A fishy mystery: Student's discovery unravels hunt for source of an invasive fish

Wesley Gerrin traces lines on a computer screen with his fingers. Some lead to the left, toward the Ocmulgee River. Others cut to the right, toward the Middle Oconee.

Along them, red dots mark the GPS locations where evidence was found.

Gerrin (BSFR ’15, MS ’21) is part of a team of researchers at the University of Georgia Warnell School of Forestry and Natural Resources who are picking up the breadcrumbs of clues left by an invasive fish called the weather loach. Native to Asia, the fish are typically found in North American aquariums. But in past decades, weather loaches have been reported in the wild.

Visit the Approach of the Loach website

Georgia had been spared the spread, but in 2020, while sampling a creek in Athens-Clarke County, a Warnell student discovered a weather loach. In the years since, Warnell research professional Gerrin and associate professor Jay Shelton, with the assistance of the Georgia Department of Natural Resources, have assembled a team of students and faculty to create a comprehensive picture of this invasive fish and its effects on Georgia’s waterways.

“This is one of many examples of aquatic nuisance species—weather loach, snakeheads, zebra mussels—this is a cause for serious concern, and you can do so much harm when you release non-natives. It changes everything,” said Shelton. “It creates more consequences down the line; we lose diversity, and we start to see things that are already rare disappear. Invasive species are a big, big problem.”

Scene of the crime

Warnell faculty and students eventually collected 15 weather loaches from McNutt Creek, just below an obsolete low-head dam. But based on their preliminary investigations, they suspected this was only the start of the mystery—the scene of the crime was likely upstream, and the fish they discovered were stuck below the dam.

So, Gerrin began the hunt.

He traced McNutt Creek to identify public land along its tributaries, working with local governments for permits to search. Then, he and other faculty members created a protocol to follow each time a fish is found: Record its location, note the date and time, photograph the area. Gerrin built a website with ArcGIS where data can be uploaded directly from his phone in the field.

The clues began to mount: One weather loach found in a nearby creek, another Middle Oconee tributary. Two more found near Beech Haven and another found in a stream at Ben Burton Park, both in Clarke County. One found in another county property, Tallassee Forest. Heading east, three more were found in another tributary, Bear Creek.

Gerrin pinched his map to zoom in on an area in Jackson County that’s glowing red. “This is what I’m starting to think is the initial introduction site for the Middle Oconee,” he said. “We have so far pulled out 63 from here—10 when we first found them here, another 29 last June and then 24 in August.”

But there’s more evidence to gather than just the fish. Gerrin took a step back to connect some dots.

Using the GPS locations of the fish, he created a topographical map and compared the slopes of the stream bed and the surrounding areas to other places that had barriers. “They correspond with a drop-off and they wouldn’t be able to swim upstream,” he said. Using the map data, Gerrin is now looking at other sites with higher possibility of loach occurrence. This modeling is also helping to narrow down potential sites where the fish might have been introduced.

COLLECTING THE EVIDENCE

Finding the point of introduction isn’t the only goal of the project, though. There is a whole investigative team.

“What we’re trying to do is, simultaneously, train our students in this area, expose them to working with other professionals like DNR who work with aquatic nuisance species, also answer some of DNR’s questions and then do something about it,” said Shelton. “So, it’s multiple faculty, multiple students, both undergraduate and graduate. And when field season is coming to an end, we have a lot of other parts that will continue.”

First, the researchers want to gain a better understanding of how weather loaches behave in Georgia waters. As the fish are identified and removed, they are dissected; Sarah McNair (BSFR ’20, MNR ’22) is removing stomachs and intestines for a gut analysis, and otoliths and fin clips for DNA and environmental analyses. She started with the project last year after earning her chops volunteering on other sampling events as an undergraduate student.

“Pretty much wherever Wesley and the crew went, I went,” she said. “This whole experience has been really awesome. It really depends on the day; that’s one of the fun things about this—every single day is different.”

McNair has also worked with Warnell associate professor Elizabeth Pienaar on outreach materials to educate the public about invasive species.

The stomachs and intestines extracted by McNair go to undergraduate student Adam Musolf, who is working under the guidance of Warnell associate professor Pete Hazelton (MS ’08, PHD ’13) to analyze the weather loaches’ gut contents and stable isotopes. This work will help the team understand what the fish are eating and where they fall on the food chain. Musolf began working on the project last summer as part of a new Warnell internship program that combines research and outreach. Now he’s turning the project into his senior thesis.

“And to go along with that, we’ve collected a bunch of other fish groups to compare to the loaches. And so many bugs—because they are generalists, we expect some bugs to be part of their diet,” said Musolf.

Last fall, Musolf worked in a lab to identify and categorize bugs—not something he expected when he switched his major from marine biology to aquatic sciences. But it’s great experience, he added.

“I found out that electrofishing is a very fun thing to do, and it’s kind of what I wanted to be spending the summer doing,” he said. “And now I know how to look at bugs and I’ve gotten much better at fish ID and so this has been very helpful groundwork for whatever I move on to do in the future. And I’ve had fun along the way—that’s what everything is supposed to be like.”

Associate professor Marty Hamel will analyze otoliths, which can reveal the environment in which a fish lived for different years of its life. This way, researchers can learn whether a weather loach spent its entire life in the river—or was raised in an aquarium environment.

And then there’s the fin clips, which will be used for DNA analysis. Associate research scientist Brian Shamblin is using them to determine whether the fish are related, and if they can be linked to other invasive populations or potential captive sources. 

“We ran the mitochondrial DNA, which is the maternal lineage. We also ‘fingerprinted’ them—that is the nuclear DNA coming from both parents. The ultimate goal there is to see if we can match offspring to parents and perhaps match the fish in one tributary to fish elsewhere,” said Shamblin. But while preliminary results show the fish in the Middle Oconee system are related, he has yet to find direct offspring linking tributaries. “I was hopeful we’d be able to say this male in this creek came from a female that was sampled upstream, but so far we don’t have that—which is kind of scary,” he added.

He also plans to use eDNA to test water samples for weather loach DNA. This is a new process, but based on preliminary work he’s done for a different project, he’s hopeful this new technology can shed some additional insights on the extent of the loach invasion.

The Middle Oconee weather loaches are genetically distinct from weather loaches purchased from area aquarium stores. And then there’s a third, distinct population.

THE PLOT THICKENS

“This is where the story takes a very interesting twist,” said Shelton.

Warnell alumna Katie Lamp’l (BSFR ’17, MS ’21) was collecting water samples in nearby Gwinnett County when she spotted something that didn’t belong.

It was a weather loach.

Having worked on sampling events as a student, she knew what it was as soon as she saw it. But in Gwinnett County? That posed a problem, said Gerrin.

“This is in the Yellow River watershed, which drains to the Ocmulgee River—which is different from where we’ve been working,” said Gerrin. “To get from the Middle Oconee to the Yellow River, you’d have to go down through two dams, three lakes and up through one dam and lake—it’s not happening.”

So, the investigative team secured permits from Gwinnett County and began the process again. Using what they learned from the other population, they focused efforts on topography and barriers to movement.

“We basically just started at the confluence of Sweetwater Creek with a smaller tributary and worked our way up toward a series of ponds. And the first half or so was really nice, fast-flowing habitat with a lot of gravel and rocks; they don’t like that,” said Gerrin. “We didn’t find a single one until things started to flatten out and turned into a bed of silt that had collected tons of organic matter. When we stepped in, we sank almost to our knees. When we got to that, they just started popping up everywhere.”

The creek, adjacent to a public park and neighborhood, flows from a pond ringed by houses. There’s no direct route upstream out of the pond, so it’s possible it’s an entry point. 

The discovery adds another layer to the mystery. Could one population of weather loaches exist from an aquarium release, while a second was released into a pond for a food source? In Asia, where weather loaches are native, they are cooked in soups or sold as street-food snacks.

With this new theory added to the casefile, the team is widening its search and retooling its outreach efforts. They’re also trying to check aquarium supply stores and local supermarkets for weather loaches—anything that can provide additional DNA.

It’s all part of the investigation. Beyond their native range, little is known about how weather loaches respond to new habitat. But through the efforts of the Warnell experts, we will have an extensive set of data for the Southeast.

“We have reason to think they could live in ponds and wetlands. What we’re seeing is this preference for particular habitat here that may or may not be the same as other places in the world,” said Shelton. “So, the plot has thickened.”