9591

• UCREFRP Catalog Number: 9591
• Author: Nesler, T. P.
• Title: Interactions Between Endangered Fishes and Introduced Gamefishs in the Colorado River, Colorado, 1986-1991.
• USFW Year: 2002.
• USFW - Doc Type: 91-29,
• Copyright Material: NO

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collected and the perceived need to add sampling effort in backwaters to capture larger fish.

Seine sampling was conducted using a 1.2 x 4.6 m beach seine with 3.2 mm ace mesh.
The 30-55 mile reach was stratified into 5-mile segments within which the first and second
backwaters encountered in each segment were sampled with two seine hauls per backwater.
Minimum depth of any sample should be 0.3 m, and area sampled had to comprise approximately
25 % of the total area of the backwater. Width and length of each seine haul was recorded.
Within the seine sample area, maximum depth was recorded, and also depth halfway between ~h
shoreline and the maximum depth point. Other data recorded included river mile location,
backwater type (main or side channel), backwater length, backwater and mainchannel temperature,
and maximum depth of the backwater. In 1988, if the backwater sampled was triangular or oddly-
shaped, measurements were also made across the mouth, mid-section and tip to better approximate
surface area (backwater length x average width). Total area seined and total area of backwaters
sampled were taken for each year from McAda et al. (1994: Table B-4, Appendix E-Table B-1).

All fish collected per seine haul were examined briefly in the field to identify and
enumerate common, nongame fish species, and release these fish back into the river. All gamefish
identified were separated and kept for length measurement and stomach samples. Stomachs were
removed from larger fish (>25 mm total length) and preselVed in 10% formalin solution.
Smaller fish were preseIVed whole. The remainder of the fish sampled were preselVed in formalin
for identification or screening at the CSU Larval Fish Lab in Fort Collins, Colorado. Only the
first seine sample in each backwater sampled within ISMP and the 15-mile reach was processed
at the Larval Fish Lab in Fort Collins, Colorado for total enumeration of all fish species. Apart
from the tw~seine sample requirement of ISMP, extra seine hauls were made in some backwaters
if the presence of gamefish species warranted the effort to increase sample size. Stomach contents
were analyzed for the number and taxonomic identity of all fish species ingested. Other food
items were identified only to Order or Family when possible, but not enumerated.

Adult fish monitoring results in the study area acquired through ISMP shoreline
electrofishing from 1986-1988 indicated a lack of largemouth bass in mainchannel habitats. Adult
largemouth bass, among other adult gamefish predators, were suspected to occur in backwater
habitats in the Colorado River, but were being missed by ISMP sampling. In 1989 and 1990,
sampling was expanded to include adultllarge fish sampling trips in April and July using a block
and shock technique. This sampling technique involved blocking of a backwater where it joins
the river channel with a trammel net (150 mm mesh outer walls, 25 nun mesh inner panel). With
the blocking net in place, the backwater habitat was sampled using electrofishing equipment
(Coffelt VVP-15, 5000 watt generator, 25 em diameter stainless steel sphere anodes) mounted on
a flat-bottomed john boat. Output power was lowered during an initial pass to induce fish to
attempt to leave the backwater to avoid the electric field and become entangled in the trap net.
Output power was increased on subsequent passes to increase attraction, stunning and dip net
capture of remaining fish. During the approach to the blocking net under increased output
voltage, power was decreased near the net to minimize electrofishing injuries to fish already
captured in the blocking net. The target of this sampling included all juvenile and adult fish

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