7334

• UCREFRP Catalog Number: 7334
• Author: Valdez, R. A., J. G. Carter and R. J. Ryel
• Title: Drift of Larval Fishes in the Upper Colorado River
• USFW Year: 1985
• USFW - Doc Type: \
• Copyright Material: YES

Ira

Small-mesh dri ft nets des i gned to capture dri ft i ng ichthyofauna were
first used in the rivers of the upper basin to locate spawning areas of
endangered fish species. Haynes et al. (1984) first conducted drift studies
in the Upper Colorado River within Colorado and in the Yampa River, starting
in 1979; and Radant et ale (1983) used drift nets in the White River as
part of the Colorado squawfish investigations within the state of Utah. The
increase in water withdra\'Ials, flow modifications, and hydropower projects
in the Upper Colorado River Basin prompt the need to more thoroughly
describe the phenomenon of ichthyofaunal drift in that system.

METHODS

This paper describes two independent studies of larval fish drift in
the Upper Colorado River. The first, conducted in 1983, was designed to
assess the potential impact of a midchannel water intake structure near
Parachute, Colorado (Fig. 1). The second study was conducted in 1984, about
35 river miles downstream, to assess the potential impact of a small,
shoreline, hydropower project near Palisade, Colorado. The findings of the
two studies are synthesi zed in thi s treatment because of the similarity in
results.

Both studies were conducted from mid-June to early September. Five
gear types were used at each study site, including large-mesh seines to
document adult and juvenile species composition; small-mesh seines, dip nets
and light traps to assess the composition of non-drifting larvae and young-
of-the-year along shorelines and in shallow areas; and drift nets to assess
species composition and relative densities of drifting larvae. Tow nets
were also used at the Parachute Site to assess midchannel surface drift, and
boat el ectrofi shi ng was used at the Pal i sade Si te to further document adul t
and juvenile species composition.

The drift nets used were made of 560 micron mesh Nitex plankton
netting. Each net had a rectangular opening of 30.48 x 45.72 cm and a
length of 3 m, which tapered to a removable collection cup. Each net was
set with the open end facing directly into the current. The top of each net
was even with the surface of the water, except for paired, stacked sets to
assess vertical drift distribution. Time, water temperature, and velocity
were recorded when each dri ft net was set and removed. The total vol ume of
water filtered by each net was computed to provide estimates of larval
densities, according to the following equation:.----

Ne = (N/(A x Vm x T)) x 1000

where: Ne = estimated number of drifting fish in 1000 cubic feet of
wa ter ,
N = actual number of fish recovered from the drift sample,
A = area of net opening in square feet (1.5),
Vm average water vel oc ity in feet per second at the net
opening during the set, (initial + ending)/2, and
T = total time of set in seconds.

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