|
<br />December 2008
<br />
<br />Bestgen et at-Ecology of Gila elegans
<br />
<br />489
<br />
<br />The endangered bonytail Gila elegans is the
<br />rarest native fish in the Colorado River Basin.
<br />One of four main-stem, large-bodied fishes that
<br />are federally listed as endangered in the basin
<br />(Carlson and Mmh, 1989), only a few bony tails
<br />have been collected in the past 20 years, and wild
<br />populations may be extirpated (Vanicek and
<br />Kramer, 1969; Vanicek et aI., 1970; Holden and
<br />Stalnaker, 1975a, 1975b; Kaeding et al., 1986;
<br />United States Fish and Wildlife Service, 2002).
<br />Last verified captures of the bony tail occurred in
<br />the main stem of the Colorado River, Colorado,
<br />and the Green and lower Yampa rivers, Colorado
<br />and Utah. including reaches in Dinosaur Na-
<br />tional Monument (Vanicek and Kramer, 1969;
<br />Vanicek et aI., 1970; Holden and Stalnaker,
<br />1975a, 1975b; Kaeding et aI., 1986). A few
<br />suspected bony tails were captured in the Colo-
<br />rado River, Cataract Canyon, Utah, in the late
<br />1980s (United States Fish and Wildlife Service,
<br />2002; R. A. Valdez, pers. camm.). Reasons for
<br />demise of the formerly widespread and abundant
<br />bonytail are poorly understood because few
<br />ecological studies were conducted prior to its
<br />precipitous decline (Vanicek and Kramer, 1969;
<br />Holden and Stalnaker, 1975a; Behnke and
<br />Benson, 1983). Native fishes in the Colorado
<br />River Basin have declined due to disruption of
<br />natural flow and temperature regimes by main-
<br />stem dams and negative effects of nonnative
<br />fishes and these conditions likely also affected
<br />bony tails (Dill, 1944; Vanicek and Kramer, 1969;
<br />Holden and Stalnaker, 1975a; Carlson and Muth,
<br />1989; United States Fish and Wildlife Service,
<br />2002; Olden et aI., 2006; Bestgen et aI., 2006,
<br />2007, in litt.).
<br />The depleted state of Vloild bony tails requires
<br />stocking of hatchery-produced individuals to
<br />advance recovery efforts. Bony tails present in
<br />hatcheries originate from six female and five
<br />male fish captured in Lake Mohave Reservoir,
<br />Arizona and Nevada (Hanlman, 1982; Johnson
<br />and Jensen, 1992). Recent stocking efforts in the
<br />Upper Colorado River Basin emphasize releasing
<br />relatively large indhoiduals (ca. > 150 mm total
<br />length) in river reaches where last-known Vloild
<br />individuals were captured. During 1996-2004,
<br />44,472 bonytails (most >150 mm total length)
<br />tagged \~ith passive-integrated-transponder (PIT)
<br />tags have been released throughout the Upper
<br />Colorado River Basin, and many other, mostly
<br />smaller, coded-\~ire-tagged indhoiduals also have
<br />been released, but only limited information is
<br />
<br />i
<br />
<br />'--\l
<br />
<br />
<br />Green
<br />
<br />R<5a3~
<br />~ River
<br />
<br />
<br />
<br />t
<br />
<br />Utah
<br />
<br />Colorado
<br />
<br />N
<br />
<br />lOk.m
<br />
<br />FIG. I-Hatchery-reared bony tails (Giln eiegans) were
<br />released in the Green River at Echo Park and Browns
<br />Park and in the lower Yampa River. Sampling was
<br />conducted during 2002-2007 to determine their
<br />distribution and abundance (RK= river kilometer).
<br />
<br />available on survival or ecology of these fish (P.
<br />V. Badame and]. M. Hudson, in litt.). This paper
<br />reports sampling observations of recently
<br />stocked bony tails in the Green River to better
<br />understand their ecology and assist with recovery
<br />efforts.
<br />
<br />STUDY AREA--The study area includes the Green River
<br />from Browns Park (river km 607, 53 river km
<br />downstream of Flaming Gorge Dam) downstream
<br />through Island-Rainbow Park (river krn 528), a river
<br />reach of 80 river km (Fig. 1). Most of the study area
<br />(river km 586-528) is within Dinosaur National
<br />Monument, Colorado and Utah. The Green River is
<br />regulated by Fhuning Gorge Darn, which is 105 river km
<br />upstream of the confluence of the Yampa River (river
<br />!un 555.6). Regulation resulted in lower peak flows,
<br />higher base flows, and reduced temperatures in the
<br />Green River, conditions that are attenuated down-
<br />stream, in part, by the mostly unregulated Yanlpa River
<br />and flow and temperature re-regulation at Flaming
<br />Gorge Darn (R. T. Muth et al., in litl.).
<br />
<br />MUERlALS AND METHODs-Our sampling was part of a
<br />larger study conducted to evaluate effects of flow and
<br />temperature regimes and recent drought on the fish
<br />community of Green River downstream of Flaming
<br />Gorge Dam (K. R. Bestgen et al" in lilt.). We seined
<br />low-velocity channel margins and backwaters through-
<br />
|