National Park Service biologists planned to close off and poison a slough connected to the Colorado River upstream of the Grand Canyon to kill young, non-native bass this weekend, the agency said. It’s the second time that officials have used rotenone, a fish-killing agent, as an emergency measure to slow a mushrooming smallmouth bass invasion from Lake Powell that threatens native humpback chubs that swim the Colorado farther downstream. This time they’re seeking hundreds of young bass, instead of the handful first detected in the slough between Glen Canyon Dam and Lees Ferry last year.