7729

• UCREFRP Catalog Number: 7729
• Author: Valdez, R. A., W. J. Masslich and A. Wasowicz.
• Title: Annual Summary Report - 1990\
• USFW Year: 1991.
• Copyright Material: NO

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

minnow (103.7) and sand shiners (102.4). Native species represent the next four highest catch rates
and included, 14.7 for flannelmouth sucker, 8.6 for round tail chub, 6.0 for speckled dace and 2.1 for
bluehead sucker.

Based on catch rates, most species seemed to prefer backwaters. Only specked dace and channel
catfish were caught at higher rates in riffles and side channels, respectively (Table 23). However,
comparison of species composition from 1990 seining data must be made with discretion due to the
disproportionate sample effort between backwaters and all other habitat types.

4.2.4 Fish Species Composition - Past and Present

In order to address Objective 1 of this study, a comparison was made between the results of the
USFWS Dolores River study of 1980-81 and the 1990 results of this BIO/WEST investigation. Tables
24 and 25 present comparisons of species composition by subreach. These preIiminary comparisons
are presented as a means of indicating possible changes that have occurred in the Dolores River
within the past 8 years. Since BIO/WEST's data represents only one year of effort, the effect of
inherent variation within fish populations could not be assessed. Additionally, differences in gear
types, efficiency and methods used by the two studies could not be evaluated and may account for
some of the differences between the two data sets.

Differences in composition of fish species captured in gill nets, trammel nets and by electrofishing
are presented in Table 24. Apparent increases in the relative numbers of red shiners and fathead
minnows in Subreach 1, as well as changes in composition of these species in other subreaches, are
probably due to high variability associated with collecting these smaller species with the represented
gear types. Generally, no major changes in fish species composition appear to have occurred in the
8 years between the two studies with the possible exception of channel catfish. Substantially lower
catch rates for this species in Subreaches 1 and 2 suggest that abundance of channel catfish in the
Dolores River has decreased since 1981. Although changes in composition appear to have occurred
in Subreach 4, limited sample sizes from both studies make it difficult to drawn conclusions about fish
composition in this reach.

Table 25 presents a comparison of species composition for fish captured by seining during the two
studies. With the exception of compositional shifts between sand shiners, red shiners and fathead
minnow, seining data also suggest very few changes in species composition in the Dolores River since
1981. The shift in composition between red shiners, sand shiner and fathead minnows probably
represents natural variation in populations of these prolific species.

A comparison of combined catch rates for gill nets, trammel nets and electrofishing for samples that
were conducted during similar times of year for the two studies is presented in Table 26 Differences
in catch rates for the smaller species such as red shiners, fathead minnows, red shiners and speckled
dace are probably indicative of differences in gear type, efficiency and methodology between the two
studies rather than actual differences in the densities. A comparison of catch rates for large species
show little disparity between the two studies that can not be explained by either natural population
variation or inherent differences between the studies.

Table 27 presents a comparison of seining catch rates for similar sample periods for the two studies.
Substantial differences exist between the catch rates of roundtail chub for the spring sample (0.13 in
1990 vs. 59.8 in 1980-81) and flannelmouth sucker for the summer sample (38.2 in 1990 vs. 1.2 in

10