<< BACK TO ARTICLES

As the Rio Grande raged and swelled from its banks last month, Big Bend National Park biologists rushed to save one of the park’s most endangered residents.

The Big Bend gambusia is a tiny silvery fish with a minnow-like appearance, somewhat unique in the piscine world for bearing live young. They were first identified at a spring near Boquillas Crossing in the 1920s, but the species has lived along the Rio Grande since at least the last Ice Age. Over eons, they’ve adapted to a life of gobbling up mosquito larvae and other insects in the consistently warm temperatures of spring-fed pools. Their sensitivity is both a blessing and a curse: they flourish and outcompete other fish at balmy spring heads, but their numbers fluctuate in cooler, more distant reaches of a pond.

“They can’t tolerate variation in the water temperature,” says Raymond Skiles, the national park’s chief of science and resource management. “Certainly they don’t tolerate cool water either. They just start going belly-up dead before it starts freezing.”

The fish now live in only a few spring-fed ponds near the park’s Rio Grande Village, and last month, Skiles expected all four ponds to be completely submerged when the National Weather Service predicted the river might rise to 33 feet. While river water might not seem a dire threat to a fish, the flood had the potential to flush the tiny gambusia from their tepid ponds, and inundate their only habitat on earth with exotic fish that could gobble or hybridize them to the brink of extinction.

Faced with this ominous situation, the park’s management, in consultation with U.S. Fish and Wildlife, launched a Noah’s Ark-style rescue mission the night before the expected deluge. Skiles and a crew of biologists worked into the dark catching about 350 Big Bend gambusia—the number needed to maintain genetic diversity. They stashed the fish in ice chests filled with pond water and sent the rescued gambusia to a Fish and Wildlife Service hatchery in Dexter, New Mexico. There, the fish could be kept safe in the event they were needed to repopulate the park’s ponds.

“We felt like this was a critical operation,” says Skiles, who has spent 20 years working with other scientists to protect the fish. “That history among a number of the staff meant that it was not just a remote scientific commitment but a personal one too.”

Fortunately, the flooding was not quite as bad as predicted. Three of the four gambusia ponds were submerged, including the one containing the largest population. But the highest-elevation pond—where the fish are thought to be the most genetically pure—was spared.

While the crisis seems to have been averted, there’s still cause for concern, Skiles says. The pond that was spared depends on an electric pump to replenish it with warm spring water. But during the flood, electricity had to be turned off at Rio Grande Village, leaving the pump powerless. With mild temperatures at this time of year, Skiles hopes the gambusia can go several weeks without spring water being pumped back into their habitat.

Also of concern is the condition of the still-submerged ponds, which won’t be known for several weeks. Skiles worries the largest pond may be structurally damaged or eroded as the flood water recedes. Even if the pond is still suitable gambusia habitat, exotic fish have probably moved in and will have to be removed for the native fish to survive there.

He is more optimistic though about the other two submerged ponds. He hopes beavers may rebuild the dam that formed the lowest pond, as has happened after smaller floods in the past. And the newest pond, built a few years ago, had fortunately not yet been stocked with gambusia. Skiles says that pond can be drained to remove any exotics that rode in with the river, and an earthen berm will probably be raised to prevent another mighty flood from inundating it in the future. Then, whenever the weather is warm enough, gambusia can be introduced.

“That’ll be, we hope, our second pure and quite well-protected population,” he says.

What could have been a catastrophe may now, in some ways, help the park’s gambusia conservation efforts. Having seen how a major flood affects the ponds, park managers can do some hydrological modeling to figure out how to protect the gambusia habitat in the future. Also, Skiles says it’s good that a new, robust population has been safeguarded at the Dexter hatchery. Another small group of Big Bend gambusia has been kept there for the past 20 years, but having bred with itself for two decades, that older population wouldn’t be ideal candidates for restocking the park’s ponds after a disaster. Preserving multiple, separate populations will ensure genetic diversity, which will improve the species’ long-term chances of survival, Skiles explains.

Regardless, the endangered fishes’ future will still be precarious.

“The fact that you could draw a 400-yard-diameter circle and encompass the entire world’s population of Big Bend gambusia, that’s pretty remarkable,” he says. “Even if we have four different habitats, with some fish in each one and with different quality levels, it remains a desperate situation. Something catastrophic could still threaten all four of those ponds and the species as a whole.”

Last month wasn’t the first time Big Bend gambusia faced extinction. Since the species’ Ice Age heyday, much of their spring-fed habitat has gone dry, and human-introduced exotic fish have further jeopardized their chances of survival. The Boquillas Crossing spring, where they were originally found, dried up in the 1950s, taking the fish there with it. Soon after, the other remaining wild population at Rio Grande Village was devoured by exotic sunfish. At that point, with their natural habitat dried-up or over-run with exotics, the only Big Bend gambusia in the world were one female and two males living in an aquarium at the University of Texas at Austin.

There was concern the species might not survive the genetic bottleneck, but those three fish managed to produce enough offspring to restock a man-made, pump-powered pond at Rio Grande Village. There, the species has essentially been on life support ever since, with park staff working hard to restore them to other ponds and rushing to save them whenever a pump fails during a freeze or exotics sneak into gambusia habitat.

“As far as we can see, we’ll be sustaining them and on occasion, taking more heroic actions,” Skiles says, adding that because of the Endangered Species Act, it’s the law that the park sustain the species.

Aside from the law, some might question the value of keeping a species on life support, when most people in the world probably wouldn’t notice if the Big Bend gambusia disappeared from life’s ranks. Arguments for saving species often revolve around their potential value to humans, but Skiles believes there are less utilitarian reasons for protecting earth’s endangered.

“It reflects our overall value system, that even the weakest among us are worth preserving,” he says. “We should see a species as a component of a larger suite of species. They’re connected. They help support other things. We need to see that diversity is good, keeping an individual species is good. All these things have taken so long to develop.”

“The first rule of intelligent tinkering is to save all the parts,” he continued, referring to an Aldo Leopold quote. “We have the power to tinker, to alter things, to make decisions about what survives and what doesn’t. But that statement gives us reason, even if we don’t know why, to keep all the parts we’ve got now.”

<< BACK TO ARTICLES