NATURE NOTES:
The Big Bend Gambusia: Hot Springs Sustain the Ultimate Big Bend Native

By Andrew Stuart

For many Big Bend National Park regulars, it’s an essential stop after a day of desert adventuring. To ease into the soothing waters of the park’s hot springs, near Rio Grande Village, is pure animal satisfaction. Indeed, these springs were one of the region’s first tourist attractions.  

But the park’s thermal springs – there are many – conceal a wonder most blissful soakers never guess at. The Big Bend gambusia is found only in a few warm pools in the park. The lovely, 2-inch fish has escaped brushes with extinction time and again. It’s a small creature – with an epic story.

Dr. Sean Graham, of Sul Ross State University in Alpine, monitors the Big Bend gambusia.

“They have had about the worst set of cards dealt to them of any endangered species I can think of,” Graham said.

The fish was first identified in 1929, in a single warm-springs pool. Its beauty – the males have bright orange fins – made it a candidate for aquarium use.

Later, biologists returned to study the species – and found that the first pool had dried. 

The gambusia was located in a second pool. But this pond had been invaded by western mosquitofish. These non-natives outcompete gambusias for food, and were likely introduced by park staff. At the time, western mosquitofish were being released around the world, in an effort at mosquito control.  

The scientists collected 25 gambusia – placing some in a nearby spring-fed pond, and returning four to a fish tank in Austin. Only three – in the tank in Austin – survived. 

“It was a female and two males,” Graham said, “’Adam, Eve and Steve’ – and all the Big Bend gambusia that are alive today came from those three. We call that a genetic bottleneck, and that might be the most dramatic genetic bottleneck ever documented.”

Let’s take a step back – to consider the history of a fish that, for a time, had among the smallest ranges of any creature on Earth.

Many endangered species have been driven to the brink by recent human activity, but the gambusia’s trials began long ago. 

In the Ice Age, our region was cooler and wetter, and wetlands more widespread. The gambusia is likely a relic of that time. As desert conditions intensified, wetlands vanished. In a few warm ponds, the gambusia found its final sanctuary.

Today, descendants of the three survivors number in the thousands – in two man-made ponds at the park, fed by spring water, and in a natural pond. The National Fish Hatchery, in New Mexico, also maintains an “insurance population.”  

The fish are still vulnerable. A hard freeze could decimate them. Graham would like to see pumps at the man-made ponds on wind or solar power, to guard against an electricity outage. And Graham hopes to remove western mosquitofish and other invasives from natural areas – to expand the gambusia’s habitat.

The gambusia’s singular ordeal could unlock biological secrets. Studying their DNA could shed light on how species regain genetic diversity, after reaching near-extinction.

“It would be this contribution to science,” Graham said, “that this small fish that nobody cares about in the backwaters of Big Bend National Park provided an answer for all of biology – that’s the kind of thing that that sort of unnatural experiment provided for us.”

The gambusia largely swims under the radar. Park officials have likely feared that publicizing its presence could threaten it. And, indeed, people have dropped pet fish into the gambusias’ ponds. 

But Graham said celebrating the fish could support its conservation. Death Valley National Park, for example, sells stuffed toys of its endangered pupfish. The gambusia, Graham said, could be similarly showcased.

“In every case where people have been worried about that, it’s kind of the opposite,” he said. “If you tell more people about it, they’re more likely to care about it. People get behind endangered species – people love them.” 

Finally, Graham said, the gambusia needs to be preserved as a singular part of West Texas’ natural heritage.

“It has to be our decision that we save this fish for future generations,” he said. “It’s the same sort of philosophy that the park service has and the national park system has for preserving the parks in the first place, and that extends to species like tiny little fish.”

The next time you soak in the hot springs here, know that similar places sustain the ultimate Big Bend native.