Comparison of scales and otoliths for aging wild rainbow trout from East Tennessee streams

Age estimation is essential to properly managing the wild rainbow trout Oncorhynchus mykiss populations in east Tennessee. The age structure of a population can be used to quantify growth, longevity, and total mortality, as well as to assess environmental or management impacts. Scales have traditionally been used to age rainbow trout, but estimates using these structures may be inaccurate. As an alternative to scales, otoliths are increasingly being used to determine age estimates in fish populations. Otolith ages have been obtained and verified for wild brown trout Salmo trutta in East Tennessee but not for rainbow trout. To address this need, two readers aged scale and otolith samples from 621 wild rainbow trout from 12 East Tennessee streams. Scales identified fish up to age-3, whereas otoliths documented fish up to age-8. Reader agreement (precision) was much higher with otoliths (95%) than with scales (79%). Agreement between the two structures was only 69%. Scale accuracy also declined from 93% for age-1 fish to 50% for age-3 fish, while otoliths were 100% accurate when compared with known-age fish collected. Despite the greater accuracy and precision of otoliths as compared with scales, growth and mortality rates derived from the two structures did not differ substantially, primarily because fish older than age-3 were relatively uncommon (3.2%). Therefore, no changes in the basic management strategies applicable to wild rainbow trout populations in East Tennessee are currently necessary.