How Does This Aquarium Fish Thrive In Waller Creek?

Austin is the unique home to a thriving wild population of one of the most common pet fish in the world: the variable platy fish (on sale for $1.59 at Petco).

The platy is colorful, cheap and now the most abundant fish in Waller Creek, which runs about seven miles from North Austin down to Lady Bird Lake. There is no other known established wild population in the U.S.

“For whatever reason, right here in Austin, Texas, on Waller Creek we have what seems to be perfect conditions for them,” says Adam Cohen, fish collection manager for the Biodiversity Collections at UT Austin.

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Platys, as they’re known by fish fans, jumped the tank as early as 2004, and now Waller Creek is full of them. Colter Sonneville, who saw them downtown, sent a question to KUT’s ATXplained project. He wanted to know how the fish got here, and how they’ve eked out an existence in the creek. (For the record, this is Sonneville's second ATXplained question that KUT's investigated.)

Sonneville is a landscape architect and self-described “aquarium nerd,” who used to keep three tanks in his bedroom. Seeing the platys swimming in the wild surprised him.

When he kept the fish growing up in Chicago, he says, he knew the tank couldn’t get below 45 or 50 degrees or the platys would die.

“To see them in the wild still just kind of blows my mind,” he says.

Fair-Weather Fish

Sonneville guessed the platys in the creek were an aquarium species gone wild, but how they were surviving the cold was another question.

This fish is native to northern Mexico, right along the Gulf Coast. But they get shipped around the world as a cheap, hardy aquarium species. While Austin’s climate is within the same subtropical zone as northern Mexico, platys shouldn’t survive the winters here.

“They’ve been introduced all over the world and most of the introductions do not actually take,” Cohen says. “So someone’ll drop their aquarium fish in the creek nearby, and they might live for a summer, maybe a couple years, and then a winter comes along and knocks them back.”

In Austin, though, they stuck. That’s probably because of the sewers.

Waller Creek is a relatively straight line from North Austin down to Lady Bird Lake. But the straight line is met by a bunch of perpendicular tributaries, mostly underneath roads. In the wintertime, rainwater flowing into the sewers and under those roads stays relatively warm.

“By the time it flows back into the main stem of Waller Creek, it’s warmer than the ambient water in the creek,” Cohen says.

Temperatures in 2010 reached as low as 17 degrees Fahrenheit, and that should have killed the platys. Cohen uses that winter as a sort-of litmus test for the fish. Because they lasted the winter of 2010, he’s more certain of their permanence in Austin.

The Birds And The Bees And The Platys

Learn about the anatomy of the platy fish and one perk for offspring living in the creek, with Adam Cohen, fish collection manager at UT's Biodiversity Collections. Hint: The babies can escape a cannibalistic parent.

A Benevolent Invader

Instead of dumping pet fish into the wild, Marc Whittle at Aquatek Tropical Fish says, customers can return fish – platy or otherwise. But he understands why people tend to dump them.

“There are very, very exotic, very expensive fish here that no one would ever release into the wild,” he says. “Platys? They’re very cheap fish and I think many people, because they seem almost disposable, they feel like, ‘What’s the harm, I’ll just let these go over here.”

The U.S. isn’t the only place where this is a problem.

In Mexico, “people were taking the platys and releasing them in the local streams to make it look like the water was sweet, like all this life and color was going on, just to sell property,” Whittle says.

"I don't really hope to catch invasive species, but it is exciting when you find something new in nature that's unexpected."

Whittle used to work for the Dallas World Aquarium and other organizations collecting fish internationally. He went to Mexico as part of an endangered species research mission and found the platys were competing with native, endangered fish.

In Waller Creek, the platys are considered invasive, but researchers haven’t found any specific problems they’ve caused, Cohen says. Harmful environmental effects or not, he draws his foot in the sand: No one should be putting invasive species in creeks just to make them look cool.

As a researcher, he’s embraced the fish.

“I don’t really hope to catch invasive species, but it is exciting when you find something new in nature that’s unexpected,” he says.

As a dad, he collects platys from the creek with his daughters for their home aquarium.

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Sonneville, however, is done with keeping fish; the maintenance became too much of a burden. He says he’s found a new passion.

“I replaced it with fly-fishing actually. That’s my new jam,” he says. “It's like A River Runs Through It meets Austin.”

To see what other species live in Waller Creek, visit the citizen science website iNaturalist. Researchers from UT and hobbyists add to the online database.

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