently found in the Colorado River, including introduced species of trout, catfish, carp, and bass. Also present in the Colorado River are nonnative fish parasites, such as the Asian tapeworm (Bothriocephalus acheilognathi) and anchor worm (Lermwa cyprinacea), which infect some humpback chub and may be a threat. Monitoring the Humpback Chub Population The USGS uses a method called the Age - Structured Mark - Recapture (ASMR) model, which combines capture data and statisti- cal modeling, to estimate humpback chub 1#1 &4/g �Ak"om/ time/ archive/ printout/0,23657,1174707,00.htm1 r,.A4A"