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New Mexico State University

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Ditch Could Be Perfect Niche for Native Fish

LAS CRUCES - Hundreds of miles of twisting agricultural irrigation drains and ditches snake their way across the fertile Rio Grande valley in a cascade of links from the 1,900-mile-long waterway. Now, in an ambitious project to monitor New Mexico's native fish, scientists have begun sampling these river offshoots to determine if they can be used to foster conservation of native species.



New Mexico State University scientists have begun a three year study of New Mexico's agricultural irrigation drains and ditches to determine if these Rio Grande offshoots can be used to foster conservation of native fish species. (07/19/2002) (Courtesy Photo by Clifford Hohman)

"I know the idea is a little bit counter to what some people would propose," said David Cowley, project leader and endangered species expert with New Mexico State University's department of fishery and wildlife sciences. "Some just want to keep the water out of the irrigation system and in a river, but our research is looking at ways to use water for both purposes: agriculture and conservation."

Using the Middle Rio Grande Conservancy District as its testing ground, the NMSU team has established a series of monitoring stations in the Socorro division where they intend to sample not only the fish, but also aquatic insects, plants and algae that might be present. Insect and algae samples are already being taken, while fish collection permits are forthcoming from state and federal agencies. A critical question the researchers are attempting to answer is whether irrigation systems can provide enough food to support native fishes.

The U.S. Department of Agriculture's Rio Grande Basin Initiative funds the project, which will take three years to complete. The project is also part of NMSU's Water Task Force, a group of 75 university specialists on water-related issues who support research and educational programs to improve irrigation efficiency in agriculture and urban landscaping in the Rio Grande Basin.

In 1994, when the district's irrigation system was last surveyed, scientists found 20 fish species, eight of which were native to New Mexico. The tally included such colorful-sounding native fish as the gizzard shad and the high-profile Rio Grande silvery minnow.

The silvery minnow, endangered for eight years, is found only in the middle Rio Grande from Cochití to Elephant Butte, a stretch of river has that has gone dry in recent years due to drought.

Recently, a federal judge upheld a ruling that for the first time confirms the Bureau of Reclamation has the authority to use water from the Middle Rio Grande Conservancy District and San Juan-Chama Project, which includes water for the city of Albuquerque's long-term drinking water supply, and to protect minnow habitat if it becomes threatened.

A potential water alternative is New Mexico's complex agricultural irrigation system, Cowley said. In terms of miles, there are far more miles of irrigation drains than river within the valley, he said.

The system is divided into two parts that deliver and drain water. Delivery ditches, which originate at major river diversion sites, branch off into smaller and smaller so-called laterals, ultimately ending up in fields. Most of the laterals also have an ending point at an interior drain or riverside drain where irrigation water is returned to the river after it percolates through soil.

Cowley said that prior research in the Las Cruces region indicated that even during the off-year irrigation season, typically from mid- to late October through February, many delivery ditches stayed wet. Known as siphons, these wet spots can be found underneath another ditch pipeline or highway and appear to offer very good conditions for supporting fish.

"We're expecting to find similar sorts of things in the Middle Rio Grande Conservancy District," Cowley said.

The size of native fish in the irrigation system is expected to vary widely, he said. The largest will likely be the river carp sucker at 10 inches in length, while the smallest would be the fertilized eggs of the Rio Grande silvery minnow, which float in the current downstream until they hatch.

Many of the species in the ditches and drains will tend to be smaller specimens since the water level isn't enough to support large fish growth, Cowley explained. "But that's not to say we couldn't make simple modifications to the drains that would make them friendlier to fish," he said.

The NMSU researchers are particularly interested in determining whether once native species, especially those threatened or endangered, get in, can they find their way out of the irrigation system. Alternatively, if the fish are found in the drains, "Can we make those conditions conducive for them to survive for a year or more at a time, especially during times of drought?" Cowley said.