Can some 130-year-old specimens from the Smithsonian Institution help resolve a modern-day controversy in New Mexico?
by Ellen Davis
Maybe, says David Cowley, an assistant professor in NMSU’s Department of Fishery and Wildlife Sciences.
Cowley and graduate student Patrick Shirey turned to the Smithsonian to help with a research project on the Rio Grande silvery minnow, a tiny fish that is at the center of a debate that pits environmentalists against farmers in the middle Rio Grande area.
The silvery minnow, which gets its name from its shiny silvery skin, used to be plentiful in the Rio Grande from northern New Mexico to the mouth of the Gulf of Mexico, some 1,600 miles away. Today, the fish lives only in the Rio Grande from Cochiti Dam to Elephant Butte Reservoir, a stretch of river that has gone dry in recent years due to drought.
The 1994 placement of the silvery minnow on the endangered species list has led to an ongoing argument between environmentalists – who want to keep water in the river for the fish – and farmers, who need water diverted from the river for irrigation.
Cowley and Shirey thought that by comparing the contents of the stomachs of early silvery minnows with some later samples, they might be able to learn whether environmental conditions in the Rio Grande have changed, and whether that is what has contributed to the decline of the minnows.
The Smithsonian Institution’s Museum of Natural History in Washington, D.C., had 20 silvery minnows that were collected in 1874 by Edward Cope and Henry Yarrow, naturalists accompanying a surveying party led by Lt. George M. Wheeler. The Wheeler Surveys were sponsored by the U.S. Army Corps of Engineers in the 1870s to produce topographic maps of the western territories and to document archaeology, geology, botany, zoology and Native Americans of those areas.
Cope and Yarrow collected the minnow samples in the Rio Grande around San Ildefonso, just south of Española. They were shipped back to the Smithsonian and preserved in alcohol.
As luck would have it, Eastern New Mexico University had some silvery minnow specimens that were collected in 1978 from the same area of the river at the same time of year – a crucial criterion for the study. This was about the last time that silvery minnows were found in this part of the river.
Silvery minnows are herbivores that eat only plants – primarily algae and leaf litter that is found on the bottom of the river.
“No one had ever looked at their food habits before because it is very difficult to do,” Cowley says.
Along with their food, Cowley explains, minnows ingest tiny organisms called diatoms that have a hard glass “case” around them. Microscopic analysis of these diatoms can tell scientists whether they came from water that was pristine or polluted.
In preparation for the project, Shirey spent months learning how to identify different species of diatoms. Then, he faced another challenge.
“The Smithsonian agreed to let us dissect three specimens, so Patrick got very proficient at determining whether a fish was likely to have a full stomach or not,” Cowley says. “Fortunately all three that we picked did.” NMSU has the specimens on loan until 2005.
Shirey was able to identify thousands of diatoms from the three Smithsonian specimens. He then compared these to diatoms found in stomachs of the specimens at Eastern New Mexico University. So far, Cowley says, the research shows that conditions in the Rio Grande above Cochiti Reservoir have really not changed much since 1874.
He believes that the real culprit in the fish’s demise is a long series of events dating all the way back to the 1860s, when New Mexico went from being a Mexican territory to a U.S. territory.
“When the U.S. Army came in to secure the territory they brought in huge herds of cattle and draft animals,” Cowley explains. “Then from 1870 to 1890 there were cattle drives that brought even more cattle into the state. Overgrazing of the grasslands along the Rio Grande caused sediment to erode into the river. Because Colorado was diverting so much water off the Rio Grande, there wasn’t enough water to carry this sediment downstream. As the sediment rose, the river channel jumped as much as a mile or two across the valley and flooding became more and more of a problem, especially in the 1930s and 1940s.”
From 1954 to 1975, the Army Corps of Engineers built four sediment control dams – Jemez Canyon Dam on the downstream end of the Jemez River, which enters the Rio Grande just above Bernalillo; Galisteo Dam on Galisteo Creek south of Santa Fe; Abiquiu Dam on the lower end of the Chama River; and Cochiti Dam on the Rio Grande.
While these dams “did a fabulous job of capturing sediment and providing water for irrigation,” Cowley says they caused several problems for the minnow. One problem was that they broke up its habitat.
“Silvery minnow eggs float downstream, which is historically what enabled this fish to stay alive in years where some parts of the river were dry and some parts were wet,” Cowley says.
The elimination of sediment also meant that there was nothing to pick up food and carry it downstream for the minnow. In addition, the rivers have been engineered in such as way that there are no longer “benches” along the edges where algae can grow.
“We think we can find a win-win situation where we can use water to grow crops, intercept that water in irrigation drains, and then provide auxiliary habitats for the silvery minnow.”
Restoring the species
Cowley believes that the best way to restore the silvery minnow population is to remove or re-engineer Cochiti Dam, which took away the fish population that lived furthest upstream.
Short of this, Cowley has another idea that he believes might help save the silvery minnow – “naturalizing” irrigation drains along the Rio Grande to make refuges for the fish. Irrigation drains are ditches that take excess water from irrigated fields and return it to the river.
“There are 400 miles of irrigation drains between Cochiti Dam and the headwaters of Elephant Butte Reservoir,” Cowley says. “Even when the river dries up, these drains remain wet year-round.”
Cowley says “refuges” for the fish would need to provide enough depth so that the minnows could escape from blue herons that like to prey on them. They also need to have shallow areas where algae, diatoms and other types of phytoplanktons could grow.
In addition, Cowley says, conservancy districts would need to refine their management practice along the ditches and perhaps not mow as much, so that some vegetation falls into the drains to produce the food that the fish need.
“Much of the controversy – especially in the middle Rio Grande – deals with who is going to give up the water to save the minnow. We think we can find a win-win situation where we can use water to grow crops, intercept that water in irrigation drains, and then provide auxiliary habitats for the silvery minnow,” Cowley says.
If enough refuges could be built along the middle Rio Grande, Cowley notes, they could also provide habitat for the southwest willow flycatcher, a species that is listed as threatened.
Cowley says the research on the Smithsonian samples will aid in the refuge project.
“If we want to build naturalized refuges on irrigation drains, we need to know what the minnow eats and we need to know whether these conditions can be provided on irrigation drains,” he says.
Cowley says his research also should tell what kinds of features river restoration needs to provide so the minnows have places to eat.
“We’re spending about $10 million annually to restore the Rio Grande, but this may not benefit the minnow. When we make modifications to the river channel, we need to think about whether these modifications will promote areas that algae can grow in,” Cowley says.
Another alternative Cowley is studying is whether it might be possible to reintroduce the silvery minnow into the Rio Grande around Big Bend National Park – where it was last seen in the 1950s – or into the Pecos River, from which it disappeared in the 1970s.
To do so requires studying the effects of sediment and salinity on the buoyancy of the minnow eggs. Minnow eggs rupture in saline water, and some river areas have become increasingly saline due to runoff from irrigated lands and diversion of snow melt that would dilute the salt.
For more information: David Cowley – firstname.lastname@example.org