7137

• UCREFRP Catalog Number: 7137
• Author: Jones, R. L. and H. M. Tyus.
• Title: Recruitment of Colorado Squawfish in the Green River Basin, Colorado and Utah 1979-1984.
• USFW Year: 1985.
• USFW - Doc Type: Vernal Utah.
• Copyright Material: NO

14.
An analysis of these data suggests that elevated flows during the
primary nursery period (Aug-Sept) are detrimental to the survival of
larvae. This supposition is supported by a regression of C/E values on
mean August and September discharges at the Jensen gauging station for
all years of the study (1979-84). C/E values were negatively correlated
(r=-0.89, p=0.05) with combined August and September discharge (Figure
11). About 80% of the variation in C/E was attributable to the mean
daily discharge during August and September. This is possibly because the
quantity of backwater habitat available to larval Colorado squawfish
also varies inversely with discharge. In 1983, FWS measured the area of
backwater habitat at three sites on the Green River when flows released
from Flaming Gorge Dam were 800 cfs and 4000 cfs (add inflow of tributaries
for study site discharge). The surface elevation of the river changed
by 1.79 ft. at RM 315, 2.12 ft. at RM 291, and 1.94 ft. at RM 250.
Although the resultant change in area of backwater habitat was difficult
to quantify (in every case in which a backwater area was measured under
800 cfs flow there was flow-through at 4000 cfs) there was a marked
change in available habitat between the different flows.
River discharge during the nursery period may also affect the average
size of Colorado squawfish larvae collected in FWS fall monitoring
program samples. Table 3 gives the mean total length of larvae captured
in both the upstream (Strata E & F) and downstream .(Strata A & B) nursery
areas and a combined mean of both areas. Although these data show.no
consistent differences in growth rate between the two areas, in years of
high discharge (1983-84) the mean total length of larvae at the end of
the growing season was reduced by 27% from that recorded in years of
more favorable flows (1979-80). As in the case of C/E, mean total
length was negatively correlated (r=-0.87, p=0.05) with mean August and
September discharge (Figure 12.).
Information about fish community structure, collected during CRFP larval
sampling and fall monitoring program, may help explain the effects of
various flow regimes on such factors as interspecific competition and
predator/prey dynamics. These data are currently being analyzed but
results and conclusions are not available as of this time.
Based on data collected by FWS and Colorado Division of Wildlife researchers
it is suggested that two important relationships between river discharge
and Colorado squawfish reproductive success exist. These are: a) high
flows in late spring may enhance spawning success (Haynes and Muth 1984)
and; b) abnormally high flows in late summer are detrimental to larval
survival in nursery areas. The best hydrographic conditions for survival
of larvae in the green river was obtained by reviewing conditions which
existed in 1980, since this was the year of highest larval survival on
record. It should be noted, however, that since FWS fall monitoring
program data extends back only six-years all that can be stated with
authority is that 1980 was the best among those years (optimum conditions
for Colorado squawfish reproduction can only be determined by further
research).