9694

• UCREFRP Catalog Number: 9694
• Author: Hawkins, J., C. Walford, and A. Hill
• Title: Smallmouth bass control in the middle Yampa River, 2003-2007.
• USFW Year: 2009
• USFW - Doc Type: Contribution 154 Larval Fish Laboratory, Colorado State University.
• Copyright Material: NO

1982, only one smallmouth bass was captured among the almost 4,000 juvenile and
adult fish collected and additional boat electrofishing from 1986 through 1988 produced
none (Wick et al. 1985; McAda et al. 1994). There was no detectible reproduction of
smallmouth bass in the river based on the absence of young bass in seine and dipnet
collections of approximately 35,000 small-bodied fish from the lower 121 miles of the
Yampa River in 1981 and 1982 (Wick et al. 1985). Absence of smallmouth bass
through 1988 suggested that escapement was minimal even after high peak-flows in
1983 and 1984.

Fish sampling in several Yampa River reaches downstream of Craig first detected the
species 45 miles downstream of Elkhead Reservoir in 1981, then 70 miles downstream
of the reservoir in 1990 and 1991, and 95 miles downstream of the reservoir in 1992
(McAda et al. 1994). Although relative abundance of smallmouth bass remained low,
the number captured increased from two in 1989, to five in 1990, 14 in 1991, and 49 in
1992 (McAda et al. 1994). Because of their rarity, smallmouth bass were not
considered a problem by Colorado River Basin biologists and managers surveyed
about problematic nonnative fishes in 1990 (Hawkins and Nesler 1991). However,
smallmouth bass abundance increased dramatically in the Yampa River in late summer
1992 after rapid draining of Elkhead Reservoir through the unscreened outlet structure
(Nesler 1995). By 2003, the riverine population of adult small mouth bass comprised
18% of the fish collected at Little Yampa Canyon, 13% of the fish at Maybell, and 5% of
the fish at Lily Park (Anderson 2004). Riverine reproduction was confirmed in 2001
when sub-adult smallmouth bass (< 150 mm TL) comprised 98% of the fish seined at
Little Yampa Canyon and 58% of the fish seined at Maybell (Anderson 2004). In 2005,
Elkhead Reservoir dam construction included a screened notched spillway to reduce
escapement of reservoir fishes. However, on May 17th the screen clogged with debris
and failed allowing unscreened discharge into Elkhead Creek. Construction on the
reservoir was completed in 2006 with a 6.4 mm screened outlet structure to reduce fish
escapement during draw-down events and a redesigned unscreened spillway.

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