WSP10995

• File Number: 8220.101.09
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 12/21/1994
• Author: USDOI-USFWS
• Title: Final Biological Opinion-Operation of the GlenCanyon Dam as the Modified Low Fluctuation Flow Alternative of the Final Environmental Impact Statement-Operation of Glen Canyon Dam
• Water Supply Pro - Doc Type: Biological Opinion

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The inability of larvae to recruit into young-of-yearor: juvenile .life stages in wild populations
has been recognized as a major limiting factor for the survival of the razorback sucker, and
Minckley et al. (1991j identified three possible explanations: "(1) transport from the system, (2)
nutritional constraints resulting in. starvation, and (3); loss of early life-h,istory stages to.
predation" with the last one thought the most probable.

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To examine all possible factors, however, food habitat investigations oflarval razorback suckers
have been initiated to determine whether food may be limiting such as availability of appropriate
size food items for larvae that have just consumed their. yolk sack or have changed orientation
of their mouth (Minckley et aI. 1991, Papoulias and Minckley 1992). An intensive study of
larval razorback suckers spawned and raised in an isolated cove in Lake Mohave is the subject
of an ongoing investigation (Paul Marsh,Arizona State.University, personal communication).

Critical Habitat

The total length of critical habitat designated for the razorback sucker within the Colorado River
basin is 2776 kIn. Within the project area, this includes the Colorado River and the l()()..year
flood plain from the confluence with the Paria River (about RM 1) to Hoover Dam (555 kIn).
Known constituent elements (defined above for the humpback chub) include water, physical
habitat, and biological environment as required for each particular life stage of the species.

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BALD EAGLE

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The Colorado River corridor within the Grand Canyon has become an important winter
concentration area for the bald eagle. Although not a separate subspecies, bald eagles in the
southwestern United States and northern Mexico are considered a distinct population for
purposes of recovery efforts and section 7 consultation (U.S. Fish and Wildlife Service 1982).
Prior to 1984, Floyd Thompson (Service, retired) reported locating three possible bald eagle
nests in the Grand Canyon region (Hunt et aI. 1992). However, whether the wintering bald
eagles in the Grand Canyon are part of the southwestern breeding population or migrate from
northern latitudes is not known.

Bald eagles were not recorded in concentrations in the Grand Canyon until after the
establishment of the mainstem rainbow trout (Oncorhynchus mykiss) fishery following
construction of Glen Canyon Dam. Prior to Glen Canyon Dam, low winter flows and the
presence of large native fishes may have offered similar or even superior conditions to those that
exist today for wintering bald eagles, but no documentation. exists to support this premise.
Wintering bald eagles ,were first. documented (n=4) in.thewinter of 1985-1986 (Brown et al.
, 1989), and observations increased to a high of 26birds.counted in a single day at Nankoweap
Creek in late February 1990 (National Park Service 1992).

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Bald .eagle use .of the river corridor-is opportunistic anlt ;currently . concentrated around
Nankoweap '€reek (RM 52) where' the birds '.utilize an abundant food source in the form of
winter-spawning trout. These trout have been found to comprise greater than 99% of eagle
foraging attempts (Brown and Leibfried 1990; National Park Service 1992). The number of bald
eagles at Nankoweap Creek appears to be directly related to the abundance of spawning trout.

IS Deeember 1994 fInal biological opinion 1.11.93.F.167

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