When crews drained portions of the San Antonio River this month for its routine biannual maintenance, a familiar problem surfaced once again: thousands of invasive apple snails were scattered across the riverbed of the River Walk.
More than 3,000 snails were removed during the weeklong cleanup — over 1,000 more than during the last major maintenance cycle in January 2024.
The increase reflects a broader trend, according to data from the San Antonio River Authority, which shows crews recorded 11,212 adult apple snails removed in 2025 through the end of November, compared with 8,359 over the same period in 2024.
The snails, native to South America, were first discovered in the downtown stretch of the San Antonio River in 2019 and are believed to have been introduced through the illegal release of aquarium pets. Since then, the species has spread throughout portions of the river system, prompting years of removal efforts by the San Antonio River authority staff and volunteers.
Adrian Arroyos, an aquatic biologist with the San Antonio River Authority, said this year’s totals reflect the steady growth the agency has tracked in recent years — but with one notable difference.
“During the last drain, almost every snail we picked up was alive,” Arroyos said. “This year we found ourselves locating just shells, which is definitely a positive thing. That means they’re being preyed on.”
While the snails are not being targeted by their natural predators, Arroyos said the increased predation is helping slow the population in a natural way. Alongside that development, river authority biologists are now testing two experimental barrier systems at key points along the River Walk aimed at stopping the snails from reproducing in the first place.
Apple snails are able to spread quickly because of their aggressive reproductive cycle.
According to river authority officials, a single female can lay as many as 20 to 25 clutches of eggs each year, with each pink-colored clutch containing anywhere from 500 to 2,000 eggs. The eggs are laid above the waterline on hard surfaces along the river walls and hatch within about two weeks, dropping back into the river once they emerge.
That process allows populations to grow rapidly if left unchecked, even as crews continue removing adult snails and eggs from the riverbed. In 2025, from January to November, more than 18,000 apple snail eggs were removed and recorded by river authority officials and volunteers.
To interrupt that cycle, the new barriers were installed in areas where snail densities have historically been highest, including the River Center Turning Basin and portions of the Museum Reach between Josephine and Grayson streets.
One barrier design features an angled lip extending outward from the river wall. As snails attempt to crawl up toward the surface to lay their eggs, the angled edge is intended to stop them from reaching the top. When the snail’s shell hits the lip, it is expected to fall back into the water.
The second design mimics bird deterrent spikes commonly placed on buildings; tiny plastic spike-like protrusions are used to create an uncomfortable surface for the snails to cling to. When the snails encounter the spikes, biologists expect them to detach and drop back down before reaching egg-laying areas.
“If we can prevent them from coming up and laying their eggs, then we basically interrupt their reproduction, making it easier for us to manage and also for us to have to cover less ground,” Arroyos said. “It’s experimental. We don’t know how they’re going to react to them, but we hope it does affect them.”
River authority staff plans to monitor how the snails respond over time to the initial 800 feet of barriers to determine whether the approach is effective, before potential expansion to other parts of the river.
Beyond their rapid spread, apple snails pose a significant threat to the river’s ecological health. Arroyos said the invasive species feeds heavily on native underwater vegetation — an essential component of the river’s ecosystem.
“They don’t ever finish their meal,” he said. “They’ll munch on one plant, damage it, and then move on to the next one. In high volumes, that becomes really problematic.”
Native aquatic plants provide shelter for fish, help stabilize riverbanks and play a key role in maintaining water quality by producing oxygen and filtering pollutants. As vegetation disappears, erosion worsens and habitat for native species declines.
Apple snails weren’t the only invasive species crews pulled from the channel during the drain. River authority teams also removed hundreds of non-native fish, including tilapia, Rio Grande cichlids, redbreast sunfish, plecostomus — commonly known as plecos — and common carp. In all, crews documented 1,657 non-native animals removed during the maintenance period, not including apple snails.
Plecos, Arroyos said, remain one of the most physically damaging invasives in the system because of what they do to the riverbanks. Plecos are commonly sold in pet stores and often illegally dumped from aquariums into other waterways. During this year’s operation, 59 Plecos were removed in total.
“They are pretty detrimental to the banks,” he said. “They like to burrow underneath them, and it disrupts the structure of the bank, allowing it to erode way worse than it would naturally. It messes with stability and it creates sediment which affects turbidity in the water.”
That erosion compounds the same water quality and habitat challenges officials are already working to manage. During the 2026 maintenance operation, the Public Works Department removed about 1,770 tons of sediment from the San Antonio River’s main channel. Two years ago, the city was focused on a smaller area and removed 46.6 tons of sediment.
As water levels dropped, river authority crews also worked to relocate native fish out of isolated pockets of water and into designated downstream areas where water remained at safer levels. Arroyos said teams rescued native species including channel catfish, yellow bullhead and largemouth bass, scanning drained portions of the channel for signs of fish stress and moving them as needed.
Some native fish were found dead during the multi-day process. In a statement, river authority environmental science teams said the fish were not left in shallow areas but were found already deceased, likely due to changing conditions within the channel over time.
In total, 3,237 native fish from 12 species were unable to be relocated during the operation. Officials did not provide the number of live fish that were relocated during the maintenance process.
With the draining completed and barriers installed, officials plan to assess the effectiveness of the new systems in the coming months.
Those interested in helping the river authority manage its invasive species problem, can sign up to volunteer with the River Warrior program.
