9489

• UCREFRP Catalog Number: 9489
• Author: Mueller, G.
• Title: The Role of Stocking in the Reestablishment and Augmentation of Native Fish in the Lower Colorado river Mainstream ‎(1998-2002)‎.
• USFW Year: 1998-2
• USFW - Doc Type: 03-288,
• Copyright Material: NO

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Executive Summary

The Colorado River has experienced dramatic physical and biological change. Rated as the fifth largest
river in the USA by volume, today its waters seldom reach the sea. Water diversions gradually reduce its flow
to a point where its last remaining waters are diverted at Morales Dam leaving nearly 100 Ian of historic
channel dry. In contrast, lower basin storage reservoirs cover 36% ofthe historic channel. Remaining portions
ofthe flowing river have been channelized and straightened to a point where it now resembles a large canal.
Levees, mechanical dredging, and the natural forces of erosion have degraded the river channel nearly 2 m in
some locations, isolating it from its floodplain and affecting local water tables. The river no longer functions as
a natural stream system characteristic of spring run-off, summer spates, and droughts. Today it serves as a
water storage and conveyance system to meet human needs.
Physical change has been severe, but not as devastating as the biological pollution. More than 80 nonnative
fish species have been introduced to the lower basin. Today, over 20 fish species have established, many
forming economically important sport fisheries. As these alien species expanded their range, native
communities rapidly declined and disappeared from much oftheir historic range. By 1930, most had become
rare. The last remnant populations ofbonytail, razorback sucker, and Colorado pikerninnow in the lower basin
were taken downstream of Davis Dam during the 1960's and 1970's. Today, Colorado pikeminnow, and it
appears, wild bonytail are extixpated downstream of Glen Canyon Dam, and wild razorback suckers are
extremely rare. The Colorado River and its fish assemblage is a totally different ecosystem than it was a
century ago.
State and federal agencies have been attempting to reestablish native communities for nearly three decades.
More than 12 million razorback suckers, most of them small, were stocked between 1981 and 1991. Few of
these fish survived and during the past decade managers have switched to stocking larger suckers to improve
survival. Since 1995, nearly 18,000 bonytail and 30,000 large razorback suckers have been stocked in Lake
Havasu. There was also a single stocking (611) of flannelmouth suckers in 1976. These programs have
produced mixed results. The single introduction of flannelmouth sucker has resulted in a thriving community,
estimated at more than 4,000 fish. This success spirited hopes by many that other natives would respond
similarly but unfortunately, that has not occurred.
Initial stocking returns suggest that stocking survival ofbonytail and razorback sucker is relatively poor
(< 12%) and the absence of any detectable recruitment indicates present reintroduction efforts are falling short
of anticipated survival or potential recovery. In contrast, the single introduction of wild flannelmouth sucker,
out-performed millions of hatchery produced razorback sucker. This suggests hatchery reared fish may be
inferior to wild fish in terms of survival skills, which has been found to be the case for terrestrial animal
introductions. A review of culturing, stocking, and repatriation techniques is warranted which examines ways
to better prepare fish to convert to natural foods, recognize predators, and be physically conditioned to cope
with currents and hopefully avoid or escape predators.
Comparison of flannelmouth sucker success and the razorback sucker's failure provides compelling
evidence that helps explain the dramatic physical habitat changes that have occurred and the possible role of
habitat selection and predator communities. It mimics conditions observed in portions of the upper basin where
flannelmouth suckers are still common but razorback suckers have been extirpated. Both sucker species are
successfully spawning in the lower basin, however, recruitment can only be detected for flannelmouth. Habitat
preference and associated predation pressure of those habitats appear to be the primary factors responsible for
recruitment. Flannelmouth suckers prefer channel habitat that supports a fraction of the predators found in off-
channel habitats where razorback suckers reside. The dependence of razorback sucker young on slack water

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