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

All drift net samples were preserved in individually-labelled plastic
bags with 10% formal in. The fi sh were sorted from the debri s as soon as
possible and transferred to labelled glass vials with 5% formalin. Each
larval fish was examined microscopically to identify the specimen, measure
its total length, and determine its stage of development. Identification of
specimens was based on criteria presented by Snyder (1981).

RESULTS OF PARACHUTE STUDY

Species Composition and Relative Abundance

Of the 13 fish species captured at the Parachute Site, only 8 were
found as larvae in drift nets (Fig. 2). In addition to these 8, carp
(Cyprinus carpio), black bullhead (Ictalurus melas), rainbow trout (Sa1mo
airdneri), mountain whitefish (Prosopium williamsoni), and razorback sucker
Xyrauc en texanus) were captured with other gear types, but not as drifting
larvae. Only seine hauls yielded the full complement of 13 species. Three
of the five native species -- f1anne1mouth sucker, b1uehead sucker, and
speck1 ed dace -- made up nearly 78% of the observed shore1 i ne dri ft at the
parachute Site. The remaining two native species -- roundtail chub and
mottled sculpin -- raised the composition to nearly 84%. The single
specimen of razorback sucker was only tentatively identified as the federal
'candidate' species, since the fish was only 21.5 mm TL (total length) and
in the meta 1 arva1 phase of development.

Shore1 ine surface dri ft was assessed with stationary dri ft nets, and
midchanne1 surface dri ft was assessed with tow nets and stationary dri ft
nets affixed to the water intake structure. The midchanne1 benthic zone was
not sampled because of high water velocity and excessively deep sample
conditions. The species composition of the shore1 ine and midchanne1 surface
zones was very similar. Four species (f1annelmouth sucker, b1uehead sucker,
speckled dace, and white sucker) made up the bulk of the catch (84.0% in
shoreline sets and 78.5% in midchanne1 samples). Other species captured in
shoreline drift nets were roundtai1 chub (6%), fathead minnow (3.2%),
mottled sculpin (0.2%), and green sunfish (0.2%). Other species in
midchanne1 samples were mottled sculpin, fathead minnow (each 4.8%), and
roundtail chub (2.4%). Although the composition of drift in the shore1 ine
and midchannel surface zones was similar, the density of shoreline drift was
nearly seven times greater. This difference was significant for samples
between July 18 ~nd September 2, using a randomization test of paired
differences of weekly means (p=O. 026). It appears from these data that the
majority of surface larval drift in the Upper Colorado River occurs along
the shoreline.

Weekly Drift Densities And Patterns

Although larval and juvenile fishes were found in the drift at the
Parachute Site for a period of 9 weeks, nearly 71% were found in a 3-week
period, from July 25 to August 15, 1983 (Fig. 3). Mean weekly densities
over the 9-week period varied from 0.0 to 1.91 fish per 1000 cfw (cubic feet
of wa t e r ) .

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