Does the Timing of Tributary Emigration Drive Recruitment Success in the Grand Canyon Humpback Chub?

The Grand Canyon reach of the Colorado River has undergone substantial biological and physical changes concurrent with the completion of the Glen Canyon Dam and subsequent regulation of the river.  Biotic changes include fish population reduction or extirpation of endemic fish species and abiotic changes include lower water temperature regimes.  In 1973, the humpback chub was declared “Endangered” and has failed to recover despite ongoing management activities.  The largest remaining humpback chub population is found in the Little Colorado and Colorado rivers near their confluence.  The Little Colorado River is the only documented spawning and nursery habitat.  In this study, we used natural microchemical tags and daily growth rates obtained from otoliths to estimate the age at emigration from the Little Colorado River to the Colorado River.  Results suggest that adult and sub-adult humpback chub spend at least ~70 days post-hatch in the Little Colorado River before emigrating to the Colorado River.  Although young juvenile humpback chub that have emigrated from the Little Colorado River are frequently found in the Colorado River, few adults that have recruited to the population emigrated as young juveniles, suggesting that early emigrants are unlikely to survive.