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

the middle Stillwater Canyon reach, 76 from the San Rafael River confluence reach, and 1 larva
was caught in the Green River Valley reach.
Capture dates of razorback sucker larvae over all years ranged from 16 May to 21 July in
the middle Green River and from 7 May to 9 July in the lower Green River. In most years, larvae
were first collected 20-30 d after the earliest estimated date of spawning and were usually most
abundant in samples collected before mid-June. Earlier first occurrence of larvae in collections
from the San Rafael River confluence or middle Stillwater Canyon reaches compared to
collections from the middle Green River suggested that razorback suckers reproduced in the
lower Green River each year during 1994-1996.
Estimated initiation of spawning by razorback suckers in each year during 1993-1996
was generally associated with the beginning of spring-runoff flows and was probably triggered by
a suite of interacting environmental cues that could not be detected by analysis of single water
temperature and discharge parameters. Duration of spawning in either the middle or lower Green
River varied among years but usually spanned 4-6 weeks each year. Spawning occurred during
increasing and highest spring flows and encompassed a wide range of mainstem mean daily
discharges (78-696 m3 Is) and instantaneous daily water temperatures (8-21 OC).
Most larval razorback suckers (11-18 mm total length, TL) contained food items, and
mean percent fullness of digestive tracts increased with fish length (ranged from 35 to 65%).
Principal dietary components were chironomid larvae, cladocerans, rotifers, algae, and
miscellaneous debris. Estimated mean daily growth of razorback sucker larvae less than
35 d old collected from either river section during 1993-1996 was lowest in 1994 (0,31 and 0.27
mm TL/d for the middle and lower Green River, respectively) and greatest in 1996 (0.35 and
0.33 mm TL/d). Over all years, specimens from the middle Green River grew 6-21 % faster than
those from the lower Green River. Although food abundance appeared adequate to meet the
minimum nutritional requirements for larval survival, the growth potential of razorback sucker
larvae is greater than we observed. Poor growth can significantly reduce the survival of fish
early life stages if size-dependent processes regulate year-class success. Extremely low survival
was suggested by the apparent disappearance of larval razorback suckers from Green River
nursery habitats by early or mid-July each year.

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