Validation of a relationship between statolith size and age of larval Great Lakes sea lamprey (Petromyzon marinus)

Sea lampreys (Petromyzon marinus) are parasitic pests in the Great Lakes that have negatively affected game fish populations. Accurate aging of pest species such as sea lampreys can provide estimates of growth and mortality rates, which can direct control efforts. Because growth of larval sea lampreys is both slow and variable, determining age based solely on visual assessments of length-frequency distributions is subject to considerable uncertainty. Otoliths have been used to estimate age in teleosts through annuli counts and otolith size metrics. Lampreys do not have otoliths, having instead an analogous structure called a statolith. Determining age based on statolith annuli counts has been found to be imprecise and inaccurate. Therefore, we evaluated whether statolith size was correlated with ammocoete age using known-age populations of ammocoetes from two Great Lakes streams with contrasting physical conditions that affect larval sea lamprey growth. A morphometric system was used to measure length, width, and height of statoliths from these known-age populations. Statolith width was found to be the measurement that best distinguished the age-classes within the populations. A likelihood-based statistical model was used to assess ammocoete population age composition. Even though statolith width was clearly associated with age, combining statolith width data and length-frequency did not improve estimates of proportions at age relative to those estimated using only length-frequency information.