Evaluation of Pacific Lamprey statoliths for age estimation across their life cycle

Abstract Objective The anadromous Pacific Lamprey Entosphenus tridentatus is an ecologically and culturally important fish that has experienced declines in abundance and distribution throughout large portions of its range. However, the lack of fundamental biological information, such as age and growth, impedes management and conservation efforts. Methods We used known‐age hatchery and wild Pacific Lamprey to evaluate the potential of statoliths (apatite concretions found within the auditory capsules) to provide estimates of individual size and age throughout ontogeny. Statolith size and growth were assessed with comparisons of morphometrics (area, perimeter, length, and width) in relation to fish length. Traditional band‐reading methods and random forest (RF) analysis were evaluated for their utility in estimating age. Result We observed a strong, positive relationship between statolith size and fish length for larvae that were hatchery reared. However, there were no positive relationships between statolith size and fish length for any of the postmetamorphic life stages, which indicates that statolith growth slows dramatically or ceases after metamorphosis. Band‐reading methods of statoliths from known‐age lamprey (1‐ and 2‐ to 9‐year‐olds) resulted in accurate age estimates for 28% of the fish, and 57% of mean age estimates fell within 1 year of the true age. While we found that the statolith was not very useful for traditional age estimation techniques, its inclusion in morphometric‐based age classifications was promising. A RF model based on body length, collection river, and statolith area accurately assigned 76% of the larval and freshwater juveniles to their known age. Additionally, an RF model using only body length and river of origin correctly classified 70% of the known‐age samples. Conclusion Multivariate approaches, including those that include lethal and nonlethal predictor variables, provide potential approaches to estimate age in larval and early juvenile Pacific Lamprey.