Factors influencing precision of age estimation from scales and otoliths of bluegills in Illinois reservoirs

We examined the effects of population-specific variation on age estimates from scales and otoliths of bluegills Lepomis macrochirus from Illinois reservoirs. We divided 12 reservoirs into four groups (north stunted, north quality, south stunted, and south quality) to examine the effects of latitude and fish size distribution on the precision of estimated ages. Ages of 40 bluegills from each reservoir (total N=480) were independently estimated by two readers. Otoliths provided more precise age estimates than scales. Population size structure and sex had no effect on precision of ages estimated from either structure; however, latitude, age, and maturity stage all affected precision of ages estimated from scales. Age also affected precision of estimates from otoliths. Percent agreement, coefficient of variation, and age bias plots all provided useful interpretations of the data. Our results demonstrate the importance of examining population-specific sources of variation with multiple statistical methods when comparing ages of fish from different populations.