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• UCREFRP Catalog Number: 7300
• Author: Tyus, H. M. and C. A. Karp.
• Title: Habitat Use and Streamflow Needs of Rare and Endangered Fishes, Yampa River, Colorado.
• USFW Year: 1989.
• USFW - Doc Type: Biological Report 89‎(14)‎,
• Copyright Material: NO

(Archer and Tyus 1984; U.S. Fish and Wildlife Service,
unpublished data). There is substantial field and
laboratory data showing that Colorado squawfish and
other squawfish species require cleaned cobble surfaces
for successful egg adhesion (Burns 1966; Patten and
Rodman 1969; Hamman 1981). Hamman (1981) noted
hatching of Colorado squawfish larvae from cobble
surfaces. The need for cleaned cobble and boulder
substrates is supported by the repeated spawning of
Colorado squawfish following peak flows and peak
sediment transport (Figs. 10 and 11).
Three hundred eight Colorado squawfish (including
208 ripe adults) were collected by Service biologists at
the two confirmed spawning sites in the Yampa and
Green rivers during spawning periods from 1981 to 1988
(Table 5). The fish were classified ripe if milt or eggs
could be expressed from the vent with light hand
pressure on the abdomen. Ripe males (N = 194) were
bronze colored and heavily covered with breeding
tubercles. Twenty-five additional fish were classified as
suspected males because of the presence of these two
characteristics, even though milt could not be expressed.
Robust tuberculation in ripe male Colorado squawfish
was also noted by Seethaler (1978) and Hamman (1981).
Only 14 ripe or spent female Colorado squawfish were
positively identified based on the expression of eggs.
However, 42 additional fish were classified as suspected
females because of their-large size and the absence of
heavy tuberculation and bronze coloration. These data
indicate a paucity of adult female Colorado squawfish
in the Green River system, which may be due to
differential mortality (Tyus et al. 1987). A high male to
female ratio was previously noted for both Colorado
squawfish (Seethaler 1978) and other squawfish species
(Patten and Rodman 1969).
Larvae and Postlarvae
Larval Colorado squawfish emerge as sac fry from
cobble bars in the Yampa Canyon and drift downstream
(Tyus et al. 1982b; Haynes et al. 1984; Fig. 4) to
concentrate in shallow backwater habitats in the Green
River (Tyus et al. 1982b, 1987; Fig. 12). About 16 days
are required for transport of newly emerged Colorado
squawfish fry to the mouth of the Yampa River from the
midpoint of the spawning grounds (RK 26.4-29.1; U.S.
Fish and Wildlife Service, unpublished data). From 1979
to 1981, peaks of abundance of young Colorado
squawfish were noted to occur about 160 km
downstream of the Yampa River spawning reach (Tyus
et al. 1982b). Presumably, young fish use river current
Table 5. Spawning collections of Colorado sgicatvfsh, Yampa artd Green rivers, from June to August, 1981-88.

Year

River

IVa
Ripe
nb TLc Males
Suspectedd
n TL Females
Ripe Suspectede
n TL n TL
1981 Yampa 35 20 538 6 528 1 779 2 748
1981 Green 4 1 478 0 - 0 - 0 -
1982 Yampa 1 1 547 0 - 0 - 0 -
1982 Green 11 6 509 0 - 0 - 2 642
1983 Yampa 22 13 596 1 560 3 722 2 662
1983 Green 14 11 569 0 - 0 - 1 625
1984 Yampa 38 20 560 1 510 3 666 11 714
1984 Green 29 14 574 4 544 1 750 6 671
1985 Yampa 13 10 571 0 - 1 723 1 639
1985 Green 36 24 574 5 549 0 - 2 626
1986 Yampa 12 ~ 7 535 0 - 1 485 3 702
1986 Green 24 22 541 1 559 0 - 1 781
1987 Yampa 19 13 539 1 510 0 - 4 621
1987 Green 25 16 533 4 520 0 - 4 666
1988 Yampa 5 4 544 0 - 0 - 1 725
1988 Green 20 12 563 2 588 4 565 2 684
Total Yampa 145 88 555 9 528 9 683 24 693
Total Green 163 106 555 16 546 5 602 18 667
a N =all fish captured on spawning grounds.
b n =ripe or suspected male and female.
c TL =average total length in millimeter.
d Heavily tuberculated and bronze-colored fish, but with no expressible sex products.
e Large fish with little bronze coloration, little or no tuberculation, and large vent.
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