<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 />
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