Age Determination for Lampreys

Age structures of several populations of parasitic and nonparasitic lampreys were estimated from statolith band number and length‐frequency distributions. In larval populations of the parasitic sea lamprey Petromyzon marinus and nonparasitic American brook lamprey Lampetra appendix , which display strong seasonal patterns ofgrowth, one narrow dark band or annulus and one broad opaque band was produced each winter and summer, respectively. The formation of an annulus during slow growth was validated in two populations of sea lampreys with a chemical tissue marker, oxytetracycline. Length‐frequency distributions required large samples to identify age groups, and the older age groups within a population were often difficult to distinguish because of overlap in size ranges. Thus, the assignment of age to an individual from its total length was likely to be imprecise unless that individualˈs length was close to the modal length of an age group. Some discrepancies in the assignment of population age structure were noted between the two methods. Annuli were not recognizable in the statoliths of larvae or adults from a population of southern brook lamprey Ichthyomyzon gagei that displayed an even growth rate throughout the year, thus making length‐frequency distribution analysis the only alternative.