Debunking the myth of the sand-burrowing minnow
It's a popular refrain here in central New Mexico come summer: The silvery minnow can hunker down, bury itself in a dry streambed and outlast drought.
Whenever the river slows and its bed begins to dry, I'm inevitably informed that the Rio Grande has always dried, and the four-inch long minnow has always survived. This year, I received a letter pointing out that "old-timers" and "local observers" know that minnows can bury themselves and their eggs in the sandy river bottom. I was also told that "desert fish have evolved to deal with drought."
I've seen the bumper sticker that says "A woman without a man is like a fish without a bicycle." But in my mind, a fish without water is just a dead fish, one that's likely to be eaten by a bird or maybe even run over by a bicycle. Not being an ichthyologist, however, I decided to consult with biologists, geneticists and fisheries scientists and ask whether the minnow can live in sand to swim another day.
"BS," began a message that came to my inbox within the hour. This fisheries biologist, who had certainly been asked the question more than once, wrote that when the Middle Rio Grande dried in 1996, the manager of Bosque del Apache refuge took a backhoe to the dry riverbed. He, along with a handful of farmers invited to watch, dug six feet into the riverbed — and nary an egg was found.
"Fish don't live in sand — dry or wet," the testy biologist continued in his e-mail.
Fish do burrow in sand to survive drought in Africa and Australia. Known as aestivating fish, they include the lungfish and salamander fish. But aestivating fish, one geneticist assured me, do not live in North America.
The minnow used to swim throughout much of the 1,850 mile-long Rio Grande; in fact, it was once the most populous fish species in the Middle Rio Grande. It also lived in the Pecos River, which flows through eastern New Mexico. The fish is now found in small numbers in a 157-mile stretch of the Rio Grande, and it is completely gone from the Pecos, which nowadays also dries each summer.
Another biologist told me that most of the fish species that occupy the Rio Grande are closely related to Mississippi River-drainage fish and are not really desert-adapted. He added that "these fish occupy medium-to-large perennially flowing rivers that flow through arid areas." This bears repeating: The fish live in rivers that flow through dry areas.
Not only that, but even if sections of the Rio Grande dried up in the past, they never dried completely. When one portion of the riverbed dried, water likely remained in an adjacent bend or oxbow. Without dams to stop them, minnow eggs and larvae from upstream waterways would have drifted downstream, re-colonizing the reaches that had suffered drying.
Today's Rio Grande does not bend and meander throughout the valley; it is carefully managed to flow within a relatively narrow channel for short stretches between diversions and dams. Now, there are 16 major dams and diversions on the Rio Grande between its headwaters in Colorado and the Gulf of Mexico. When one portion of the river dries, the fish can't simply swim up or downstream. When, say, 80 miles of the river dries, as it did in 2003, biologists must salvage what fish they can, pack them in plastic bags with water, then truck them to a wet portion of the river. Then, from the fall through the spring, when flows are higher, it's up to biologists to stock the river from fish they've raised in tanks.
A second geneticist pointed out that we didn't collect much historical flow data before dams and diversions.
"That timescale (from when water managers started recording flows) is pretty negligible when it comes to evolution," she said, "even for a minnow with a short generation time." In other words, even the short-lived minnow can't evolve fast enough to be able to live without water. That's like expecting humans to evolve within a few hundred years to survive while breathing carbon monoxide.
It's critical that journalists report many sides of any story. But let's lay to rest the myth of the amazingly resilient silvery minnow. There's absolutely no proof that it burrows into the sand and hangs on in the dry streambeds of the river, and as far as I can tell, there's little proof that the "old-timers" who tell such tales exist either.