7877

• UCREFRP Catalog Number: 7877
• Author: Anderson, R. M.
• Title: An Evaluation of Fish Community Structure and Habitat Potential for Colorado Squawfish and Razorback Sucker in the Unoccupied Reach ‎(Palisade to Rifle)‎ of the Colorado River, 1993-1995.
• USFW Year: 1997.
• USFW - Doc Type: FA Proj. SE-3,
• Copyright Material: NO

The direct influence offlow on the fish community was studied by comparing density, size and
age structure of two river reaches with flow condition over the two-year study period. Admittedly,
two years of data is insufficient to statistically determine relationships between flows and the fish
community. However, implications made from only two years of fish data were strengthened by the
fact that the fish sampling done in March 1994 was conducted following a very high flow year and
the March 1995 fish sample immediately followed a very low flow year. The minimum flow prior to
fish sampling in 1994 was 1430 cfs, and it was 1200 cfs prior to sampling in 1995. Density and
biomass estimates were very similar between these two years, which suggested that the observed
minimum stream flows had not differentially impacted survivorship of any species or older life stage
in the fish community. A relationship was indicated for peak flow conditions and numbers of young-
of -year fish. In early June 1993 flow peaked at 22,000 cfs (fish were sampled the following March),
while peak flow in 1994 was only 11,000 cfs. Fish larvae and fry numbers were much higher in
March 1995, following the low flow year, compared to fish of this size collected in March 1994. It
was postulated that roundtail chub and white sucker nursery habitat conditions in the main channel
were improved in the 1994 summer compared to 1993, which could explain the increase in number
of small fish in March 1995 compared to March 1994. However, since neither Colorado squawfish
or razorback suckers are currently part of this fish community, no empirical data was available for
studying effects of maxima or minima flows on the habitats of these species. In spite of this, the
author speculated that the minimum flow, which apparently was adequate to protect habitats of the
native sucker and chub (1,200 cfs), would also probably provide adequate protection for adult
Colorado squawfish and razorback sucker habitats.

Insufficient data was collected to determine the impacts of peak flows on habitat availability
for endangered, other native and nonnative fishes. It is believed that razorback sucker exploit low
velocity backwater and flooded habitats during runoff for their nursery and rearing habitats and
therefore were vulnerable when these habitats were severely impacted by channelization activities and
reduced peaked flows: Remaining floodplain ponds and backwaters were found to be dominated by
nonnative species. Further reductions in peak flow, without compensatory channel restructuring,
could likely result in further habitat degradation and concomitant increases in nonnative fish numbers
thereby complicating recovery efforts. Availability of spawning and nursery habitat for Colorado
squawfish was not considered limiting. Adults residing above the diversion dams may chose to move
to downstream spawning sites. Even if squawfish spawn above the dams, e.g. in Debeque Canyon,
larvae would probably display a strong tendency for downstream dispersal to nursery areas. It was
suggested that experimental stocking of small razorback sucker and Colorado squawfish could be
useful for studying effects of peak flow on habitat availability for fish of this life stage.

Key words: Colorado River, fishery assessment, habitat assessment, endangered fish, speCIes
composition, flow recommendations, endangered fish stocking

Vlll

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I