Historical changes of a native fish assemblage in an unregulated western river: a case study of nonnative fish invasion.

Authors: John Hawkins, Camerob Walford, Edmund Wick

Year: 2016 (xlviii)

Abstract

The Yampa River, located in northwestern Colorado, flows 205 miles to confluence with the Green River, a major tributary to the Colorado River. It descends from 12,000 to 5,087 ft. elevation and drains > 8,000 mi2 of the Southern Rockies and portions of the Wyoming Basin. The Yampa River is one of the few western rivers with a natural flow regime and is often cited as the last remaining unregulated river in the Upper Colorado River Basin. Although largely unregulated, it has two relatively small rese rvoirs at high elevations on the mainstream (Stagecoach and Catamount) and several other reservoirs on tributaries, all of which contain introduced sportfish. Persistent, but low levels of nonnative fish escapement from those reservoirs had minimal effect on the riverine fish assemblage for years after reservoirs were built. Sampling in the early 1980s revealed a largely intact native assemblage with native taxa comprising over 70% of the large-bodied portion of the fish community. In the mid-1990s, after Northern Pike, Esox lucius, and Smallmouth Bass, Micropterus dolomieu, escaped and established reproducing populations in the Yampa River, we observed drastic changes to the riverine fish assemblage. By 2000, the fish assemblage had changed to the point that natives comprised less than 4% of the fish community in most reaches. We describe the historical and current fish assemblages in the Yampa River, the invasion sequence for Northern Pike and Smallmouth Bass, and how restoration management is informed by understanding how certain reaches have maintained native fishes even in the presence of predatory nonnatives.