Cooperative Fish and Wildlife Research Units Program: Idaho
Education, Research and Technical Assistance for Managing Our Natural Resources

Watkins, C. J., T. J. Ross, R. S. Hardy, and M. C. Quist. 2015. Precision of hard structures used to estimate age of mountain whitefish. Western North American Naturalist 75:1-7.


The mountain whitefish Prosopium williamsoni is a widely-distributed salmonid in western North America that has decreased in abundance over portions of its distribution due to anthropogenic disturbance. We examined precision of age estimates obtained with scales, pectoral fin rays, and sagittal otoliths from 167 mountain whitefish from the Kootenai River in northern Idaho. Otoliths and pectoral fin rays were mounted in epoxy and cross-sectioned before examination. Scales were pressed onto acetate slides. Between-reader precision (i.e., between two readers), confidence rating and relationships with precision of age estimates, and between-reader agreement rates were compared among hard structures. Coefficient of variation (CV) was lowest and percent exact agreement (PA-0) was highest for scales (CV = 5.9; PA-0 = 70%) compared to pectoral fin rays (CV = 11.0; PA-0 = 58%) and otoliths (CV = 12.3; PA-0 = 55%). Mean confidence ratings for scales were significantly higher than for pectoral fin rays and otoliths (P < 0.001). In addition, mean confidence ratings for pectoral fin rays were significantly higher than for otoliths (P < 0.001). Reader confidence decreased with fish age for scales and pectoral fin rays, but reader confidence increased with fish age for otoliths. In general, age estimates were more precise and reader confidence was higher for scales compared to pectoral fin rays and otoliths.