Precision of Structures Used to Estimate Age and Growth of Apache Trout from the White Mountains of Arizona

Obtaining reliable information on the age structure of fish populations is important for making conservation and management decisions. We sought to evaluate precision and reader confidence in age estimates from scales (two body locations), sectioned fin rays (pectoral, pelvic, anal), and sectioned sagittal otoliths from Apache Trout Oncorhynchus apache (n = 78 fish) sampled from the East Fork White River, Arizona, in 2017. Structures were aged independently by two experienced readers without prior knowledge of fish length. Each reader provided a confidence rating of zero (no confidence) to three (completely confident) as a measure of readability. Both readers were unable to estimate age from scales collected from the area just posterior to the insertion of the pectoral fin. Scales removed from an area just dorsal to the lateral line and posterior to the dorsal fin were used in all analyses. Percent exact agreement between readers was highest for scales and otoliths (> 72.0%) and lowest for fin rays (31.8–58.1%). Average confidence rating was highest for sectioned otoliths (mean ± SE; 2.1 ± 0.07), and lowest for anal fin rays (0.3 ± 0.06) and scales (0.7 ± 0.05). Consensus ages from otoliths were compared to the other structures. Percent exact agreement with otolith age was low and varied from 21.6–35.7% among structures. Similarly, percent agreement within one year was also low among structures (58.0–70.2%). Scales consistently underestimated age of age-4 and older fish (based on otolith age); whereas fin rays tended to overestimate age of younger fish and underestimate age of older Apache Trout. Although sectioned otoliths require lethal sampling, they produced the most precise age estimates for Apache Trout with the highest reader confidence. Dorsal scales may be a suitable non-lethal alternative to otoliths if ages for only young fish (age-3 and younger) meet study objectives.