Validation of Scales and Otoliths for Estimating Age of Rainbow Trout from Southern Appalachian Streams

Abstract

Annuli counts from scales and otoliths are commonly used to estimate the age of rainbow trout Oncorhynchus mykiss in southern Appalachian streams. However, validation of annuli counts from these two structures for rainbow trout has not been reported. Oxytetracycline (OTC) injections (50 mg OTC/kg fish) were used to mark otoliths of visible-implant-tagged rainbow trout in a mark–recapture study. The validation of scales for age estimation was performed by comparing the number of annuli on scales of individual rainbow trout at capture with those collected 12–15 months later upon recapture. Scale accuracy declined from 72% for age-2 fish to 0% for age-4 fish. In contrast, and regardless of age, all fish formed an additional annulus outside the OTC mark on otoliths. We conclude that annuli counts from otoliths will provide accurate age estimates for rainbow trout in the southern Appalachians. The use of otoliths and consequential sacrifice of fish may not be an option in some cases. However, annuli counts obtained from the scales of rainbow trout in southern Appalachian streams may not provide accurate age estimates for fish older than age 2.