By Mueller, Gordon; Carpenter, Jeanette
Endangered Species Update , Vol. 25, No. 3
A group of dedicated biologists are restoring two endangered fish species, the bonytail chub (Gila elegans) and the razorback sucker (Xyrauchen texanus), to portions of the Colorado River in the southwestern United States. Habitat alteration and the introduction of nearly 80 non-native species have brought these native fish to the brink of extinction. With their young being eaten by the introduced fish, none of the razorbacks or bonytails survive to replace adults as they die of old age. The last confirmed wild bonytail was captured two decades ago, and fewer than 1,000 wild razorback suckers remain.
Since the early 1980s, nearly 1 million bonytail and 12 million small razorback suckers have been stocked into the river. Unfortunately, these young, small fish were preyed upon by channel and flathead catfish, sunfish, and black bass that were introduced for recreational purposes. Today, resource managers must stock large (at least 14 inches or 35 centimeters long) bonytail and razorback suckers simply to maintain their presence in the river system. Some younger fish do survive and spawn, but their young continue to be eaten by the introduced fish.
Recognizing the imminent loss of these unique fish, a small group of biologists led by Tom Burke of the U.S. Bureau of Reclamation and Gordon Mueller of the U.S. Geological Survey (USGS) started a program in 1989 to save the razorback sucker in Lake Mohave, a 67-mile (108-kilometer) stretch of the Colorado River straddling the southern tip of Nevada and northwestern Arizona. Biologists from the U.S. Fish and Wildlife Service, National Park Service, Nevada Division of Wildlife, Arizona Game and Fish Department, and Arizona State University joined the effort, and this recovery work has now been expanded to the entire lower river.
One of the first steps in recovering these fish was to improve the survival rates of larval fish. Researchers discovered that razorback sucker larvae, like moths, are attracted to light. These fish spawn in late winter; so, before the wild larvae can be eaten by predators, volunteers venture out onto Lake Mohave with lights and small aquarium dip nets to capture them. Tom Burke, who has led this effort for more than a decade, explains: "We fondly refer to these 1/4 -inch larvae as 'two eyes and a wiggle.'" The larvae are then transported to the nearby Willow Beach National Fish Hatchery, where they are raised to at least 14 inches in length before being released. The approach of using wild-born fish is unique in that it maintains the genetic diversity of the reservoir population; thus, these stocked fish originate from hundreds of wild parents rather than from a few dozen, which is usually the case with hatchery production.
The process of working with these species revealed other critical secrets about their biology. Fish and Wildlife Service biologist Chuck Minckley (now retired) discovered that both bonytails and razorback suckers spawned and produced young in an off-channel pond at Cibola National Wildlife Refuge, near Blythe, Arizona. Prior to this, biologists believed these fish required river conditions to successfully complete their life cycle. …