Evaluation of Humpback Chub Gila Cypha Egg Maturation Using Ultrasonic Imaging and OvaprimŽ

Although nine known aggregations of humpback chub (Gila cypha) currently exist in the mainstem Colorado River, little is known about their reproductive potential. It has been hypothesized that water temperatures in the section of the mainstem Colorado River that runs through the Grand Canyon are too low for female chub to develop eggs for spawning. Here, we evaluate use of ultrasonic imaging and Ovaprim® to identify the degree of egg development in humpback chub to potentially identify sites where spawning is occurring in the mainstem Colorado River of the Grand Canyon. We first show how surrogate fish, either roundtail chub (Gila robusta) or common carp (Cyprinus carpio), can be used to evaluate the efficacy of the technique in general. Potentially gravid females can be injected with Ovaprim, a synthetic hormone that induces spawning, to confirm results obtained via ultrasound. This hormone does not affect females without developed gametes and has an incidental mortality rate of 1.3%. Ultrasound is then used to image eggs of humpback chub occupying known spawning sites in tributary streams of the Colorado River. Ultrasonic imaging is non-lethal and has been used to successfully determine sex and ovarian maturity of a variety of fishes, including Pacific herring, halibut, haddock, and shovelnose sturgeon.