7779

• UCREFRP Catalog Number: 7779
• Author: Tyus, H. M. and J. M. Beard
• USFW Year: 1990
• Copyright Material: YES

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1990]

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GREEN RIVER FISH ECOLOGY

35

to potential interactions of these species with
sympatric endangered Colorado River fishes.

METHODS

Northern pike and walleye were primarily
collected by electrofishing. Sampling was
conducted from April to November 1979-
1981 and trom April to June 1984-1988 in 517
km of the mainstream Green River. The study
area included the mainstream Green River
trom its confluence with the Yampa River in
Dinosaur National Monument to a point 35
km above the confluence of the Green and
Colorado rivers (Fig. 1). The lower 73 km of
the Yampa River was also sampled 1984-1989.
In 1989, spring sampling was conducted in the
lower 73 km of the Yampa River and in the
Green River in a 208-km reach below its con-
fluence with the Yampa River (Fig. 1).
In 1979-1981, fishes were sampled with a
variety of gear, including electrofishing,
seines, trammel nets, and wire traps, depend-
ing on gear suitability; sampling was con-
ducted during pre runoff, runoff, and post-
runoff conditions. In 1984-1988, sampling
included only alongshore electrofishing in the
prerunoff and early runoff period and in-
volved continuous downstream coverage with
a pulsed DC unit. Electrofishing collections
in which all shoreline habitats were sampled
were considered representative collections,
and catches of fishes per hour (C/h) sampled
were recorded. Some opportunistic spring
electrofishing was also conducted in sus-
pected northern pike and walleye habitats;
however, no C/h data were reported for these
samples.
All northern pike and walleye collected
were measured for total length (TL). Location
of capture was also noted. After 1983, all fish
were sacrificed and stomach contents identi-
fied to the lowest possible taxon with the aid of
a 25X binocular dissecting scope. The date,
location, and water conditions at the point of
capture of all females with ripe eggs and fully
developed ovaries were recorded. We also
obtained 49 northern pike and 11 walleye
stomachs trom other workers and identified
their contents.

RESULTS
Abundance and Distribution
Eighty-four northern pike were collected

TABLE I. Catches of adult northern pike (Esox /ucius)
and walleye (Stizostedion vitreum) per hour of elec-
trofishing (C/h), in the Green River, Utah, April-June
1984-1988 (n = number of fish). Upper Green River =
km 337.8-552, lower = km 35-337.7.
River Hours Northern pike Walleye
location fished n CIh n C/h
1984
Upper IOI.2 20 .20 15 .15
Lower 32.3 0 0
1985
Upper 886.9 4 .005 8 .01
Lower 28.9 0 0
1986
Upper 753.2 5 .01 17 .02
Lower 35.9 0 0
1987
Upper 760.1 16 .02 8 .01
Lower 37.2 I .03 1 .03
1988
Upper 441.5 9 .02 0
Lower 43.0 3 .07 1 .02

from 1979 to 1989, including 33 females in
breeding condition (mature ovaries with ripe
eggs). Ripe females were captured in April-
June in the mainstream Green River at water
temperatures of 10-19 C. All pike were con-
sidered adults or large juveniles based on size
(average = 619 mm TL, range 321-1,045 mm;
Carlander 1969). Average catch of north-
ern pike increased 0.05-0.14 fish per hour
from 1984 to 1988 (Table 1). Seventy-eight
percent (n = 59) of the pike were collected in
the upper Green River in 1984-1988, but
many of these (43%) were taken in shallow,
low-velocity, shoreline habitats at the mouth
of Ashley Creek. Northern pike were spotty
in distribution but sometimes abundant in
semiimpounded habitats. Their captures
were often associated with prominent aquatic
and bank vegetation.
We captured two tagged northern pike in
this study. These adult fish (594 and 820 mm
TL) were tagged by Colorado Division of
Wildlife personnel in the Yampa River in 1982
and 1988 (E. J. Wick and T. P. Nesler, per-
sonal communication). One fish had moved
about 110 km between 15 April 1982 and
10 May 1983 when we recaptured it in the
Yampa River at km 18.4. The other pike had