Monday October 2, 2023

High Country News

On July 1, 2022, a National Park Service biologist named Jeff Arnold was hauling nets through a slough off the Colorado River, several miles downstream from Glen Canyon Dam, when he captured three greenish fish lined with vertical black stripes. He texted photos of his catch to colleagues, who confirmed his fears: The fish were smallmouth bass, voracious predators that have invaded waters around the West. Worse, they were juveniles. Smallmouth weren’t just living below the dam — they’d likely begun to breed. 

It was a grim discovery. Smallmouth bass, whose native range encompasses rivers and lakes in much of the Eastern United States and Great Lakes, have long plagued the Colorado River. State agencies and anglers probably began stocking them in the watershed in the mid-1900s, and they’ve since conquered much of the basin, including Lake Powell, the reservoir that sloshes above Glen Canyon Dam. Downriver from the dam, however, lies the Grand Canyon, whose sandstone depths have historically provided a bass-free haven for native fish — most of all, the humpback chub, a federally threatened species endowed with an odd dorsal bulge. Now, biologists realized, neither the canyon nor its chub were safe.

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