8068

• UCREFRP Catalog Number: 8068
• Author: Bestgen, K. R., R. T. Muth and M. A. Trammell.
• Title: Downstream transport of Colorado squawfish larvae in the Green River drainage
• USFW Year: 1998.
• USFW - Doc Type: temporal and spatial variation in abundance and relationships with juvenile recruitment.
• Copyright Material: NO

1
1

1
1

1

1

stained by debris pigments. This protocol recovered 100% of a known number of larvae (range
9-10) from five experimental samples.
Laboratory procedures. --Colorado squawfish larvae (< 20 mm TL) captured in drift nets
were measured to the nearest 0.1 mm TL. Ages of selected specimens collected in 1990-1992
were estimated by counting daily increments in otoliths. Counts of otolith increments, which
were first formed at hatching, were estimated two to four separate times in the left sagitta or
lapillus and averaged (see Bestgen and Bundy [ 1998]), for details on aging techniques). Otoliths
from all but 9 of 2537 fish sampled in 1991-1992 were readable. Laboratory studies showed that
ages of Colorado squawfish reared in fluctuating temperatures of 18, 22 and 26°C for up to 165 d
post hatching were estimated without bias (Bestgen acid Bundy 1998). Therefore, age
underestimation of wild fish was discounted as a source of error.
Initiation of reproduction was estimated by subtracting the average age of larvae at
capture (6 d, Bestgen 1997) and average incubation time of fertilized eggs at 18°C (6 d, Bestgen
and Williams 1994) from the date that larvae were first captured in drift nets set in the lower
Yampa River and the lower Green River. Date of cessation of reproduction was similarly
estimated by subtracting 12 d from the date that the last larvae were captured at each sampling
site. The first peak in reproductive activity was qualitatively determined by subtracting 12 d
from the first substantial increase in capture of larvae in drift nets. Most temperature data for the
Yampa River from 1990-1996 were recorded just upstream of the Green River with a
continuously recording thermograph (G. Smith, pers. comm., U. S. Fish and Wildlife Service,
Denver, Colorado). Lower Yampa River data were estimated in 1993 and 1994 by adding 2.1 °C
to temperature measurements from an upstream gauge at Government Bridge upstream of
Maybell, Colorado. That value was the average difference between the two stations in the month
of June in 1991, 1992, 1995, and 1996. Degree-day accumulation in the lower Yampa River was
estimated from temperature data collected near Jensen, Utah because this was the most
geographically proximate gauge with a full year of records. Most lower Green River temperature
data were estimated by adding 2.3°C to temperature measurements from an upstream gauge at the
Ouray National Wildlife Refuge (G. Smith, pers. comm.). That value was the average difference
between the two stations in the month of June in 1992. All 1991 and 1995 temperature data and
degree-day accumulation data for all years were from instantaneous records collected by the U.S.
Geological Survey near the town of Green River, Utah. In general, we avoided use of data
6