Repeated home aquarium dumps complicate the Pahrump poolfish’s improbable comeback from the brink of extinction

The Pahrump poolfish, endemic only to the Pahrump Valley in Nye County and the last remaining native fish species in the valley, has been fighting an uphill battle of survival for decades. As a desert fish species, Pahrump poolfish are largely susceptible to changes in the single-spring habitats in which they are found. Consequently, the Pahrump poolfish has had to overcome a series of setbacks across several decades in the battle against extinction.

“The Pahrump poolfish are not only native to Nevada, but they’re not found anywhere else,” Brandon Senger, supervising fisheries biologist for the Nevada Department of Wildlife in Las Vegas said. “Historically, they have two other related species in the same genus, the Raycraft Ranch springfish and the Pahrump killifish, but both of which went extinct by the mid-1950s.”

The Pahrump poolfish likely would have suffered the same fate as its relative species, if not for area biologists in the 1960s that realized Manse Spring, the Pahrump poolfish’s one and only habitat at the time, was desiccating due to excessive groundwater pumping. 

“When Nevada started having a lot of groundwater pumping withdrawal taking off in the Valley in the 60s, biologists at the time started to notice that some of these spring sites, and Manse Spring in particular, was being affected with less water flow,” Senger said. “If [Manse Spring went dry], the species would go completely extinct off the face of the earth. So in the early 70s, they salvaged some of those fish and took them to different sites as a last-ditch effort in case the spring went dry.”

Those efforts proved critical for the Pahrump poolfish’s survival, as Manse Spring did in fact go dry in 1975. Ever since then, the Pahrump poolfish has become Nevada’s “homeless fish,” with populations in several sites across the Las Vegas Valley.

Aside from a refuge site in White Pine County, the two primary locations they’re found in now are at Corn Creek Field Station at the Desert National Wildlife Refuge and Lake Harriet at Spring Mountain Ranch State Park. The most recent site to be established for the species is at a number of ponds in Springs Preserve. 

Although these sites have been successful in keeping the endangered Pahrump poolfish population in existence, they have not been without unexpected challenges. One particular challenge for the species has been the introduction of non-native fish species, oftentimes tropical aquarium fish, to their refuge habitats. 

“Corn Creek was the first site [Pahrump poolfish] got moved to from Manse Spring back in the 70s and they were there for decades until non-native fish were introduced,” Senger said. “These were your typical aquarium or backyard pond fish species like Mollys, guppies, crayfish and goldfish. When those species got introduced at Corn Creek by the late 90s, our entire poolfish population at Corn Creek had been extirpated.”

The Corn Creek refuge site went without Pahrump poolfish until 2015, once site restorations to the spring system were fully completed. The restoration process for a spring is lengthy, which after removing any surviving Pahrump poolfish, requires a Rotenone treatment to eradicate the non-native fish populations that had been introduced. 

According to a press release from the Nevada Department of Wildlife, Rotenone is a naturally occurring substance from the roots of a South American plant that blocks oxygen for gilled organisms; yet, according to NDOW, Rotenone decomposes quickly, so it does not remain in the environment for an extended period of time. Therefore, its use is a common strategy to remove non-native fish species from an invaded habitat. 

“So in 2014, after they did that [restoration at Corn Creek] but before we had a chance to put poolfish back in, somebody had introduced non-native fish again [to the site],” Senger said. “So our agency did another Rotenone treatment to eradicate those non-native fish and we were successful at the time. So later on in the summer of 2014, we moved some poolfish from our other sites back into Corn Creek and by 2018, our estimated poolfish population at Corn Creek was over 2,000 fish.”

However, the recent resurgence in the Pahrump poolfish population was short-lived, as newly-introduced non-native fish once again appeared at Corn Creek by 2019.

“We think it might have been some aquarium fish because the species we saw are called the weather loach or dojo loach fish, which are known to be a pretty common aquarium fish,” Senger said. “So while our population estimate was over 2,000 poolfish in 2018, our estimate last year was less than 200 fish.”

Consequently, Senger and NDOW are back at square one in having to eradicate non-native aquarium fish from Corn Creek to keep the Pahrump poolfish population there alive. However, it’s not just the Corn Creek site that has been challenged by frequent aquarium dumps of non-native fish species. 

Lake Harriet has historically hosted one of the biggest refuge populations for the Pahrump poolfish. But once non-native crayfish, goldfish and mosquitofish populations were found in the lake, Pahrump poolfish populations dropped from over 12,000 fish to just a couple hundred within a year. 

“At that point we did our population survey and realized that the population had totally crashed,” Senger said. “We were worried that we would lose all those fish, so we went in to salvage all the remaining poolfish at Lake Harriet. We were able to bring back over 600 and take them back to our Lake Mead Fish Hatchery, where we reared them while we tried to figure out how to get rid of the non-native fish.”

Lake Harriet – photo: Nevada Department of Wildlife

Eradicating non-native species is further complicated by the presence of crayfish, which can withstand the Rotenone treatments. 

“Eradicating non-native fish is fairly straightforward using Rotenone,” Senger said. “But crayfish are kind of a new and more challenging twist, as Rotenone is not lethal to crayfish and they can burrow under the ground to escape chemicals in the water like that. Then they can still live in a desiccated, drier mud environment for a long time.”

Eradicating these non-native species called for more extreme measures at Lake Harriet, which meant completely drying out the lake.

“Working with the Nevada Division of State Parks, we decided that we’d kind of go nuclear to try to eradicate [the lake],” Senger said. “But the only way you can do that is to totally dry out the system and get it 100% dry for well over a year. So we did that and while it was dry, the State Parks did some site improvements and infrastructure maintenance that they needed to do.”

A crayfish in Lake Harriet – photo: Nevada Department of Wildlife

Lake Harriet was re-filled early this past spring, with the plan for a re-introduction of Pahrump poolfish shortly thereafter. That plan, however, was further complicated by the COVID-19 pandemic that shut down the country in March.

“When COVID hit, we couldn’t get out to do a whole lot of work so the pond had just been sitting there all spring, which is actually good as that allowed for some vegetation to develop in the system,” Senger said. “But once we got back to work this summer, we took our fish that we’ve been holding at our hatchery that originally came from Lake Harriet, as well as from our other refuge sites, and finally got them back into the lake.”

The re-introduction of Pahrump poolfish to Lake Harriet in June of 2020 has so far been successful, with reports that thousands of larval, newly hatched poolfish can be seen around the shorelines. 

“Those fish we put in there obviously had a big spawn which was great, we saw just by the naked eye thousands of freshly spawn larval fish,” Senger said. “Those are immediate results of that restoration project and we are very enthused about that.”

But while another restoration project is now underway at Corn Creek and Lake Harriet has begun its rebound, NDOW is pushing to educate the public not to dump their aquarium fish into these sites. 

“It seems like the most common thing is people have these fish at home and they become attached to their pets,” Senger said. “So they may think they’re doing something good by releasing their fish out somewhere so they can live out the remainder of their natural lives. Typically it’s not anything nefarious, but it’s just a lack of understanding of what can happen when you dump fish in the site like that.”

For pet owners who can no longer keep or care for their fish, Senger wants the public to know there are other options out there. Many pet stores, for example, can take fish and find a new home for them. 

In order for the Pahrump poolfish population to continue its rebound, it’s vital that non-native aquarium fish are not dumped into their refuge sites. Introduction of aquarium fish to desert spring habitats are not only dangerous to Pahrump poolfish, but also to the aquarium fish themselves, which are accustomed to year-round warm waters. 

“One or two things are going to happen [when you dump your fish], either you’re putting those fish into a site that isn’t warm year-round and they will die,” Senger said. “Or their aquarium fish may live, but it can be at the cost of potentially thousands of native fish. So it’s really a common education approach and just trying to get people to be aware of what type of effect [aquarium dumping] could actually have.”

What makes non-native aquarium fish so harmful to native populations of Pahrump poolfish is their aggressive nature and ability to breed rapidly. For the Pahrump poolfish, which is accustomed to being the only fish species within a single spring, they simply haven’t evolved to be able to adapt to these threats and competition introduced into their habitat.

“When [aquarium fish] get dumped into one of our natural spring sites that has year-round warm water, that’s just a haven for them because that’s what they’re used to,” Senger said.  “A lot of them are very fecund; they breed very readily in a home aquarium and even more rapidly in a warm spring. So it’s really hard on the springs when these types of tropical fish are introduced.” 

But despite these frequent challenges, Senger is optimistic for the future of the Pahrump poolfish. NDOW is near-completion on a number ongoing restoration projects, including a revamped refuge site in White Pine County, which will bode well for the endangered species. 

“Once we get the Rotenone treatment done at Corn Creek in the next year, we’ll get poolfish back in there,” Senger said. “We just have to get through this next year or two because we’re right on the cusp of having all these sites doing really well with a lot of good restoration being completed.”

If all goes well and no more non-native fish species are introduced to these refuge sites, Senger foresees a population boom for the Pahrump poolfish over the next couple of years. For the long-abused fish, this opportunity has been decades in the making.  

“If we can keep non-native fish out of these sites, I think we’re really close to a species-wide, tens of thousands of poolfish,” Senger said. “Last year, we were at 6,000 poolfish among all the sites total and I think tripling that over the next couple of years is totally reasonable.”

Scott King is a graduate student at the University of Nevada, Reno, pursuing his Master’s degree in Media Innovation. Originally from Cleveland, Ohio, Scott recently returned from Grenada, where he served for two years as a literacy teacher with the Peace Corps. Support his work in the Ally.

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