Searching for the needle in the haystack: Identifying natural otolith tags to determine natal origins of the humpback chub in the Grand Canyon

The humpback chub (Gila cypha), is a long-lived fish species that requires high-gradient and turbulent riverine habitats.  Prior to the completion of the Glen Canyon dam on the Colorado River, humpback chub were distributed throughout the Grand Canyon ecosystem.  Sharp declines in humpback chub populations coincided with the completion of the Glen Canyon dam and subsequent regulation of the Colorado River.  Despite intensive rehabilitation efforts, populations have failed to recover and species distribution is limited to an area near the confluence of the Colorado and Little Colorado Rivers.  Many abiotic and biotic factors have been identified to explain the lack of population recovery, however little is known about spawning locations and early life stage habitat use.  Therefore, we have evaluated a suite of naturally occurring isotopic and elemental otolith tags to identify natal origins.  Results suggest that individuals residing in the Little Colorado River tributary obtain unique otolith δ¹³C and δ¹⁸O isotopic signatures.  Furthermore, otolith microstructural analyses reveal that Little Colorado River residency results in higher growth rates compared to Colorado River residency.  Understanding the importance of the Little Colorado River as spawning and nursery habitat is critical for development of successful recovery plans for this species.