9533

• UCREFRP Catalog Number: 9533
• Author: Muth, R. T. and E. Wick.
• Title: Field Studies on Larval Razorback Sucker in Canyonlands National Park and Glen Canyon National Recreation Area, 1993-1995.
• USFW Year: 1997.
• USFW - Doc Type: Fort Collins.
• Copyright Material: NO

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aged to determine growth rates and exact time of spawning. That information can be used to
document environmental parameters during spawning and early rearing and evaluate growth
and survival potentials of larvae under present regulated flow and habitat conditions.

CONCLUSIONS AND RECOMMENDATIONS

The discovery of razorback sucker larvae in the middle and especially the lower Green
River is significant and raises important questions about the presumed status of razorback
sucker populations in the upper Colorado River basin and management of flows and habitats to
enhance the species throughout the Green River system. Documentation of successful
razorback sucker reproduction through collection of larvae has occurred annually since 1992,
but, although difficult to assess, recruitment appears to be insufficient to sustain populations.
The apparent bottleneck is poor survival of age-O fish. Obviously, present environmental
conditions are not conducive to good growth and survival of young razorback sucker, but
information on influencing factors is limited. Management of flows to create and maintain
nursery habitats during the criticallarval-early juvenile period (e.g., FLO Engineering, Inc.
1996) is essential. Actions to improve the quality of nursery habitats through artificial
enhancement of productivity or control of nonnative fishes should be considered.
Sampling for razorback sucker in the middle and lower Green River systems will
continue through at least 1998 under the razorback sucker monitoring program, which in 1996
and 1997 includes sampling for adults in the middle Green River, but additional studies are
needed. Recommendations:

. The collection of larval razorback sucker in the lower Green River strongly suggests
that spawning is occurring somewhere in that river section; previously, conventional
thought was that razorback sucker reproduction now occurs only in the middle Green
River system. Dates of first captures of razorback sucker larvae in reaches of the lower
Green River in 1994-1996 occurred about 14-30 d before dates of first captures in
corresponding years in reaches of the middle Green River (R. T. Muth, unpublished
data).

Implement a sampling program for adult razorback sucker in the lower Green
River to document their abundance and distribution and the existence of
spawning aggregations. Survey sampling for adult razorback sucker in selected areas
of the lower Green River will begin in 1998 as part of the razorback sucker monitoring
program.

. In the middle and lower Green River systems, conduct studies to characterize
biological (e.g., food resources, fish diets, sympatric species, and interspecific
fish interactions) and physical (e.g., water temperature, turbidity, D.O., and pH)
attributes of habitats presently used by larval razorback sucker as nursery areas.
Studies could incorporate aspects of the planned NPS/LFL experiment on use of
enclosures to study the growth and survival of razorback sucker larvae described in the
methods section of this report, a mark-recapture approach using techniques for marking
otoliths with fluorescent chemicals, and(or) otolith aging of wild-caught larvae preserved
in 100% ethanol. Information gained would help determine existing limiting factors and

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