In 2014, Arizona used fish poison to clear an invasive fish from Virgin River after mechanical removal methods failed. Then the red shiner came back

For years, conservation crews battled an invasive fish that had taken hold in the Virgin River and threatened two endangered species found nowhere else on Earth.

They tried repeated seining and other mechanical removal methods. The efforts could temporarily reduce the invasive population, but once red shiner established a breeding population, completely removing them became extremely difficult.

Then, in 2014, conservation partners tried something more aggressive: rotenone, a broad-spectrum fish-killing chemical used to eradicate fish from a defined section of water.


The treatment worked.

But it didn't last.

Within a few years, red shiner had returned to the treated stretch — revealing just how difficult it can be to permanently eliminate an invasive species from a river.

The invasive fish threatening two rare species

The red shiner is a small freshwater fish native to parts of central North America. After being introduced into the Virgin River system, it established breeding populations and began spreading through the river.

That created a serious problem for two endangered native fish: the Virgin River chub (Gila seminuda) and woundfin (Plagopterus argentissimus).

Both species are found nowhere else in the world.

Red shiner compete with native fish for food and habitat, while also preying on their eggs and young. Their ability to reproduce rapidly makes the problem even worse.

According to the U.S. Fish & Wildlife Service, recovery of woundfin is not possible in sections of the Virgin River where established red shiner populations remain.

17 years of removal efforts

Conservationists had already demonstrated that red shiner could be removed under the right conditions.

Between 1997 and 2014, managers eradicated the invasive fish from more than 40 miles of stream habitat upstream of the Stateline Barrier in Utah.

But the Virgin River Gorge presented a much tougher challenge.

Crews repeatedly used seines to capture red shiner in the rugged gorge between 2003 and 2014. The method could suppress the population, but it was expensive, physically demanding and unable to reliably eliminate every fish from an established breeding population.

That eventually led conservation partners to change tactics.

The 2014 rotenone treatment

The Virgin River Resource Management and Recovery Program had constructed barriers to divide the river into manageable sections and prevent fish from moving freely between treated and untreated areas.

Two of the most important structures were the Stateline Barrier, about 2.2 miles above the Utah-Arizona border, and the Virgin Gorge Barrier, roughly 16.9 miles downstream.

The section between the two barriers became the target of the 2014 rotenone treatment.

Before the chemical was applied, biologists moved native fish out of the treatment area to protect them. Crews also identified isolated pools, backwaters and other areas where red shiner might escape the treatment because the water did not mix adequately with the main channel.

Those potential refuges had to be treated separately.

The strategy worked.

Red shiner were successfully eradicated from the reach between the two barriers.

For the moment, the invasive fish had been removed.

Then the river flooded

The 2014 success exposed another problem: eradicating an invasive species is only half the battle if it can return.

The Virgin Gorge Barrier had previously been modified to meet regulatory requirements. Those changes, however, introduced design problems that became apparent during subsequent high-flow events.

During flooding, red shiner were able to move upstream past the barrier.

The fish eventually recolonized the very stretch of river that conservationists had cleared.

The 2014 treatment had worked — but the containment system had failed to provide permanent protection.

Scientists redesigned the barrier

In 2015, researchers at Utah State University's Utah Water Research Laboratory used physical and numerical hydraulic modeling to examine the Virgin Gorge Barrier.

Their work produced recommendations for modifications designed to prevent red shiner from moving upstream during high-flow events.

The barrier was modified in 2018.

But conservationists soon discovered another problem that had nothing to do with the barrier itself.

It was hidden in the fish's reproductive cycle.

The 2018 treatment killed the adults — but not the next generation

After the barrier modifications were completed, conservation partners treated the reach with rotenone again in 2018.

Once again, the adult red shiner population was successfully eliminated.

Yet the eradication still did not hold.

The problem was eggs.

Red shiner can reproduce repeatedly during the warmer months, and fertilized eggs already present before treatment can survive the chemical application. After the rotenone dissipated, those eggs hatched.

The young fish then began repopulating the river.

The experience demonstrated a crucial weakness in relying on a single treatment: killing the adult population does not necessarily kill the next generation already developing in the river.

The breakthrough came in 2021

Conservation partners changed the strategy.

Instead of conducting one treatment, they carried out two rotenone treatments separated by two weeks.

The timing was critical.

The first treatment removed the existing fish population. The two-week interval gave surviving eggs an opportunity to hatch, while the newly emerged fish were still vulnerable to a second treatment.

The approach produced a far bigger result.

According to the U.S. Fish & Wildlife Service, the 2021 treatments resulted in successful eradication of red shiner from more than 65 miles of the Virgin River, including complete eradication in Utah and approximately 15 miles of the river in Arizona.

Native fish began returning

Eradication wasn't considered complete simply because crews failed to find red shiner immediately after treatment.

Conservationists conducted intensive post-treatment monitoring, particularly in the uppermost 2.2 miles of the treated reach.

That section was especially important because red shiner are upstream colonizers. Any surviving fish would have been expected to move into the area.

Researchers found no red shiner during the survey.

Instead, they found something conservationists had been working toward for years.

Native woundfin and Virgin River chub had begun recolonizing the treated habitat.

The return of these endangered native fish provided an early indication that removing the invasive species was allowing the river's original fish community to recover.

The battle isn't over

The Virgin River story also demonstrates why invasive-species eradication can take decades.

Removing an established population may require chemical treatment, physical barriers and intensive monitoring — and even then, a single flood or surviving eggs can undo years of work.

Conservation partners now plan to continue red shiner eradication farther downstream into Arizona and Nevada while working to rebuild populations of woundfin and Virgin River chub.

They will also continue monitoring the barriers for signs that red shiner or other invasive species are moving upstream from Lake Mead and the lower Virgin River.

The lesson from the Virgin River is striking: sometimes killing the invasive fish isn't the hardest part. Keeping it from coming back is.