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Scientists want to know if Rio Grande contaminants are feminizing the endangered silvery minnow

August 25, 2010, 1:00 am

Declining water levels threaten fish caught in shrinking pools during the summer months. Here, Yvette Paroz (US Bureau of Reclamation) and Jason Remshardt (Fish and Wildlife Services) use nets to search for silvery minnows.
Credits: Courtesy US Fish and Wildlife Services

As it is, the fish cannot survive on its own in the Middle Rio Grande. Not only do biologists try to save fish during summertime drying but, every year since 2001, they have released hundreds of thousands of hatchery-raised minnows into the Rio Grande.

Also, in accordance with the Fish and Wildlife Service’s road map for species recovery, state, federal and local agencies have invested in three “off-channel” facilities. Both the City of Albuquerque and the Interstate Stream Commission have built concrete refugiums where the fish are raised. In 2006, the US Bureau of Reclamation built a sanctuary in Albuquerque that theoretically will create, for the fish, habitat in the Bosque between the irrigation canal and the river during certain times of the year (the sanctuary was recently handed over to Fish and Wildlife, but has not actually been used yet). In addition, the Albuquerque Bernalillo County Water Utility Authority has built a fish passage structure for its new water diversion dam.

As part of Fish and Wildlife’s recovery plan, biologists also are trying to establish a second population of silvery minnows. That’s why, since 2008, Remshardt and his crews have been heading down to the Big Bend of Texas, using 1,000-gallon tanker trucks to dump several hundred thousand fish into the river there. So far, that project has been going well: They’ve documented the fish spawning and, this summer, caught larval and juvenile fish. Next year, they’re hoping to do a river-wide survey to determine how many fish are in the river, and how far they’ve expanded.

The official recovery plan says the fish must have three self-sustaining populations before it can be “downlisted” from endangered. But biologists remain unsure where that third population might be re-introduced: Given the prevalence of dams along the Rio Grande, it’s hard to imagine where a third population might take hold.

So, after a decade of recovery work and collaboration among federal, state and local agencies, and millions of dollars spent, how is the minnow doing?

“All indications are we’ve sort of bounced back from what most folks would say was the low period—2003, 2004—when catch rates were at the lowest,” Remshardt says.

At that point, there were basically no fish in Albuquerque and only a few fish in the Isleta Reach to the south. The few fish biologists were finding lived in what’s called the San Acacia Reach between the Sevilleta National Wildlife Refuge and Elephant Butte Reservoir.

“And now, we see fish at almost all the monitoring sites, year-round,” Remshardt says, noting that while Fish and Wildlife has increased and honed its ability to raise and stock fish, biologists are also seeing fish reproducing and surviving in the river.

That said, it only takes one bad year to blow the fish’s recovery. The minnow’s lifespan is maybe two or three years in the wild. In many cases, the fish live only one year. To survive, the minnows must reproduce each year:

“Every year, you have to have a successful number of fish reproduce to replace the ones that are lost,” Remshardt says. “So it’s a struggle to make sure you get that done every year.”

And there are two main issues that still have yet to be resolved: river intermittency and fish passage.

Because stretches of the river are divided by dams, it’s impossible for the fish to move on their own into safe waters when flows dwindle or disappear.

“Those are the two big ones we need to get answers to, or work around, before we can even talk about making this population self-sustaining,” Remshardt says.

But when it comes to water, the fish is not at the top of everyone’s priority list.
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