Temporal and Spatial Relations between the Spawning of Humpback Chub and Roundtail Chub in the Upper Colorado River

Abstract

Gonadosomatic indices (100 × gonad weight/whole-body weight) and the occurrence of expressible gametes indicated a temporal overlap in the spawning of humpback chub Gila cypha and roundtail chub G. robusta at Black Rocks, a turbulent, deepwater reach of the upper Colorado River, Colorado (39°N, 109°W). Radiotelemetry showed that roundtail chub moved throughout the upper Colorado River during the spawning season, whereas humpback chub were restricted almost entirely to the 1.8-km Black Rocks reach; however, spatial isolation of the species during spawning was not demonstrated. A principal component analysis of morphologic measurements taken from live fish supported taxonomic assignments made in the field and suggested that most of the few specimens considered hybrids or intergrades by field personnel were actually humpback chub; however, an analysis based on fin ray counts indicated that this group included roundtail chub, hybrids, or backcross individuals. The strong affinity of humpback chub for the unique habitat conditions created by water flowing rapidly among large, angular boulders and shoreline rock outcrops at Black Rocks and elsewhere suggests that the unusual morphology of this species is an adaptation to life in such habitats. The limited availability of such habitats might be an important factor controlling the distribution and relative abundance of the humpback chub, a species now listed as endangered.