Establishment of a Reproducing Population of Endangered Humpback Chub through Translocations to a Colorado River Tributary in Grand Canyon, Arizona

Translocations, defined herein as the human-assisted movement of individuals from a source population to other waters within their historical range, are prevalent in recovery plans for endangered fishes. Many translocations fail to establish new populations, however, and outcomes are often poorly documented. Endangered Humpback Chub Gila cypha persist as a self-sustaining population in Grand Canyon, Arizona, despite threats from introduced nonnative competitors and predators and modified flow, thermal, and sediment regimes due to river regulation. In the decades following the completion of Glen Canyon Dam, the Grand Canyon population has been primarily sustained through reproduction in a single Colorado River tributary, the Little Colorado River (LCR). To establish population redundancy and aid in recovery, we annually translocated between 243 and 509 juvenile Humpback Chub from the LCR to Havasu Creek, a smaller Colorado River tributary in Grand Canyon National Park. Juvenile Humpback Chub were collected from the wild and reared in a hatchery for 8-12 months prior to the translocations. Through biannual mark-recapture sampling in Havasu Creek, we estimated annual abundance for all of the translocated cohorts and found that apparent survival and growth rates met or exceeded the demographic rates that are published for the LCR. We observed reproductively mature adults each year in May, beginning in 2012, and untagged juvenile Humpback Chub beginning in the following year and every year thereafter, with results that indicated successful reproduction. Beginning in 2016, we noted recruitment to maturity of fish that were produced in situ and the population's abundance increased through 2018, indicating potential for the establishment of a self-sustaining population. As an example of the successful translocation of an endangered species that demonstrates the potential importance of tributaries in the recovery of large-river fishes, our study may help to inform future recovery planning.