8216

• UCREFRP Catalog Number: 8216
• Author: Crowl, T. A. and L. Lentsch.
• Title: Estimating Northern Pike Predation on Colorado Squawfish in the Middle Green River
• USFW Year: 1996.
• USFW - Doc Type: A Bioenergetics Approach.
• Copyright Material: NO

1

n
1
Growth of northern pike was estimated by aging, measuring (total and fork lengths, mm), and
weighing (g) specimens collected during October 1993, May and -June 1994 in the Green River.
Fish were captured with electroshockers and trammel nets. Ages were interpreted by counting
annuli on magnified projections of scales mounted between microscope slides. Annuli were
clearly defined, because rapid growth during the warm season was followed by severe winter
conditions. Although the sample size was small (N=26), the pattern of weight-at-age was clear
(Table 1) and compared favorably with growth patterns from a neighboring population upstream
in the Yampa River (T. Nesler pers. comm.). In model simulations, spawning losses (7.7% wet
on the difference in length-weight regression in June versus those in May and October.
body mass) were subtracted from pike larger than 660 mm on 15 June (simulation day 45), based
The proportional contribution of different prey species in the diet was estimated from stomach
samples collected from two sub-populations within the upper basin. Fish were sampled during the
spring and fall, 1987-89 in the Yampa River, and in May 1993 -June 1994 in the Ouray reach,
Green River. Stomach contents were identified to species or the lowest possible taxonomic level.
We measured total or standard prey lengths (mm) and wet weights (mg). Diet composition was
computed separately for each age class of northern pike, and was used to partition daily rations
into estimated consumption rates on Colorado squawfish and other prey in the bioenergetic
simulations. Since nearly all prey items in the stomachs were cyprinids, we
6