An Assessment of Age Determination Methods for Great Lakes Larval Sea Lampreys

Abstract

AbstractEstimating the age composition and recruitment of populations of invasive sea lampreys Petromyzon marinus necessitates the validation and improvement of age assessment methods that rely on statoliths and length‐frequency data. Determining age based on length‐frequency distributions is subjective because of heterogeneity in the growth rates of larval sea lampreys (ammocoetes) within and across streams and the resulting overlap in lengths between age‐classes. Statolith‐based age assessment methods have never been validated for more than 1 year. We established “known‐age” ammocoete populations in two streams by introducing a single cohort of spawners above barriers and compared estimates of ammocoete age based on statoliths with the true ages. In five additional streams, we used microsatellite data from adults and ammocoetes to assign parents to ammocoetes produced the same year and compared the age determined by statolith‐based interpretation with the age based on parentage assignment. We combined length‐frequency data with age composition data from streams having known‐age populations and evaluated likelihood‐based statistical models used to estimate the age composition of the ammocoete population. Multiple independent age readings of statoliths from known‐age sea lampreys indicated that the age assessment bias (average percent error) was 24.3–36.2%. Genotype‐based ages differed from statolith‐based ages in 36.1% of cases across all streams. Bias‐corrected statolith ages, when combined with length‐frequency data, substantially increased age assessment accuracy in a known‐age population stream and increased precision in a randomly selected stream; their use, however, requires knowledge of the magnitude of the bias of statolith‐based age assessment in study streams. Further effort is needed to quantify these biases and generalize to different types of streams before a reliable methodology for age assessment will be available for sea lamprey management.