9691

• UCREFRP Catalog Number: 9691
• Author: Recovery Implementation Program.
• Title: Recovery Implementation Program For Endangered Fish Species in the Upper Colorado River Basin 25th Annual Recovery Program Researchers Meeting.
• USFW Year: 2004.
• USFW - Doc Type: Moab, UT.
• Copyright Material: NO

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have much lower densities of nonnative fish for the first year. The fish populations in
these depressions are reset to zero during these dry cycles. This study was designed to
evaluate if large numbers of razorback sucker larvae that entered a "reset" tloodplain
depression could overwhelm nonnative fish predation, survive, grow and reenter the river
during future spring floods. This situation was experimentally created by pumping water
from the river into a dry floodplain and introducing larval razorback suckers and
nonnative fish into partitioned portions of the floodplain depression. Survival of many
razorback sucker larvae was detected, reaching lengths of up to 115mm by mid-summer.
Continued monitoring of this floodplain during future spring floods will determine when
these razorback suckers enter the river. To continue building on the successes of these
studies, we evaluated survival at lower stocking densities to identify the lower stocking
threshold needed to enable detection of surviving larvae. This evaluation took place in a
"reset" floodplain that connected with the river during spring high flows and was re-
invaded by nonnative fishes. Twelve lI8-acre experimental enclosures were constructed
at the Baeser floodplain site. Larval razorback sucker were stocked into the enclosures at
six densities ranging from 36,000 to 800 larvae/acre. Survival of razorback sucker was
detected in the lowest and the highest density enclosures. However, survival among all
enclosures was quite variable. The highest survival (11 %) was observed in the highest
density (36,000 larvae/acre) enclosure. On average, the four enclosures with the lowest
density of stocked razorback sucker had the lowest estimated survival rate, indicating that
survival of larval razorback sucker in the presence of nonnative predators is density
dependent.

Field application of the 'floodplain reset' approach to enhance survival of bony tail
and razorback sucker in the middle Green River, Utah.

Modde, Tim

u.s. Fish and Wildlife Service, Vernal, UT

Floodplains provide nursery habitat for endangered fishes but also support residual
nonnatives that prevent survival of native fishes. In an effort to reduce predator pressure,
the use of resetting, or draining floodplains, prior to stocking was evaluated as a method
of enhancing larval bonytail and razorback sucker survival.

Bonytail and razorback sucker were stocked into Green River floodplains in June 2003.
Bonytaillarvae were stocked at a rate of 1,430 larvaelha into the three largest floodplains
and adult bonytail were stocked all five floodplains. Razorback sucker were stocked in
all five floodplains between 4 and 16 June at a rate of 1,945 tish/ha. All floodplains were
connected to the Green River by high flows between 21 May and 5 June 2003.
Nonnative tish accessed all study floodplains and reproduced. At the conclusion of the
study, nonnative fishes dominated fish numbers and biomass. However, the minimum
number ofbonytail estimated in floodplains during July ranged between 0 and 112/ha.
Bonytail reproduction was observed in three of the tive floodplains. Razorback sucker
surviving through the end of July was much lower, ranging between 1.3 and 5,6 tishiha.

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