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Silvery Minnows Return to Texas
Photo Credit: Aimee Roberson/FWS
by Mike Bender
One of America’s most critically endangered species, the Rio Grande silvery minnow (Hybognathus amarus), began to face a brighter future on December 17, 2008, with the release of more than 430,000 hatchery-raised fish into former habitat in the Big Bend region of west Texas. The U.S. Fish and Wildlife Service plans to release additional fish there over the next four years to establish an experimental, self-sustaining wild population in the lower Rio Grande.
A bucket brigade of volunteers met a Service fish transportation truck near Rio Grande Village, one of four release sites in and near Big Bend National Park. As hatchery biologists netted the fish from the truck’s tanks and carefully placed them into buckets, the volunteers passed them down the line to Ray Mathews of the Texas Water Development Board, who stood two-feet deep in the river. He gently dipped the minnows into a net enclosure, where they spent a day acclimating to the river before their final release. For the first time in about 50 years, silvery minnows inhabited the waters of the Big Bend region.
Photo Credit: Raymond Skiles/NPS
Jason Remshardt of the Service’s New Mexico Fish and Wildlife Conservation Office coordinated the collection, transportation, and release of the minnows. The stock for the release came from two sources: the Service’s Dexter National Fish Hatchery and Technology Center in New Mexico, and the City of Albuquerque’s Rio Grande Silvery Minnow Rearing and Breeding Facility, which is funded by the Middle Rio Grande ESA Collaborative Program and the State of New Mexico. These fish were not needed for the continuing silvery minnow augmentation effort in the middle Rio Grande of New Mexico.
Native to the Rio Grande system from northern New Mexico to the Gulf of Mexico, the silvery minnow was once considered one of the river’s most abundant and widespread species. But extensive habitat changes have reduced its range by almost 95 percent to a reach of the middle Rio Grande near Albuquerque, New Mexico. The Rio Grande silvery minnow needs free-flowing streams in which to reproduce, and much of the river has been impounded by reservoirs. Other sections of the river are subject to drying due to withdrawals for irrigation, pumping for municipal use, and periodic droughts. Water pollution, stream channelization, and introductions of non-native fish species may also have played a part in the silvery minnow’s decline.
In 2001, the Service’s New Mexico Ecological Services Field Office and New Mexico Fish and Wildlife Conservation Office began working in the middle Rio Grande region near Albuquerque to maintain the species’ last natural population. At times when water withdrawals caused parts of this reach to dry, biologists led rescue efforts to move the fish to wetter parts. Silvery minnow eggs that would otherwise drift downstream into Elephant Butte Reservoir and die were salvaged for captive propagation. The Service has stocked more than one million hatchery-raised Rio Grande silvery minnows back into the river in New Mexico to augment the wild population.
Photo Credit: Mike Bender/FWS
The draft revised recovery plan for the Rio Grande silvery minnow calls for secure wild populations at three locations throughout the species’ range. In 2003, the Service began looking for suitable habitat in which to establish a second population. The next year, a team of biologists from the Fish and Wildlife Service and the National Park Service rafted the Rio Grande in the Big Bend region of Texas to evaluate habitat and conduct fish surveys.
Scientists believe that water pollution and a prolonged drought in the 1950s caused the disappearance of silvery minnows from the lower Rio Grande, including Big Bend National Park, which lies within the Chihuahuan Desert. Since that time, however, enough water to support a minnow population has remained in the river below the mouth of the Rio Conchos, a major tributary that originates in the Sierra Madre Occidental of Mexico. The quality of the Rio Grande water also has improved due to better sewage treatment, reduced mining activity, and changes in agricultural practices.
Photo Credit: Mark Lockwood/TPWD
Rio Grande silvery minnows need low-velocity habitats with sandy or silty bottoms. These habitats are generally found in meandering rivers with side channels, oxbows, and backwaters. In recent decades, however, dense stands of non-native salt cedar (Tamarix chinensis) and giant reed (Arundo donax) have grown up along the Rio Grande in the Big Bend, anchoring the banks and causing the channel to become narrower and deeper. For a number of years, the National Park Service has been working to enhance the habitat by reducing invasive vegetation along sections of the river. It is expensive and time-consuming work, but nature lent a hand in September 2008 with the largest flood in decades. In places, it scoured much of the remaining invasive vegetation and rearranged the river channel, creating a more natural mosaic of cobbles, gravel shoals, and sand bars. As a result, conditions improved for the return of the silvery minnow.
For Raymond Skiles, a wildlife biologist for Big Bend National Park, the reintroduction is an important step toward restoring the park’s ecosystem. "It’s a flagship for the dozen or so other species that are no longer here. It’s great to have one of them back. This is one of a suite of species, and we hope there will be others that follow."
The Rio Grande silvery minnow in the Big Bend is designated as an "experimental, non-essential population," meaning that the loss of this population would not be essential to the species’ survival. Such a designation allows more flexibility in management, which helps to make species reintroductions more acceptable to the public. The boundary of the experimental population is from Little Box Canyon downstream of Fort Quitman in Hudspeth County, Texas, through Big Bend National Park and the Rio Grande National Wild and Scenic River, to the Amistad Dam in Val Verde County, Texas. Although the experimental population boundary extends up the Pecos River to the mouth of Independence Creek, the minnows are not expected to move into the Pecos.
Photo Credit: Bob Pos/FWS
Aimee Roberson, a wildlife biologist with the Fish and Wildlife Service, worked on Rio Grande silvery minnow conservation from the New Mexico office until taking a position in the Alpine, Texas, office to coordinate the Big Bend reintroduction. After five years, many public meetings, and a great deal of paperwork, she said that the release day was "like Christmas." She quickly added, "But now the real work begins." That work will include additional minnow releases for the next four years, quarterly monitoring of the fish, and annual surveys to detect spawning.
At the Rio Grande Village release site, Joy Nicholopoulos, the Service’s Texas State Administrator for Ecological Services, emphasized that the silvery minnow reintroduction was made possible by support from a wide array of partners. In addition to the Fish and Wildlife Service and the National Park Service, other partners include the Texas Parks and Wildlife Department, City of Albuquerque, Middle Rio Grande Endangered Species Collaborative Program, El Carmen Adam’s Ranch, Texas Commission on Environmental Quality, Texas Department of Agriculture, Texas Water Development Board, Texas Farm Bureau, University of Texas-Pan American, World Wildlife Fund, The Nature Conservancy, U.S. Geological Survey, International Boundary and Water Commission (including its Mexican section, Comisión Internacional de Límites y Aguas), and other Mexican agencies (the Comisión Nacional de Areas Naturales Protegidas, Departmento de Restauración Ecologia, and Instituto Nacional Ecologia).
Nancy Gloman, the Service’s Southwest Assistant Regional Director for Ecological Services, was especially pleased that young people attended the minnow release and helped with the bucket brigade. "This is why we do what we do, so that people can return in years to come, see the minnows and other wildlife, and know that we made a difference for conservation."
Mike Bender, editor of the Endangered Species Bulletin, can be reached at firstname.lastname@example.org or 703-358-2335.
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