The Grand Canyon was once home to eight native fishes, including six endemic to the Colorado River basin. Following construction of Glen Canyon Dam in 1964, which fragmented habitats and homogenized the once highly dynamic flow regime, five were extirpated, and two are now listed under the Endangered Species Act (ESA). In the cold tailwater below Glen Canyon Dam in Grand Canyon, invasive salmonids thrive, threatening the remnant fish assemblage that evolved under seasonally warmer and silt-laden waters. Experimental actions are in progress to recover the endangered humpback chub and razorback sucker, as well as conserve imperiled flannelmouth and bluehead sucker populations. Assessing the efficacy of adaptive management actions is often difficult due in part, to confounding environmental variation or unexpected responses of fish populations.
One such experiment involves suppression of invasive salmonids, followed by translocations of humpback chub, to smaller tributaries of the Colorado River within Grand Canyon National Park, where less-impaired flow and thermal regimes exist. In cooperation with the National Park Service, Bureau of Reclamation, and U.S. Fish and Wildlife Service, the Utah Cooperative Fish and Wildlife Research Unit is investigating the efficacy of these ambitious experiments.
Following electrofishing to remove trout from >15 km of one tributary, Bright Angel Creek, analyses suggested ~90% declines in invasive salmonid abundance and ~480% increases in native suckers and speckled dace, over eight years. Building on the successful suppression of trout, 120 passive-integrated-transponder-tagged (PIT) humpback chub were released in May, 2018. A fixed PIT tag antenna array and seasonal monitoring with hoop nets will be used to estimate demographic rates of humpback chub, and other native fishes, in the coming years. Data will be used to monitor how spatial-temporal variation in invasive salmonids, hydrology, and the thermal regime may influence the population dynamics of native fishes. Mixed effects regression models suggest flow and temperate may influence the recovery of native fishes following salmonid suppression. This research may assist managers in prioritizing and planning recovery actions across the Colorado River basin where flow alteration and invasive fishes threaten the unique, endemic, fish assemblage.
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