Direct Validation of Annual Growth Increments on Sectioned Otoliths from Adult Arctic Grayling and a Comparison of Otolith and Scale Ages

Abstract

AbstractAccurate ages are essential for the responsible management of fish populations, and validation of ages ensures that accurate age estimates are available to managers. Scales have typically been used to estimate ages of Arctic grayling Thymallus arcticus in Alaska; however, the recapture of marked fish after several years at large has cast doubt on the accuracy of scale ages for this species. To validate age determinations from otoliths, we used oxytetracycline (OTC) to mark 102 Arctic grayling from the Eldorado River, Seward Peninsula, Alaska, during 1994 and 1995. Fish were tagged, fin‐clipped, weighed, and injected with 25 mg of OTC/kg of body weight. Over the succeeding 4 years, 16 OTC‐marked Arctic grayling were recaptured. Fluorescent marks were visible in thin‐sectioned otoliths from 15 of the 16 recaptured fish, and annuli that were visible beyond the mark accurately recorded the passage of years between mark and recapture in all otoliths with OTC marks. Ages assigned from scales underestimated the otolith‐assigned age by as much as 20 years. We assembled a larger data set of scale–otolith pairs (n = 74) by combining data from sampling mortalities collected in nearby rivers during 1988–1995 with data from this study. We used the Fisheries Analysis Simulation Tool (FAST 2.1) to estimate growth parameters, and we used an age bias plot and a simple population model to illustrate the mismanagement of Arctic grayling fisheries that could occur if scale ages are relied upon to describe stock structure. We recommend the use of otoliths as the standard of age determination in Arctic grayling populations until a nonlethal method (e.g., fin rays) can be validated for age determination in this species.