NATURE NOTES:
In a Watershed in Crisis, Scientists Work to Understand, and Save, the Desert Fish of Big Bend

Think of West Texas wildlife, and aquatic creatures aren’t what comes to mind. But the Rio Grande and its tributaries are home to a stunning diversity of fish, mussels and turtles. Many are found nowhere else on Earth. 

But the Rio Grande is also one of the world’s most endangered rivers. Images this spring from Big Bend National Park, where the river has gone dry for miles, make that plain. With 95 percent of the river’s water diverted for human use, what’s remarkable is how much aquatic life survives.

That survival is far from guaranteed. Scientists are working now to understand the Big Bend’s native fish, and to preserve, and even strengthen, their populations.

Dr. Brian Laub is a professor at the University of Texas-San Antonio, and an expert in the ecology of desert streams.

“We have these persistent impacts,” Laub said of the Rio Grande, “the declining flows, the increasing salinity and the changing habitat – but an event like a severe drought can potentially, if the population has been hit by some of these other things, be the final nail in the coffin.”

In a 2016 report, Laub tracked changes to fish communities in the Big Bend, to develop a roadmap for preserving native aquatic life here.

The most obvious changes have been local extinctions. Of 36 fish species once native to the Big Bend, four are gone. But even as some species have vanished, or been driven to the brink, others have increased in abundance. 

Those changes are tied to the transformation of the river itself.

Historically, the Rio Grande ran high here for six months each year – first as spring snowmelt surged down from the Rockies, then as summer monsoons swelled the Rio Conchos, which joins the Rio Grande at Presidio-Ojinaga. The growth of irrigated agriculture in the 19th Century, from Colorado to El Paso, effectively cut the Big Bend Rio Grande off from its headwaters. By the mid-20th Century, damming and diversion in Mexico had sharply reduced flows from the Conchos.

The Rio Grande silvery minnow was one species that paid the price. In the 1950s, drought and drying events, like the one occurring now, were the “nail in its coffin.” But its disappearance here was tied to the phenomenon of “hydrologic forcing.”

Native fish are adapted to the river’s ancient patterns. The silvery minnow’s reproduction was cued to, or “forced” by, annual floods – in the absence of those floods, populations plummeted. Other fish reliant on hydrologic cues are imperiled now.

With declining flows, water quality has also declined. Laub found that the diversity and abundance of native fish is now greatest near springs, where water quality improves. The species that thrive today are “generalists” – fish, like the red shiner, that can tolerate differences in water quality and temperature, and aren’t tied to specific niches in the river. 

Because one fundamental change to the river has been the loss of “complexity.”

There’s a human tendency, at least in industrial societies, toward uniformity: sameness and consistency are seen as strengths. But nature testifies otherwise. Life thrives best, and is most resilient, amidst diversity and variegation. 

Ecologist Demitra Blythe did her master’s thesis here, and she quantified how “complexity” in the river equates with native fish abundance.

“In terms of the ecology, of these broader ecological process, this was some of the first work being done on the Rio,” Blythe said, “of actually linking the geomorphic template to the ecological one. It was really exciting to see that, in a lot of ways it translated – basically, complex habitat actually may matter for broader ecological processes.”

What does the “simplification” of the river look like? 

In its passage through open desert, the Rio Grande was once a braided stream, with a broad floodplain, bounded by cottonwoods and willows. As flows declined, and regular flooding ceased, sediment carried in by local tributaries choked the channel. Invasive plants like tamarisk and giant river cane took hold. The river changed from a wide, meandering stream, to an ever-narrowing ditch, hemmed it by thickets of non-native vegetation. 

That’s meant the disappearance of riffles and rapids, eddies, pools and backwaters – specific niches that diverse fish rely on to live out their lives.

But a degree of complexity endures – especially in the Big Bend canyons. Blythe found the canyons, where the rivers twists and tumbles amidst bedrock and boulders, are now hotspots for native fish. Food webs are more robust here. Blythe’s research suggests that managing the river to maintain habitat complexity could be key to preserving aquatic life.

Megan Bean is Texas Parks & Wildlife’s native fish coordinator for the Chihuahuan Desert. 

“It’s incredible,” Beans said of the region. “It’s one of the most diverse areas in Texas, and also the most imperiled. We have more fish in the Chihuahuan Desert at risk and imperiled than anywhere else in the state.”

While fish are her speciality, and her passion, Bean works beyond the river corridor. 

“It’s really important to manage not just the species, but the habitat,” she said. “When I look at native fish conservation in the Rio Grande, what’s important is how the system is managed. We like to try to address issues where they’re starting, and not when they end up in the stream.”

With other conservation groups, and with state funding, Bean partners with Trans-Pecos landowners on restoration projects – often focused on tributaries like Alamito and Terlingua creeks, and on the high grasslands.

How could restoring the Marfa grasslands impact fish in the Rio Grande?

Healthy grasslands absorb rainfall, releasing it slowly into arroyos and creeks. But on overgrazed, denuded land, rain runs off in a torrent. Big Bend creeks that were once perennial are now intermittent or dry – except during flash floods.

Bean are her partners are helping landowners remove creosote and other shrubs, to allow grasses to rebound. And they’re returning cottonwoods and willows – which were logged out a century ago for mining operations – to creek sides.

Because the Rio Grande’s tributary creeks are critical for fish – for spawning, and as sanctuaries when the river floods. A decade ago, Alamito Creek was home to diverse native species – Mexican stonerollers, roundnose minnows, Conchos pupfish. Today, those fish are gone. The Conchos pupfish once lived throughout the region – it’s now found only in the Devils River.

The Big Bend Rio Grande is among the most remote river stretches in the United States. That wild isolation is part of its beauty. But it also means it can be overlooked. “In-stream flows” are controversial, but allocating water for environmental purposes may be necessary – if the river and its life are to endure. 

Science can provide the context, but that decision lies with policy makers, and people, Blythe said. 

“Water is just going to become more and more important,” she said, “and balancing wanting to maintain aquatic life with human needs is just going to become more and more critical as we move into the future. Having a better gauge on the ecology allows you to actually make more informed decisions. This is a really cool river. Let’s work toward saving it.”