The Grand Canyon’s Small Mouth Bass Problem
With all the conversation going on around falling water levels in Lake Powell, one issue that has garnered less press is how small mouth bass are passing through Glen Canyon dam and into Grand Canyon. Certainly, there have been some stories, but not to the degree that maybe the issue deserves, given the severity of the concern for the sheer disruption it could cause in the aquatic ecosystem of Grand Canyon.
Small mouth bass is the most recent, and alarming, arrival among a number of non-native fish that have been identified in the Colorado River below Glen Canyon dam. There is evidence that green sunfish, carp, and a few other species exist in Grand Canyon, and all these fish are, in one way or another, a threat to the delicate balance that exists in the canyon for native fish, some of which are on the Endangered Species list. Humpback chub is the Grand Canyon fish that states and the federal government have spent the most effort and money to recover over the past few decades, and recent data shows that the canyon’s humpback chub population has staged quite a comeback.
Green sunfish were discovered in Grand Canyon in 2015, and their presence in the canyon has generally been from being pulled through the hydropower generation tubes in Glen Canyon Dam, and to a lesser degree from being flushed into the canyon during flash floods that race down the Little Colorado River. Small mouth bass were introduced into Lake Powell in 1982 and thrive in the warm water layers near the top of the lake. Small mouth bass (and green sunfish) are both voracious feeders, and adults can grow to between 12-22 inches long. Humpback chub is also a reasonably sized fish, with adults getting up to about 20 inches in length. They can live to be up to about 30 years old in the wild and are uniquely adapted to surviving in their whitewater habitat.
Small mouth bass pose an urgent two-pronged threat that could put the decades of work (and investment) that has gone into recovering humpback chub populations in real jeopardy. First, small mouth bass would prey on the much younger humpback chub juveniles, creating a “generational” problem with mature humpback chub getting older over time with a lot fewer young chubs coming up behind them. The second problem, which sets small mouth bass apart from green sunfish, is that the small mouth bass’ “gape” or ability to open its mouth really wide, is considerably larger than the green sunfish, making it possible for the bass to eat even adult humpback chub. So, the threat of decimating the population of chub actually exists for both juveniles and adults.
A logical question then might be – “how are these fish getting into Grand Canyon when they have never been there before?” The answer points directly back to lake levels in Lake Powell. Take a look at the chart below…
What this chart shows is the temperature of the water from the lake’s surface to the depths, by year. Lake elevation is the vertical axis, with time on the horizontal axis from the year 2000 on the far left to 2023 on the far right. The big change is within the big red circle on the upper right – showing how the warmer temperature water has steadily declined past the black line that identifies the elevation of the “penstocks” or the tubes through the dam where water flows to produce hydropower. As the lake has fallen, small mouth bass have been pulled lower and lower in their comfy warm layer until they can pass through the hydropower tubes, through the dam, into Grand Canyon.
Additionally, since that water is so warm, once they get below the dam they have a new playground (and breeding ground) that is warm and full of food. And this is where the real concern is – both that small mouth bass can now reproduce in that warm water and have an abundant food source in both young brown and rainbow trout (in the usually cold waters just below the dam – a very productive trout fishing destination) and native fishes like humpback chub.
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Concern has been raised over the past couple of years as lake levels have fallen, but it wasn’t until last spring that there was evidence that the fish had passed through the dam. But now they are there, and the National Park Service, Arizona Game & Fish, Bureau of Reclamation, and other stakeholders are all trying to figure out the best way to address this issue. Upon discovering that the fish were actually in the canyon, officials began poisoning the fish to try to tamp down the population, but that solution is not a long-term answer, and is highly troubling for tribal people who object to the taking of life.
Currently, the Bureau of Reclamation is conducting an assessment is exploring four different proposals for how to flow more cold water through the dam to hopefully discourage the small mouth bass from reproducing. In the meantime, there are other proposals for how to screen or block the fish from getting close enough to the dam to be pulled through the hydropower tubes. The public can comment until March 10 by submitting your comments to this email address.
It’s a complicated issue, and one that many people are trying to figure out the best path forward. Millions of dollars have been spent recovering humpback chub and other native fish in Grand Canyon. We must figure out ways to sustain the native fish population within the canyon and help keep that ecosystem in balance.