The Role of stocking in the reestablishment and augmentation of native fish in the Lower Colorado River mainstream (1998-2002)

Abstract

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 km of historic channel dry. In contrast, lower basin storage reservoirs cover 36% of the historic channel. Remaining portions of the 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 of their historic range. By 1930, most had become rare. The last remnant populations of bonytail, razorback sucker, and Colorado pikeminnow in the lower basin were taken downstream of Davis Dam during the 1960&rsquo;s and 1970&rsquo;s. Today, Colorado pikeminnow, and it appears, wild bonytail are extirpated 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 of bonytail 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&rsquo;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 habitat puts the species at a much higher predation risk. Through a process of trial and error during the past two decades, managers are now stocking large natives to increase their survival. Small native fish simply have not survived. While this improves short-term stocking survival, it ignores or at least delays dealing with the predation issue. Current stocking programs have reestablished or augmented relatively small populations of bonytail, razorback, and flannelmouth suckers between Davis and Parker Dams. All three species are better off than they were a decade ago in this section of the river. Unfortunately, bonytail and razorback sucker will only maintain a presence in the Colorado River main stem through continued stocking and it remains to be seen if management agencies will make that long-term commitment. While the gains for the bonytail and razorback sucker have been difficult, the successful reintroduction of flannelmouth sucker highlights the ecological changes that have taken place and suggests this, and possibly other channel oriented species (i.e., Gila robusta) could be established. In contrast, there is no evidence to suggest we can expect similar recruitment or expansions for bonytail and razorback sucker. Their dependence on slack water habitat leaves their young vulnerable to overwhelming predation. Recovery in the main stem will only be accomplished with a dramatic decrease and possibly a total removal of nonnative species. After ten years and over $6 million in expenditures to remove nonnative fish it appears this philosophy is neither technically nor politically viable. In the meantime, stocking is the only alternative available to insure these species don&rsquo;t disappear. The only viable option appears the creation and maintenance of small, isolated refuge communities where these species have shown they can produce young.