Download Pikeminnow, Ptychocheilus lucius, is a torpedo

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.