A New Simple Method for Age Determination of Harbour Porpoises (Phocoena phocoena)

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Abstract
Age determination of marine mammals is important for understanding the impact of anthropogenic disturbances as well as for population management. Toothed whales are usually age-determined by counting annually formed layers in their teeth. This includes a time-consuming sequence of preparations, usually involving chemical treatment. This study tested a quicker and simpler method for age determination of harbour porpoises (Phocoena phocoena), originally developed for age determination of foxes and other terrestrial carnivores. The tooth was ground with fine-grained sandpaper, and the age lines were directly read using a binocular microscope. To evaluate the usability of the grinding method for harbour porpoises, three tests were used: (1) the number of growth layer groups (GLGs) in teeth from 66 harbour porpoises by the grinding method were compared by two readers; (2) GLGs in teeth from six harbour porpoises prepared by the grinding method and by the decalcification method were compared in a blinded set-up with two readers; and (3) the GLGs in teeth from two individuals with known ages prepared by both the grinding method and the decalcification method, respectively, were compared. A Bland–Altman plot showed high agreement between the determined age of individuals by the two different methods. The average age difference was -0.56 years, and the 95% confidence interval for the average difference was [-4.3, 3.2] years. The grinding method is therefore considered to be a valid alternative and quicker method for age determination of harbour porpoises.

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