Humpback chub (Gila cypha) monitoring & translocations in the Little Colorado River, Arizona

Authors: Michael Pillow, Pilar Rinker, David Ward

Year: 2023

Abstract

The humpback chub (Gila cypha) is a large-bodied, federally protected Colorado River cyprinid with populations in Colorado, Utah, and the Grand Canyon in Arizona. The Little Colorado River (LCR) is a warm water, spring-fed, Colorado River tributary at the Grand Canyon that provides important spawning and rearing habitat for a large portion of this population each year. In 2000, the U.S. Fish and Wildlife Service began conducting a series of annual, two-pass, closed Chapman Petersen mark-recapture efforts in the spring and fall in the LCR to track abundances of humpback chub and other native fish. Since 2006, there has been a significant increase of adult humpback chub in the LCR. During spring 2022 the estimated abundance of humpback chub ≥150 mm in the lower 13.57 km of the LCR was 10,563 (SE = 727). Of these fish, it was estimated that 8,525 (SE = 665) were adults (≥200 mm). During fall 2022 the estimated abundance of humpback chub ≥150 mm in the lower 13.57 km of the LCR was 2,320 (SE = 367). Of these fish, it was estimated that 1,888 (SE = 321) were ≥200 mm. These numbers indicate that the spring spawning and fall abundances of humpback chub in the LCR have remained relatively stable since experiencing significant post-2006 increases. In addition, since 2003 the U.S. Fish and Wildlife Service has translocated 4,871 juvenile humpback chub from the lower reaches of the LCR to above Chute Falls, a travertine waterfall complex within the LCR that acts as a barrier to some upstream migration. By the time they are two years old, the majority (74.5%, n = 760) of translocated fish that remain above the falls are adults (≥200 mm), compared to only 12.6% (n = 539) of the captured but not translocated humpback chub. A warmer annual temperature profile and abundance of food above the falls are thought to contribute to the higher growth rates of these translocated fish compared to their non-translocated cohorts in the lower LCR.