Outstanding in the Field (Ep 5): Fish in the Grand Canyon

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Narrator: Welcome, and thanks for joining us for this episode of Outstanding in the Field, the U.S. Geological Survey’s podcast series produced by the Ecosystems Mission Area. We’ll be highlighting our fun, fascinating, often grand fieldwork studying ecosystems across the country. I’m Marisa Lubeck.

Today we’re talking about one-of-a-kind native fish species that call the Grand Canyon segment of the Colorado River home. These native fish species can’t be found anywhere else in North America, meaning they are endemic – or truly natives – of the Colorado River Basin. However, changes in the river system over time have altered the habitat and impacted native fish populations, such as those of North America’s largest minnow - the endangered Colorado pikeminnow. Growing up to six feet in length, it was once the top predator in the Colorado River, but is no longer found below Glen Canyon Dam.

David Ward: In the Colorado River in Grand Canyon there were formally seven species of native fish—this was prior to the dams. We currently have five species that are present in the Colorado River in Grand Canyon, and two of them are missing. The two that we don’t have any more are the Colorado pike minnow and the bonytail chub.

Narrator: That’s David Ward, a USGS fish biologist with the Southwest Biological Science Center’s Grand Canyon Monitoring and Research Center in Flagstaff, Arizona. He and his colleagues are studying the past and present conditions of the Colorado River to understand the impacts to fish populations.

David Ward: The five species that we have existing in the Colorado River in Grand Canyon today are the speckled dace, the flannelmouth sucker, the bluehead sucker, the humpback chub, and the razorback sucker, which we thought was gone for a long time, but was just recently detected in the Colorado River near the Lake Mead inflow in the last few years.

Narrator: Six-foot predatory minnows? Humpback chub that can live up to 30 years? What made the native fish in the Colorado River so unusual? And how have the river’s characteristics over time helped them evolve into truly incredible and specialized species?

Ward: Prior to Glen Canyon Dam, the Colorado River was really a river of extremes. Extremes of temperature, so, in the summertime the water got really warm and, in the winter, it was below freezing. In the summer, raging torrents, in access of 100,00 cubic feet per second during the summer. And in the winter, it would go down to a trickle of just a couple thousand cfs. So, the unique sort of features that we have for our Colorado River native fish, things like humps in humpback chub, and the bony keel in the razorback sucker, are a product of millions of years of surviving these environmental conditions and these environmental extremes that were present all the time in the Colorado River.

Narrator: Today the Colorado River generally runs clear, but in the past, it was often a murky brown mixture of sandy sediment. This was especially true during spring snow melt and rains, and during the strong summer monsoon storms that are common to northern Arizona and other parts of the Southwest. This sediment-laden water greatly limits visibility and makes it hard for nonnative fish to see the young native fish they prey on. The clear river we see today is the result of the Glen Canyon Dam, which was completed in 1963. The dam changed the river and river affects the fish.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     

Ward: When you put a large dam on a river and it backs up the water into a reservoir, the water that’s released below the dam is really cold because it’s coming out of the bottom of the reservoir. And so, it changes the river a lot because it changes the thermal regime, it changes the temperature of the river, it also stops all the sediment, it also changes the flow patterns. So, instead of flooding, we get a steading stream of water coming out below the reservoirs.

Narrator: Water temperature is very important to fish. Some nonnative fish, like rainbow trout, do well in colder water. Remember David saying the water released just below the dam is really cold? Well, rainbow trout, an economically important sportfish, are abundant directly downstream of Glen Canyon Dam. However, they compete with and prey on native fish downstream of the dam. The Colorado River below the dam is getting warmer and that warmer water is good for native fish like the endangered humpback chub, which right now is found about 75 miles downstream of the dam.  

Ward: One of the things we’re studying in the Colorado River to help managers is how the effects of drought and warmer conditions are affecting the native fish, particularly the humpback chub, in the Colorado River. So, with current drought conditions, reservoir levels in Lake Powell have dropped considerably, so the water that’s coming out below the dam is much warmer than is used to be. All of our Colorado River native fish, including humpback chub, are warm water fish, so they’re doing a lot better right now with warmer water because of drought conditions.

Narrator: But, humpback chub isn’t the only fish that likes warmer water temperatures. Some unwanted invasive fish species do as well, and this could be a problem in the future.

Ward: One of the things we’re worried about is that the invasive species like smallmouth bass and catfish also do better with warmer water conditions. So, we’re worried that those species are going to become established in the Colorado River, and through competition and predation, the invasive species will outcompete the native fish. So, one of the things we’re studying is the relationships between our invasive predators and our native fish to try and give managers some information on what to expect as drought conditions continue and the water continues to be warmer coming out of Glen Canyon Dam.

Narrator: We don’t have a crystal ball to tell us how the Colorado River will change looking forward, but USGS science helps managers respond to change and conserve the remarkably unique native fish populations of the Colorado River Basin.

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This podcast is a product of the U.S. Geological Survey, Department of the Interior. Music is by Marty Fitzpatrick and artwork by Jeffery Kemp. A big thank you to David Ward and Todd Wojtowicz from the USGS Southwest Biological Science Center, and the rest of the Outstanding in the Field team: Suzanna Soileau, Hannah Hamilton, Sue Kemp, and Catherine Puckett. I’m Marisa Lubeck. Thanks for listening.

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