A little fish with a bright future

The silvery minnow, once thought to be on the verge of extinction, is enjoying the Rio Grande's deep waters.

The Rio Grande is flowing deep and strong this year, giving hundreds of thousands of endangered silvery minnows all the wiggle room they need — for love.

"We're anticipating a lot of successful reproduction," says Michael Porter, the U.S. Army Corps of Engineers' lead silvery minnow biologist in Albuquerque.

Which is great news for the tiny fish, not to mention for farmers, builders and others whose livelihoods might be affected if the silvery minnow were to continue its historical trend toward extinction.

But the river is also giving biologists a better understanding of when, where and why the tiny silver fish reproduce — or fail to. It turns out that when the minnow was listed as an endangered species more than a decade ago, scientists didn't fully understand what it would take to nurse the species back to health in the silty Rio Grande.

Now, some scientists say, the picture is clearing up.

Hybognathus amarus once was found throughout 3,000 miles of the Rio Grande and its tributaries, but dams and habitat destruction in the 20th century nearly caused its extinction. The species was declared endangered in 1994. Now the fish, the biggest and oldest of which are about 4 inches long, are found only in pockets of the river between dams, including the stretch running through Albuquerque.

No one knows exactly how many silvery minnows are left, Porter said — probably no more than a few million and perhaps far fewer. Keeping the species alive has been a challenge that has brought together a diverse collection of agencies, from the Corps of Engineers and the U.S. Fish and Wildlife Service to the New Mexico Interstate Stream Commission, the city of Albuquerque and several surrounding pueblos.

Their efforts have included gathering eggs in the wild and moving them to Albuquerque for rearing and holding thousands of adult fish in tanks to ensure a viable breeding population. In drought years, they have rescued fish from shallow pools in the river bed so they weren't gobbled up by predators or boiled by the sun. New channels have been carved along the river's edge to provide additional habitat.

But ongoing research suggests that geology may play a bigger role than previously thought in the ultimate survival of the species, and Porter said he's optimistic about the future. It all comes down to water, he says — and particularly about having enough flow at the right time.

Starting as early as March and as late as June, female silvery minnows release several thousand eggs at a time, which quickly swell into translucent globes about 1/8-inch in diameter. The floating eggs mature and hatch in about two days, Porter said. Within a few more days, the larvae develop into juvenile fish that can swim and eat on their own.

There is some debate among scientists over how best to ensure the silvery minnow's reproductive success. The current thinking goes that the minnows need a short burst of high water flow — known as a spawning spike — to spawn, and that the eggs can float as far as 180 miles downstream and still survive. That strategy is written into the documents that guide the silvery minnow recovery.

Porter says his studies have led him to a different theory. He believes the minnows fare best when the eggs don't float away. The optimum conditions, he says, are when the river is deep enough to inundate vegetated sand bars and shallow areas along the bank for about a week — what he calls recruitment flow. He points to 40-fold increases in population survey results in years like 2004 when the Rio Grande flowed at that rate.

The good news is that Rio Grande can reach that recruitment flow about three years out of four, Porter said, though it would require an agreement with Cochiti Pueblo to raise the water level at Cochiti Dam during some years.

"I've started to realize (the silvery minnow) is the most recoverable species on the (endangered species) list," he said. "It's going to take some work," but Porter said he's more optimistic than ever that the Rio Grande silvery minnow won't go extinct.

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