7856

• UCREFRP Catalog Number: 7856
• Author: Muth, R. T. and D. E. Snyder
• Title: Diets of Young Colorado Squawfish and Other Small Fish in Backwaters of the Green River, Colorado and Utah
• USFW Year: 1995
• USFW - Doc Type: The Great Basin Naturalist
• Copyright Material: YES

102 GREAT BASIN NATURALIST [Volume 55
TABLE 2. Diet overlap by total-length (TL) intervals (mm) between young-of-year Colorado squawfish and eight other
confluence with the Yampa River, Colorado and Utah. Overlap values were calculated using Schoener's (1970) index
asterisk (*).
Fathead
Red shiner Sand shiner minnow
TL of Upper reacha Lower reach- Upper Lower Upper
Colorado
squawfish 11-20 21-30 31-40 >40 11-20 21-30 31-40 >40 21-30 31-40 >40 21-30 31-40 >40 21-30 31-40 >40
<11 0.59 0.54 0.40 0.43 0.42 0.49 0.50 0.47 0.46 0.47 0.35 0.53 0.55 0.52 0.40 0.37 0.38
11-20 0.49 0.45 0.43 0.31 0.63* 0.53 0,47 0.38 0.37 0.31 0.14 0.49 0.45 0.38 0.18 0.15 0.15
21-30 0.55 0.51 0.53 0.41 0.74* 0.57 0.51 0.42 0.42 0.43 0.27 0.49 0.47 0.38 0.23 0.19 0.23
31-40 0.40 0.39 0.40 0.27 0.73* 0.57 0.52 0.42 0.35 0.29 0.12 0.50 0.45 0.38 0.17 0.13 0.15
>40 0.39 0.37 0.39 0.35 0.65* 0.47 0.44 0.37 0.34 0.28 0.12 0.53 0.47 0.37 0.17 0.13 0.14
al3pper reach = confluence of Green and Yampa rivers at RK 555 (river kilometers above confluence of Green and Colorado rivers) in Echo Park, Dinosaur National Monument,
CO, to Sand Wash, UT (RK 346); lower reach = Sand Wash to Turks Head, Canyonlands National Park, UT (RK 35).
Alternatively, greater diet overlap and vari-
ety in the lower reach might have been a
reflection of a difference in backwater avail-
ability between the upper and lower reaches.
Tyus and Haines (1991) reported about 150%
more backwaters per kilometer in the upper
than lower reach. Fishes in the lower reach
might have been more crowded in available
backwaters, resulting in greater shared use
and broader intraspecific use of available food.
McAda and Tyus (1984) attributed reduc-
tions in diet overlap between Colorado squaw-
fish >40 mm TL and red shiner or channel
catfish to decreased consumption of immature
dipterans and increased consumption of fish
by Colorado squawfish. However, Ruppert et
al. (1993) reported fish larvae in digestive
tracts of 15% of adult red shiner (36-79 mm
TL) from ephemeral shoreline embayments
near confluence of the Green and Yampa
rivers. Unlike our study, they sampled on a
diel basis and killed fish with an overdose of
anesthetic before preservation to minimize
possible regurgitation. Their results suggest
that high diet overlap between young
Colorado squawfish >_ 40 mm TL and red
shiner might reoccur or continue with larger,
piscivorous red shiner. Although we docu-
mented high diet overlap between young
Colorado squawfish >10 mm TL and other
fishes in backwaters of the Green River, espe-
cially channel catfish (Table 2), only red shiner,
because of its extreme abundance (Haines and
Tyus 1990), is likely to be a serious competitor
for food with young Colorado squawfish. Red
shiner has often been implicated in decline of
native fishes of the American Southwest (e.g.,
Minckley 1973, Greger and Deacon 1988,
Rinne 1991).
Competition might also be a factor between
smaller specimens of both Colorado squawfish
and other fishes. Few specimens <21 mm TL,
other than red shiner and fathead minnow
11-20 mm TL, were available for comparisons
with Colorado squawfish. However, as for
smaller Colorado squawfish, zooplankton
would likely be an important component of
their diets (Joseph et al. 1977), and corre-
sponding overlap values would be high, espe-
cially for specimens < I I mm TL. Although
dense populations may develop in backwaters,
zooplankton may be limited under certain
conditions because plankton communities in
rivers are subject to dramatic spatial or tempo-
ral fluctuations in abundance and diversity
(Hynes 1970, Welcomme 1985, Ward 1989). In
support of this generalization, Grabowski and
Hiebert (1989) reported that zooplankton den-
sities were higher in backwaters than in main-
channel habitats within the upper reach and
documented both spatial and temporal fluctu-
ations in zooplankton abundance. They also
observed higher concentrations of zooplank-
ton in more confined backwaters than those
with a broad connection to the river and sug-
gested that densities were influenced by
extent of water exchange between backwaters
and the main river.
In conclusion, we found high diet overlap
between YOY Colorado squawfish and several
small size groups of other fish species in Green
River backwaters. Because of the extreme
abundance of red shiner, we speculate that
diet overlap could result in food competition
11