EDINBURG — Joshua Reaves caught something unexpected while fishing in an irrigation canal more than a week ago: an 18-inch pacu.

A relative of the piranha, the large tropical fish is becoming more common in Rio Grande Valley waterways as residents who keep them as pets dump them into canals and resacas when the fish grow too big or become a hassle to handle, according to wildlife observers.

Reaves, 28, said he was surprised to find the large silver fish swimming in a canal, near Farm-to-Market Road 495 and Stewart Road, and was even more surprise at how he caught it.

“I caught him on a piece of boneless chicken. They’re supposed to be vegetarians,” he said.

They’re also supposed to be swimming in fresh waters of South America, not South Texas.

But pacu have been bred in fish farms and sold at pet stores throughout the country, typically when they’re still just a few inches long.

Many owners initially don’t realize pacus can grow beyond 20 inches in length in a short time, outgrowing the typical 10-gallon aquariums in which they are kept.

“What happens is they sell them at Wal-Mart when they’re little,” said Eric Kite, owner of Aquarium Masters in McAllen.

“They grow based on their size potential … They get up to 19 pounds. A lot of people, when they grow, toss them in the canals.”

A fish that size would need to be placed in at least a 200-gallon tank, he said.

“It’s like going and getting a St. Bernard puppy,” Kite said. “They’re not thinking about the long term.”

Fish and other creatures not native to the area are found in the wild here in the Valley all too often, say state and federal wildlife experts.

Reynaldo Cardona, lead technician at Texas Parks and Wildlife Department’s Inland Fisheries Division office in Corpus Christi, said growing numbers of goldfish, pacus, oscars and other aquarium fish can be found swimming in waters throughout South Texas.

And fish aren’t the only would-be pets at large in the Valley.

“We found iguanas here at the Santa Ana (National Wildlife) Refuge,” said Bryan Winton, refuge manager at the Lower Rio Grande Valley National Wildlife Refuge. Iguanas are not from the Valley, so they were likely brought here as pets, Winton said.

While owners think they’re finding a humane way to dispose of the animals, they’re actually introducing a non-native species into the habitat, which can set off a cascade of problems.

The newcomers might outperform the local flora and fauna and overpopulate because there is nothing in the habitat to control them, experts said.

Such is the case with hydrilla, water hyacinths and other foreign aquatic plants that have made their way to the Valley, Cardona said.

“Now they’re hurting lakes. We don’t have native insects that can control them,” he said.

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Jennifer L. Berghom covers health, environment and science issues and general assignments at The Monitor. She can be reached at (956) 683-4462.