Validating the Use of Otoliths and Scales for Aging Rio Grande Silvery Minnow Based on Known Age Individuals

Authors: Kathy Lang

Year: 2016 (xlviii)

Abstract

Otoliths and/or scales are often used to determine the age of fishes. This practice allows biologists to estimate species-specific characteristics such as growth rates, age at sexual maturity and maximum age. These estimates can then be used to make decisions regarding the management of that species. While standardized procedures for assigning ages to otoliths and scales have been developed and accepted, there is still the possibility of error in assigning ages to a particular species unless the chosen a ging method has been validated. A validation study should: 1) determine the location of the first annulus and 2) prove that the annuli are laid down exactly once per year. These studies can be done via indirect methods (e.g. marginal increment analysis) or via dir ect methods (e.g. mark/recapture studies or holding fish for over one year). The ABQ BioPark’s Aquatic Conservation Facility maintains several year classes of the endangered Rio Grande silvery minnow, Hybognathus amarus, to serve as broodstock for the captive propagation program. Adult fish are kept in outdoor systems and exposed to near-natural temperature fluctuations for most of their lives. This captive population, therefore, can be used to validate the aging of this species. Beginning in September 2016, otoliths and scales from any naturally-occurring mortality of a Rio Grande silvery minnow are being collected and examined for validation of annulus formation on those structures. Preliminary results indicate that otoliths provide a more accurate assig nment of age than scales, and that annuli are, in fact, laid down once per year on the otoliths.