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<br />Final Report <br /> <br />humpback chub in Grand Canyon probably spawned <br />in the mainstem, and likely occurred in higher <br />concentrations in areas of more suitable habitat or <br />greater food supplies. Tributaries and tributary <br />inflows were probably also used for spawning, and <br />YOlUlg remained close to hatching sites. Humpback <br />chub were also believed to occur in canyon regions <br />upstream of Grand Canyon, including 80 kIn in <br />N arrow and Cataract canyons. Hence, historic <br />distribution in this part of the basin probably <br />extended about 535 kIn from the confluence of the <br />Colorado and Green rivers, VT to Grand Wash, AZ, <br />including all canyon regions except for Glen <br />Canyon, which was a gentle alluvial reach <br />uncharacteristic of humpback chub habitat. <br /> <br />Fish populations of the Colorado River have been <br />affected in numerous ways by land use practices, <br />water diversions, and non-native fishes starting in <br />the late 1800s (Miller 1961). By the time Glen <br />Canyon Dam was completed in 1963, the lower 70 <br />km of Grand Canyon had been inundated and <br />aggraded by Lake Mead as a result of construction <br />of Hoover Dam in 1935, Lake Powell eventually <br />inundated 52 kIn of potential habitat in Narrow <br />Canyon and lower Cataract Canyon, Occupied <br />habitat was reduced by an additional 77 km (Paria <br />River to Shinumo Wash plus Granite Springs <br />Canyon to Separation Canyon) following closure of <br />Glen Canyon Dam in 1963. The combined effects <br />of Lake Mead (13%), Lake Powell (10%), and Glen <br />Canyon Dam operations (14%) have reduced 535 <br />km of potential historic habitat by 199 km or 37%. <br /> <br />ABUNDANCE <br /> <br />The largest of nine aggregations in the Colorado <br />River in Grand Canyon was the mainstem <br />component of the LCR population, with about 3,500 <br />adults ( ~200 nun TL). The second largest <br />aggregation was in Middle Granite Gorge with an <br />estimated 98 adults, The other aggregations and <br />estimated numbers of adults (N) included the <br />Shinumo Creek inflow (N=57), 30-Mile (N=52), <br />Havasu Creek inflow (N= 13), and Pumpkin Spring <br />(N=5). The combined estimates for. the nine <br />mainstem aggregations were about 3,300 to 3,800 <br />adults. Since 98% of all humpback chub captured <br />were in these nine aggregations, the estimated total <br />numbers of adults in the mainstem were about 3,370 <br />to 3,880 (95% C.!. =:t: 25% of total), <br /> <br />Executive Summary . 5 <br /> <br />The numbers of sub adult humpback chub (<200 <br />nun TL, 1-3 years of age) in the mainstem varied <br />dramatically as young descended from the LCR <br />annually in mid to late summer. Lowest densities of <br /><1 fish/l00 m2 were usually found in late winter and <br />early spring, and highest densities (3 fish! 1 00 m2 in <br />1991 and 1992 and up to 13 fish/lOO m2 in (1993) <br />were found in mid to late summer, as young <br />descended concmrent with monsoon floods from the <br />LCR Greatest numbers of subadults were usually <br />found between the LCR inflow (RM 61.3) and <br />Hance Rapid (RM 76.6), with dramatically fewer <br />numbers downstream where shoreline habitat was <br />limited and numbers of predaceous brown trout <br />were higher. The estimated numbers of subadults in <br />this 24.6 km subreach ranged from 246,000 (11100 <br />m2) to 738,000 (3/100 m2) individuals in years of <br />lowest observed densities'( 1991 and 1992), and up <br />to 3,198,000 (13/100 m2) in the year of highest <br />observed density (1993). Subadults upstream of the <br />LCR were rare, and densities downstream of Hance <br />Rapid were much lower than in the subreach <br />immediately below the LCR Since 92% of all <br />sub adults captured were found in this 24.6-km <br />subreach, the estimated peak number of sub adults in <br />the mainstem during 1991 and 1992 were 250,000- <br />800,000 and numbers in 1993 may have reached <br />3,500,000 subadults. <br /> <br />SURVIVORSHIP <br /> <br />Survival of humpback chub in the mainstem is <br />probably limited by cold temperatures, suitable <br />habitat, food availability, and predation. Parasites <br />are not believed to be a significant factor to survival <br />in the mainstem, although an increased incidence ofoAsian tapeworms could affected the health of <br />individual fish under stress. Cold mainstem <br />temperatures likely induce thermal shock in many <br />young fish descending from the LCR, causing death <br />or aberrant behavior that can solicit predator <br />response. Cold temperatures also significantly <br />reduce swimming performance of young fish <br />(Bulkley et al. 1982), restricting suitable habitat to <br />low-velocity areas and limiting escape potential <br />from cold-water predators. Subadult shoreline <br />habitat is destabilized by fluctuating releases from <br />Glen Canyon Dam, causing greater exposure of <br />sub adults to predation. Predation on sub adult <br />humpback chub in the mainstem could be <br />substantial, and may be a primary factor limiting <br />survival and recruitment ofyoWlg, <br />