Comparison of Aging Structures and Life History Aspects of an Historical Population of the Roundtail Chub, Gila robusta (Cyprinidae), in the Yampa River Canyon, Colorado

Authors: Stephen Ross, Tim Modde, Derek Ross

Year: 2018

Abstract

The Gila robusta complex includes three large, morphologically variable cyprinid species native to the Colorado River drainage. All three members of this complex, G. cypha , G. elegans , and G. robusta, historically occurred in the Yampa River Canyon of Colorado and Utah. Both G. elegans and G. cypha are federally listed as endangered, and G. robusta (Roundtail Chub) is considered either imperiled or vulnerable to extinction by all state management agencies in the Colorado River basin. In July of 1998, 1999, 2001, and 2002, we opportunistically collected Roundtail Chubs on USFWS nonnative fish removal trips on the Yampa River in Dinosaur National Park. Roundtail Chubs were still abundant at this time. Our objectives were to determine the efficacy of scales, opercular bones, and otoliths as aging structures and to describe age, growth, ages at first reproduction, and tuberculation patterns relative to reproductive stages. Based on all four years of data, we obtained interpretable annuli for 111 fish using otoliths, 91 fish using opercles, and 111 fish using scales. Scale annuli generally agreed with otolith annuli up to approximately eight years, after which scale annuli consistently underestimated opercular annuli. Opercular annuli closely agreed with otolith annuli up to 15 years before underestimating otolith annuli. Based on otoliths, the oldest fish was aged at 22+, 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. Both sexes have similar ages at first reproduction, with females showing early maturing gonadal stages at five years, and late maturing or mature gonadal stages from 8-20 years. Our July sampling likely preceded actual spawning dates given that we did not collect females with later developmental stages of ovaries (e.g., MR or RE) stages. Males showed testicular development beginning at 6 years, with mature or running ripe testes from 6 to 22 years. Both sexes develop tubercles, with the intensity and extent of tubercles on the body related to gonadal stages. The greatest tubercle development occurred in males with maturing or running ripe testes, and in females with maturing ovaries.