9319

• UCREFRP Catalog Number: 9319
• Author: Mueller, G., M. Horn, Q. Bradwisch and L. Boobar.
• Title: Examination of Native Recruitment and Description of the Fish Communities Found in the San Jan and Colorado River Interface Zones of Lake Powell, Utah.
• USFW Year: 2001.
• USFW - Doc Type: 01-159,
• Copyright Material: NO

has probably not occurred very frequently based on the recent estimates of the
razorback population size and age structure on the middle Green River.
Based on capture/recapture of adults between the Yampa River
confluence, the farthest upstream spawning location, and the Duchesne River,
the downstream extent of nursery and feeding areas on the middle Green
River, it is estimated that about 500 adult fish remain in the middle Green
River population (Modde et al. 1996). Since closure of Flaming Gorge Dam
in 1962 until about six years after the high water years 1983-1986, there was
little detectable evidence of new recruitment into the Green River adult
population. Minckley (1989) aged ten razorback sucker captured from the
middle Geen River in the early 1980s and found them to average 29 years of
age. Spawning congregations of adults were sampled for the last 20 years.
Recently, recruitment of a few young adults into the spawning
population has been associated with the flow years 1983, 1984, and 1986,
when high peak runoff flows resulted in inundation of bottomlands adjacent to
the river. The capture of these young recruits occurred in 1990-1993. Based
on their size and growth increments between recaptures, their years of origin
were back-calculated to the 1983-1986 high flow years (Modde et al. 1996).
Only since 1993 (in association with this study) were larvae sampled
regularly through a standardized program implemented to evaluate production
of young (Muth, 1995). This program revealed that larval production is
highly variable year to year. The razorback larval sampling program offered
encouragement that good numbers of young are produced occasionally.
Loss of connectivity between the river and suitable floodplain nursery habitat
is suspected of contributing to recruitment failure. This hypothesis has been
supported recently in the finding of 28 naturally recruited juvenile razorback
in 1995 (Modde, 1996a) and another 52 young razorback in 1996 as a result
of reconnecting Old Charlie Wash to the river during spring runoff (T.
Modde, 1997, pers. comm., USFWS Vernal Field Office). Old Charlie Wash
is a specially modified floodplain habitat with fish harvest facilities.
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