7337

• UCREFRP Catalog Number: 7337
• Author: Valdez, R. A., R. J. Ryel and R. Williams.
• Title: Endangered Fishes of Cataract Canyon
• USFW Year: 1986.
• USFW - Doc Type: The Importance of the Colorado River above Lake Powell to the Colorado Squawfish, Humpback Chub, and Bonytail.
• Copyright Material: NO

In 1985, the yay squawfish handled in the Cataract Canyon area hatched
primarily the first days of July when the water temperature was about 20 C.
It is estimated that, at this time, water temperature in the Yampa and upper
Green Rivers was only 16-18 C, still too cold for spawning by squawfish.
Their relatively early appearance, the computed hatching dates, and the
absence of relatively smaller yay in Cataract Canyon during later sampling
indicates that these fish did not originate from very far upstream. The
confluence of the Yampa River is 345 river miles upstream of the Green-
Colorado confluence and this study area.

Trends In Densities Of YOY. It was observed during the six sample
trips that the number of YOY squawfish captured by seining backwater and
embayments was declining. This phenomenom is best illustrated by computing
mean CPE for each of the four months sampled (July - October) for Cataract
Canyon (RM 21625 - 201.0) (Fig. 5). A dec~ining trend in CPE was seen from
2.20 YOY/10m in July to 0.07 YOY/10m in October (ANOVA shows no
s i g n i fie ant d iff ere nee s be tw e end a tap 0 i n t sat O. 05 ) . E i the r the YO Y
underwent significant mortal ity between the larval and juvenile stages of
development, the fish occupied different habitats, or the fish were
transported further downstream. Sampl i ng downstream to upper Lake Powell
did not reveal greater numbers of yay, so the hypothesis of transport can be
rejected. If these larvae and early juvenile life stages undergo
significant mortality, identifying the source(s) could be a significant
aspect in recovering the species.

Sympatric Species. Field biologists who have encountered great numbers
of nonnative fishes while seining for YOY Colorado squawfish cannot help but
wonder if competition and predation exist that impose added stress on the
endangered species. Such interactions are difficult to assesss afield.
Nevertheless, the presence of large numbers of red shiners, channel catfish,
and fathead minnows in sympatry with young squawfish strongly suggests
interspecies interactions.

Figure 6 is presented to illustrate the relative numbers (no./1Om2) of
the three most numerous species found with young squawfish in seine hauls in
five different habitats. The highest densities of YOY squawfish occurred in
isolated pools (2.14) and backwaters (1.87), but in all cases, red shiners
were the dominant species, particularly in habitats with little or no
velocity; backwaters, embayments, and isolated pools. Most of the squawfish
in isolated pools were found in Cataract Canyon where they would have
eventually died from oxygen starvation and -desiccation. This is a
disturbing statistic that was also reported by Haynes (Upper Colorado River
Researchers Meeting; Grand Junction, Colorado, February 6-7, 1985) for the
lower Yampa River in the proximity of squawfish spawning sites.

Humpback Chub

A total of 11 humpback chub were captured during this investigation,
including 2 adults, 1 juvenile, and 8 YOY. The 8 YOY were identified by the
Larval Fish Laboratory as "possible Gila cyphall since definitive techniques
for identifying YOY Gila have not been developed. All but 2 of the 11 chubs
were captured in Cataract Canyon; the 2 were caught in the Green River at RM
41. 8 and 10.5.

10