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State’s Largest Fish a Shadow of its Former Self Jeff Mitton Natural Selections (Appeared July 7, 2005 in the Daily Camera) Centuries ago, before western rivers were choked by dams, the big, deep channels of the Green, Colorado and San Juan Rivers were stalked by an immense predator. The pikeminnow, formerly referred to as the Colorado squawfish, reached lengths of six feet and weights of 80 pounds. Pikeminnow, Ptychocheilus lucius, is a torpedo-shaped fish with an olive-green and gold back, silver sides, and white belly. Pikeminnows are denizens of both the fast, deep channels of the main rivers and the quiet backwaters. Typically, they are found in relatively warm, muddy water and do not venture up into the smaller creeks. A century ago, they were so abundant that they supported commercial fisheries in the Colorado, Green, Yampa, White, Gunnison, and San Juan Rivers. The pikeminnows were the top predators in the big western rivers. They are referred to as piscivorous, for they prey on all fish smaller than themselves. But historical accounts suggest that they would also take small mammals swimming in the rivers, for early fishermen used mice and even rabbits for bait. They have been reported to gulp ducklings. Adult pikeminnows hunt in the main channels of the big rivers and in the more quiet backwaters. But when it is time to mate, they migrate as much as 200 miles upriver, to release their eggs in placid water flowing gently over sand bars. The larvae drift downstream, and the more fortunate larvae drift into an eddy or backwater, where they feed on insects and other invertebrates. When the pikeminnow have grown, they move out into the faster water to hunt fish. Pikeminnows are the victims of changing rivers. Numerous dams have been built to tame the large rivers, and to divert water to agricultural fields. Those dams block the traditional migration routes of pikeminnows, and fragment the range of the species. Unfortunately, dams impose more changes than mere barriers. Modern, regulated flows serve agriculture and demand for electricity, not the adaptations of fish. The fish no longer experience the spring floods and the slack flows of late summer. Furthermore, dams release cold water from deep in the reservoir, not the warm water that pikeminnow are adapted to. At least 40 species of fish have been introduced into the Colorado River drainage system, and several species have pushed pikeminnow to the edge of extinction. In a fish-eat-fish world, new neighbors can be the best of news, or the worst. Northern pike, smallmouth bass, and green sunfish are piscivores that really cannot contend with the continuous flow of the main channel, so they congregate in the eddies and backwaters. There they decimate the population of young pikeminnow. However, for the few pikeminnow that escape predation, an increase in size transforms these prey into the predators; adult pikeminnow eat northern pike, smallmouth bass, and green sunfish. Declared endangered in 1967, penned in by dams, and harried by non-native predators, the pikeminnow looked like it might disappear altogether. But restoration efforts have sustained populations, and the species was downlisted to threatened in 1998. Today, 15,000 small pikeminnow are growing in the John Mumma Native Aquatic Facility near Alamosa. When they are about six inches long, they will be released into the Colorado River and the Gunnison River. It appears that pikeminnow are now self-sustaining and increasing in the Yampa River. But the enormous fish harvested by early settlers are gone; pikeminnow now attain lengths of three feet. Long ago, pikeminnow stalked bonytail, humpback chub, and razorback sucker. But now these prey species are endangered, and so rare that they cannot sustain the pikeminnow. Pikeminnow are now making a comeback, eating the pike, green sunfish and smallmouth bass that pushed them to the brink of extinction.