Use of Ultrasonic Imaging to Evaluate Egg Maturation of Humpback Chub Gila cypha in the Grand Canyon

Humpback chub Gila cypha are endangered cyprinids endemic to the Colorado River drainage and are adapted to live in fast currents of warm, turbid water. Although nine known aggregations of humpback chub currently exist in the main stem Colorado River in the Grand Canyon, little is known about their reproduction. We hypothesize that Colorado River water temperatures below Glen Canyon Dam are too low due to hypolimnetic dam releases for female humpback chub to develop mature eggs for spawning. Ultrasonic imaging is a non-lethal method that has been used to determine sex and maturity of a variety of freshwater, anadromous, and marine fishes. However, these studies typically use captive fish and not wild fish in the field. Our goal is to develop ultrasonic imaging to identify gamete development and to evaluate gamete maturity in female humpback chub in the Grand Canyon. We developed a standardized protocol for ultrasonically scanning humpback chub in the field and compared egg development in female fish from the main stem Colorado River, Little Colorado River, Havasu Creek, and Shinumo Creek. We found egg development as expected in the Little Colorado River and to an extent in the main stem Colorado River and Havasu Creek.