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Humpback chub threatened by exotic fish slipping through Glen Canyon Dam

Randall Babb
Arizona Game and Fish Department

Humpback chub in the Grand Canyon have flourished under long-running restoration efforts. But now, exotic fish that prey on chub and other native fish have begun to slip through Glen Canyon Dam. It’s an unprecedented problem caused by the drought-stricken low levels of Lake Powell. KNAU’s Melissa Sevigny spoke with National Park Service fisheries biologist Melissa Trammell about her concerns.

Most people think of a dam as pretty effective barrier for fish. Can you explain why fish are able to potentially pass through the dam when the water level is low?

Lake Powell has been declining over the last 20 years, due to reduced runoff… and as the reservoir declines in elevation, it brings the topmost warm layers of water where the warmwater fish mostly live, into closer contact with the intakes for the penstocks for hydropower production…. As many as—or as few as—25% of fish actually survive passage through the dam, and that’s usually seen as a bad thing, if you’re trying to, say, retain salmon in the Pacific Northwest. But here is a bad thing if too many do survive.

And so it’s a bad thing if these fish are getting through because they’re exotic fish, right?

They are exotic fish. These warmwater invasive species are mostly very popular game fish, or prey for the popular game fish in Lake Powell, fish such as smallmouth bass, walleye, stripers, even catfish and green sunfish.

How do these exotic fish potentially affect humpback chub populations? Are they eating them, or what’s going on there?

Smallmouth bass are voracious predator of any other fish species. They start eating fish when they are very small themselves, only a few inches long. They are able to eat fish that are up to a third or even half of their own length…. Many of the other exotic fishes that live in Lake Powell that could potentially come through are also predators… But we’re particularly concerned about smallmouth bass because they’ve been known to easily establish populations in the Colorado River and other tributaries up in the Upper Basin.

And this issue of the exotic dam coming through the dam, this is really an unprecedented problem, right? We’ve never seen these low levels before.

That’s true, we haven’t seen this since the reservoir filled 50 years ago…. The temperature of the water coming through the dam is unprecedented as well. It’s forecasted to be up to say 22-24 degrees Celsius coming out of the dam, which is well into the preferred growth and reproductive temperature for smallmouth bass…. The warm temperatures and associated invasion of these exotic species, smallmouth bass in particular, have the potential to create the biggest change in the ecosystem of the river below the dam that we’ve seen since the dam went in in the first place…. This change is something that we’ll probably never recover from. Once smallmouth bass are established, they’re impossible to remove.

What are some things we can do about this? What are some of the solutions?

There’s a few things we can do to prevent them coming through, and the major one, really, is to keep the reservoir a little bit higher….A few things that have been suggested to prevent escarpment through the dam, is some sort of screening device. There’s bubble screens, there are physical nets put in front of the penstocks… That’s not a rapid response, that’s something that will take time, it takes engineering, it takes a lot of money to put something like that in place… There are additional, more targeted efforts we could make to try to remove those fish, through electrofishing… We could implement those. We are short staffed and short funded, so it’s difficult to respond as rapidly we’d like to an invasion like this.

Melissa Trammell, thank you so much for speaking with me.

My pleasure, Melissa.

Melissa joined KNAU's team in 2015 to report on science, health, and the environment. Her work has appeared nationally on NPR and been featured on Science Friday. She grew up in Tucson, Arizona, where she fell in love with the ecology and geology of the Sonoran desert.