Invasion of the Giant Tadpoles: Wildlife Officials Say It's No Joke

They might look harmless at this stage but adult bullfrogs are known as voracious predators. (Kerry Wixted via Flickr)

Swimmers at Lake Anza in the Berkeley Hills found a slimy surprise this summer: huge tadpoles bigger than they’d ever seen there before.

They came in unnerving numbers — each five or six inches long. And they were rude.

“When you’re walking into the water, they wouldn’t even get out of the way,” recalls lifeguard Cecilia Martin, who sometimes struggled with her campers to avoid the many tadpoles underfoot.

“The kids would scream every time they would step on them—I actually stepped on them too," recalls Martin with a chill. "They would squirm out of in between your toes, which is the most gross sensation.”

The tadpoles’ appearance was mysterious, says Martin, who has worked as a lifeguard in the East Bay for four summers (her mom happens to be a news editor at KQED). “I had never seen that before, and my friends who had worked here before for five or six years, they’ve never seen it, either.”

Lake Anza, in Tilden Park, covers 8-10 acres -- too large for effective eradication of the slippery invaders.
Lake Anza covers 8-10 acres in Tilden Regional Park -- too large for effective eradication of the slippery invaders. (Daniel Potter/KQED)

To find out what they were, we called Steve Bobzien, a wildlife ecologist for the East Bay Regional Park District. He checked out the lake, and was not happy with what he saw: “Thousands — literally tens of thousands of bullfrog tadpoles in Lake Anza.”

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It’s a first, and not a welcome one. Ecological bullies to many California natives, bullfrogs are an invasive species here.

“They’re notorious for gobbling anything up that’s the size of their mouth or smaller,” Bobzien says. That could devastate the watershed’s remnant population of smaller California red-legged frogs, a species federally listed as threatened. “Bullfrogs are also known to eat other vertebrates including bats, small birds, and rodents, even," he notes. "So they’re pretty formidable predator.”

Bullfrog tadpoles can be 5-or-6 inches long, and don't yield to human swimmers.
Bullfrog tadpoles can be 5-or-6 inches long, and don't yield to human swimmers. (Douglas Mills via Flickr)

They’re also explosive breeders. How they got into Lake Anza is unclear, but getting rid of them now would be a challenge, to put it mildly.

“The goal would be to eradicate them, and that’s virtually going to be impossible,” Bobzien says. In a smaller pond, you might scoop them out with nets—or pump water out and bury them with a bulldozer. But that won’t work here. Besides the surrounding stream environment at Tilden Park, the man-made Lake Anza itself covers eight or ten acres.

“Even if you were to put on a full-on effort with a lot of funding, a lot of time, you’re still not going to be able to eradicate them.” At best, Bobzien hopes to keep the population in check, so bullfrogs don’t completely take over.

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He says it only takes two survivors to produce thousands of eggs. Likely, the titanic tadpoles are in Lake Anza to stay.

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