9586

• UCREFRP Catalog Number: 9586
• Author: Desert Fishes Council.
• Title: Proceedings of the Desert Fishes Council Volume XXVI 1994 Annual Symposium.
• USFW Year: 1994.
• USFW - Doc Type: Furnace Creek, CA.
• Copyright Material: NO

DFC PROCEEDINGS -ABSTRACTS AND CONTRIBUTED PAPERS IN ORDER PRESENTED
However, infrequent but consistent captures of
adult razorbacks in Lake Mead from the late 1960's
through 1993 clearly indicate the presence of a second
younger cohort of fish in the lake, since adults
currently being contacted would need to be greater
than 55 years old to predate significant impoundment,
considerably in excess of any reasonable projections of
species longevity. This occurrence could logically
result from either survival and recruitment in the
reservoir of juvenile fish at some point, or movement
into the impoundment of adult or juvenile fish from
adjacent riverine habitats upstream. The majority of
captured fish, based on limited knowledge of adult
razorback growth patterns, appear to have been in the
20 to 30 year age ranges at the time of capture.
Variability in individual fish growth makes it virtually
impossible to determine known age for adult
specimens from capture measurements, or if these
observed fish represent a single annual cohort or
represent moderate survival and recruitment over a
span of years. Interpretation of the available data does
indicate probable recruitment of the existing
population in the early-1960's to mid-1970's. Of
particular interest is the capture by AGFD in 1967 of
six adult fish averaging 231 mm TL (McCall, 1979).
Age estimation using available growth history curves
indicates a probable age for these fish at time of
capture of three to five years (McCarthy and Minckley,
1987). Although exact capture locations were not
available these fish were collected in upper portions of
the lake in or near Arizona.
Three primary events in the mid-1960's affected
hydrologic characteristics of Lake Mead and
associated upstream riverine habitats. The closure of
Glen Canyon Dam in 1963 significantly modified
hydrologic and limnologic characteristics of the
Colorado River downstream to Lake Mead.
Displacement downstream of resident adult and
juvenile razorback suckers from occupied river and
tributary habitats as a result of these changes is at least
a probability. Secondly, the reduced inflow into Lake
Mead from the upper Colorado River basin as a result
of the filling of Lake Powell modified reservoir storage
levels and fluctuation patterns for a several-year
period, with reservoir surface elevation reduced to as
low as 1060 feet, 120 feet below current levels.
Increased channelization of upstream portions of the
reservoir and temporary exposure of previously
submerged spawning habitats were two obvious
structural impacts of the storage reduction. Finally,
permanent modification of seasonal fluctuation
patterns in reservoir storage levels is a continuing
impact of Glen Canyon Dam operations, which has had
an identified negative impact on spring-spawning sport
fish species (Morgensen and Padilla, 1982). The first
two of these events correlate closely with presumed
recruitment dates for the Arizona specimens collected
in 1967.
Based on recent collections and observations, the
existing population of razorback suckers in Lake Mead
appears to be composed almost entirely of large adult
fish, which correlates closely to observations in Lake
Mohave. Except for a single 366 mm TL specimen
captured in 1994, recently captured fish show a range
of 467 to 765 mm TL averaging 563 mm TL,
indicating a probable age range of adults as 20 to 30
plus years old and a lack of significant recruitment to
the population. This is somewhat less than the
estimated average age of Lake Mohave adult
razorbacks (Burke and Mueller, 1993), but more
accurate age estimation would not be possible without
destructive sampling of adult fish. Although localized
spawning behavior has been observed since 1990, no
direct evidence of successful reproduction, larval
emergence, or juvenile survival and recruitment has
been observed or documented through the 1994
sampling effort. Current distribution and occupancy, at
least during the spawning period, appears to be limited
to a small number of sites in the Boulder Basin and
Overton Arm, which represents a restriction in
distribution compared to historic capture records from
the 1960's and 1970's. Contact with adult razorback
suckers at recent capture sites represents concentrated
sampling effort at known areas of use with species
specific methods, primarily trammel nets, so there may
be other areas of seasonal occupancy which have not
been located. However, intensive lakewide gill net
surveys since 1990 have shown a much lower capture
rate for adult razorbacks when compared to the same
methodology used in the 1970's, which supports the
premise that current numbers and distribution of adult
fish are very limited. Increased use of trammel net
by-sets during lakewide sampling efforts began in
1993 and this will assist in better defining possible
occupancy of suitable habitats by razorback suckers in
areas of the reservoir where presence has not recently
been documented.
LITERATURE CITED
ALLAN, R.C., AND D.L. RODEN. 1978. Fish of
Lake Mead and Lake Mohave, Base Fisheries
Data Boulder Canyon to Davis Dam. Biological
Bulletin No. 7, Nevada Department of
Wildlife, Reno. 105 pp.
BURKE, T., AND G. MUELLER. 1993. Native Fish
Work Group, 1992 Annual Report. U.S.
Bureau of Reclamation, Boulder City. 48 pp.
JONEZ, A., AND R.C. SUMNER. 1954. Lakes Mead
and Mohave Investigations: A Comparative
Study of an Established Reservoir as Related
to a Newly Created Impoundment. Final
Completion Report, Dingall-Johnson Project
F-1-R. Nevada Fish and Game Commission,
Reno. 186 pp.
MCCALL, T.C. 1979. Fishery Investigation of Lake
Mead, Arizona-Nevada, from Separation
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