8022

• UCREFRP Catalog Number: 8022
• Author: Muth, R. T., et al.
• Title: Reproduction and Early Life History of Razorback Sucker in the Green River, Utah and Colorado, 1992-1996.
• USFW Year: 1998.
• USFW - Doc Type: 34,
• Copyright Material: NO

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Conversely, the relative importance of cladocerans, rotifers, and algae tended to decrease as fish
length increased. Most digestive tracts contained debris, which accounted for moderate
proportions of gut contents (10-30% of food volume) for all TL intervals. Debris consisted of
fine, amorphous particles of organic matter, clay particles, and sand grains. Larval razorback
suckers from the lower Green River consumed slightly more algae than those from the middle
Green River. Ephemeroptera larvae were eaten by fish larger than 14 mm TL, whereas copepods,
ostracods, and invertebrate eggs were found in guts of fish smaller than 15 mm TL.

DISCUSSION

Reproduction

We documented annual reproduction by razorback suckers in the Green River through
collections of larvae (N;:::: 2,175) from reaches of the middle or lower river during 1992-1996.
These captures represent the first confirmed reproduction by the species in middle Green River
since 1984 (Tyus 1987) and the first ever records of larval razorback suckers in the lower Green
River. All but 6 of the 1,735 razorback sucker larvae caught in the middle Green River were
from reaches including or downstream ofthe suspected primary spawning area adjacent to the
Escalante Ranch. Based on the few larvae (N = 6) recorded from collections in the Echo Park
reach, reproduction by razorback suckers at the lower Yampa River spawning site during our
investigation appeared minimal, but sampling efforts in the two reaches immediately downstream
of that spawning site were comparatively low.
Earlier first occurrence of razorback sucker larvae in collections from the San Rafael
River confluence or middle Stillwater Canyon reaches compared to collections from the middle
Green River suggests that at least some reproduction occurred in the lower Green River each year
during 1994-1996. Further, the capture of only one larval razorback sucker from the Green
Ri ver Valley reach in 1996 demonstrated at most a minimal level of transport of larvae into the
lower Green River from upstream reaches. This suggests that many ofthe 439 larval razorback
suckers caught in the lowest two reaches had been produced downstream of the Green River
Valley reach. Although aggregations of ripe razorback suckers have not yet been found in the

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