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

film, and then covered with immersion oil for aging. Daily increments in each otolith were
counted three times on separate occasions at 1000X magnification, and capture date, fish length,
and otolith diameter were unknown, Counts were averaged for each larva to arrive at an
estimated age in days posthatching. Horn (1996) determined that the first increment in otoliths
of larval razorback suckers forms at hatching, and increment deposition proceeds at rate of
approximately one increment per day.
Spawning dates for razorback suckers in the middle or lower Green River each year
during 1993-1996 were estimated by subtracting estimated incubation times of embryos from
hatching dates of larvae. The hatching (date for an individual larva was established by subtracting
its posthatching age from its date of capture. Temperature-dependent incubation times of
embryos at ambient Green River temperatures on and closely preceding hatching dates were
estimated using data presented by Marsh (1985) and Bozek et al. (1990) for captive embryos.
Mainstem water temperature and discharge parameters selected for association with
spawning dates in each river section (middle or lower) and year included (1) mean (range)
discharge and temperature during the spawning period; (2) degree days, which was the sum of
recorded instantaneous daily water temperatures between 1 January and the earliest date of
spawning; (3) days ~ lOoC or ~ l40C, which were the number of days between 1 January and the
earliest date of spawning that recorded instantaneous daily water temperatures equaled or
. exceeded each respective threshold; and (4) days before peak discharge, which was the number
of days between the earliest date of spawning and the highest recorded mean daily river
discharge. Missing temperature data were estimated by linear interpolation, For this analysis, we
assumed that razorback sucker larvae caught in either the middle or lower Green River had been
produced locally.
Daily gain in total length (growth) of an individual aged razorback sucker larva between
hatching and date of capture was estimated by subtracting an approximate mean TL at hatching
of8.0 mm (Minckley and Gustafson 1982; Marsh 1985; Snyder and Muth 1990) from its total
length at capture and dividing by its age. Mean daily growth was calculated for larvae caught in
the middle or lower Green River in each year.

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