Comparison of Aging Structures and Life History of a Historical Population of Roundtail Chub (Cyprinidae) in the Yampa River, Colorado

Abstract

The Gila robusta complex includes 3 large cyprinid species native to the Colorado River drainage. All 3 members of this complex, Humpback Chub G. cypha, Bonytail Chub G. elegans, and Roundtail Chub G. robusta, historically occurred in the Yampa River Canyon of Colorado and Utah. Both Humpback Chub and Bonytail Chub are federally listed as endangered, and the Roundtail Chub is now considered either imperiled or vulnerable to extinction by all state management agencies in the Colorado River basin. In late June or early July of 1998, 1999, 2001, and 2002, we collected Roundtail Chub from the Yampa River in Dinosaur National Monument. Our objectives were to determine the efficacy of scales, opercular bones, and otoliths as aging structures, and to describe age, mortality, growth, and tuberculation patterns relative to reproductive stages. We obtained interpretable annuli for 111 fish by using otoliths, 91 fish by using opercles, and 111 fish by using scales. Otolith annuli showed the least measurement error; scale annuli generally agreed with otolith annuli up to approximately 7–8 years, after which scale annuli consistently underestimated otolith annuli. Opercular annuli closely agreed with otolith annuli up to 10–12 years before underestimating otolith annuli. Based on otoliths, the oldest fish was aged at 22 years, with 7 fish exceeding 15 years. Using a linearized catch-curve, Roundtail Chub had an instantaneous mortality rate of –0.158, corresponding to 85% survivorship. Growth in length was initially high and then declined at older ages. Slopes of mass versus length differed between sexes, and females had a slightly greater body mass per unit length than males for lengths above 398 mm TL. Both sexes have similar ages at first reproduction (5–6 years), and both develop tubercles. The greatest tubercle development occurred in males with maturing or running ripe testes, and in females with maturing ovaries.