A Simple Method to Increase Precision, Reduce Error, and Facilitate the Aging of Otoliths: the Otolith Illumination Device

Accurate and precise age estimates are critical for the effective management and understanding of fisheries resources.  Two common sources of error in age estimation are process error (i.e., error related to a technique) and interpretation error (i.e., individual reader subjectivity).  Process error has largely been reduced through the identification of otoliths as generally the most precise structure from which to estimate ages from a broad range of species and sizes.  However, interpretation error may still occur as a result of misidentification and/or enumeration of annuli.  To aid biologists in reducing interpretation error, we designed and tested a novel otolith viewing apparatus that we termed the otolith illumination device (OID) to ascertain if its use would result in a reduction of interpretation error, determined by increased precision of within- and between-reader age estimates from Walleye Sander vitreus and Smallmouth Bass Micropterus dolomieu otoliths.  Clarity of annuli on otolith subsections viewed with the OID was generally greater than clarity of annuli on subsections viewed with traditional methods.  Within- and between-reader coefficients of variation and average percent error were consistently lower (i.e., increased precision) from age estimates obtained using the OID than from ages estimated using traditional methods.  Furthermore, both readers indicated greater efficiency (i.e., speed at which ages were estimated), less eye fatigue, and greater confidence in age estimates obtained using the OID compared to age estimates using traditional methods.  Results suggest that the OID is a viable option for making structures easier and faster to age, which could lead to improvements in efficiency (i.e., labor savings) and quality of data and inferences for fisheries managers and researchers.