Fish Stocking Threatens Native Species with Invaders

Mountain yellow-legged frogs actually have creamy beige legs, and a back that mimics granite, to help them be more cryptic to predators.
Mountain yellow-legged frogs actually have creamy beige legs, and a back that mimics granite, to help them be more cryptic to predators.
Molly Peterson/KPCC

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This week, we've been examining the problem of invasive species in Southland waters. For more than 100 years, California's stocked lakes, rivers, and streams for recreational anglers. But that's getting harder to do because of more non-native species entering the state ... and more scientific research. KPCC's Molly Peterson starts where some trout do: at a hatchery in the Eastern Sierra.

Molly Peterson: Denny Redfern rolls up his sleeve and reaches into a pool of water at the Hot Creek Hatchery for a small, smooth rock. He checks its underside for New Zealand Mudsnails – an invasive pest.

Denny Redfern: The snail will come in and denude all of the vegetation through here and take the food source away from these other aquatic insects, and that's where the danger in this invasive is to change the dynamics of the stream.

Peterson: Gently, Redfern returns the rock. He's been a hatchery man for 38 years; now he runs the Department of Fish and Game's operations here. Last December, routine sampling found the tiny snails in the spring waters for the production ponds where trout grow.

Just one mudsnail can reproduce – fast. So the state won't try to purge mudsnails from rivers. Instead, Redfern says, the plan is to stock fish from the infested hatchery in waters that are already infested.

Redfern: We're bouncing fish around, still providing the opportunity up here, but it's been a logistical nightmare of moving fish around to provide good enough opportunity through there.

Peterson: Redfern says expenses are up. But there's no way around it. A law anglers lobbied hard for requires the state to stock a certain amount of trout for every fishing license sold. The state's sport fishing industry generates billions of dollars in retail sales and wages every year. Redfern says the rods and the reels and the bottomless creels of Southern Californians help to drive the economies of Inyo and Kern Counties.

Redfern: L.A.'s playground!

Peterson: State law also requires native trout for a quarter of hatchery stock. Conservationists and trout protection groups sued, claiming that non-native brown and brook trout were squeezing out golden trout – the state fish.

Scientists say there's also pretty good evidence that trout harm another freshwater resident: the mountain yellow-legged frog.

[Sound of crunching and hiking down into Little Rock Creek]

Peterson: Deep in the Angeles National Forest, Little Rock Creek is protected by knife-edge hillsides and by court order. Its yellow-legged frogs are endangered under federal law. They populate this creek and seven other places, a small fraction of the hundreds of waters they once occupied across the state. In a dry year, the creek's pretty small too.

Cindy Hitchcock: This time of year, there should be flowing water through here.

Peterson: After hiking up a rocky creek bed for more than an hour, frog biologist Cindy Hitchcock's reached a man-made fish barrier.

Hitchcock: There's sort of a natural barrier with this bedrock here. And we put this cement on top of it, a cement wall with holes coming out of it so that the water could go through still but it would enhance the barrier to the fish, so that they couldn't get over unless it was super high water.

Peterson: Above it, scientists expect frogs to thrive, but not fish. That's because to restore the area, wildlife managers pack big car batteries in here and stick a wire into the water to electroshock them.

Hitchcock: And it stuns a fish and has them float to the top until you can scoop them out.

Peterson: Above the barrier, a few trout – Hitchcock doesn't like them – and some mountain yellow-legged frogs.

[Sound of a splash]

Peterson: Now up to her waist, Hitchcock lunges for a frog kicking deep across a pool.

Hitchcock: Ooh, I got one.

Second USGS Biologist: Yeah? You did?

Hitchcock: And some rocks.

Peterson: U.S. Geological Survey biologists have been sampling here for years, to figure out whether it helps to keep fish away. Hitchcock slips a transponder under an adult frog's skin.

[Sound of frog on spring]

Hitchcock: Stop jumping!

[Sound of frog bouncing on plastic bag]

Hitchcock: I'm going to tag this one fast ...

[Sound of frog release call]

Peterson: By day's end, Hitchcock and another biologist have tagged 13 frogs, more than they'd expected at the creek.

Hitchcock: All righty, healthy bomber female frog. Off you go.

Peterson: They swab the frogs for a fungus – a known adversary. Global warming may be a threat; so might airborne pesticides from the central valley. But Hitchcock says it's clear that trout hunt tadpoles.

Hitchcock: Trout are known predators of these frogs. We think maybe that's part of the reason they're being shoved up in these headwaters. You know, places the trout haven't gotten to yet.

Peterson: Since the 19th century, that's far fewer places. Early mountaineers transplanted fish in coffee pots; long ago, the east side of the Sierra Nevada range contained no trout. Later, state agents carried fish in milk jugs on pack mules, and used planes flying overhead to boost the burgeoning recreational fishery.

The state's never reported on the environmental impacts of fish stocking. A court ruling this spring means it's got to start. Fish and Game's Jim Starr will supervise the project.

Jim Starr: The department just doesn't consider our maintaining fishing opportunities as an invasive program.

Peterson: Starr says it's possible to ensure that fish and frogs thrive – though probably in non-native locations.

Starr: You go over one valley or a couple of hundred yards. There's another lake that has no fish in it, and we can transplant yellow-legged frogs to that location and get the populations to start.

Peterson: Starr says the state doesn't know yet how much more that kind of active management like that will cost. But he says it's part of the bargain now.