ESTIMATING AGE OF SEA-OTTERS WITH CEMENTUM LAYERS IN THE FIRST PREMOLAR

We assessed sources of variation in the use of tooth cementum layers t o determine age by comparing counts in premolar tooth sections to know n ages of 20 sea otters (Enhydra lutris). Three readers examined. each sample 3 times, and the 3 readings of each sample were averaged by re ader to provide the mean estimated age. The mean (SE) of the known age sample was 5.2 years (1.0) and the 3 mean estimated ages were 7.0 (1. 0), 5.9 (1.1) and, 4.4 (0.8). The proportions of estimates accurate to within +/-1 year were 0.25, 0.55, and 0.65 and to within +/-2 rears 0 .65, 0.80, and 0.70, by reader. The proportions of samples estimated w ith >3 years error were 0.20, 0.10, and 0.05. Errors as large as 7, 6, and 5 years were made among readers. In few instances did all readers uniformly provide either accurate (error <1 yr) or inaccurate (error >1 yr) counts. In most cases (0.85), 1 or 2 of the readers provided ac curate counts. Coefficients of determination (R-2) between known ages and mean estimated ages were 0.81, 0.87, and 0.87, by reader. The resu lts of this study suggest that cementum layers within sea otter premol ar teeth likely are deposited annually and can be used for age estimat ion. However, criteria used in interpreting layers apparently varied b y reader, occasionally resulting in large errors, which were not consi stent among readers. While large errors were evident for some individu al otters, there were no differences between the known and estimated a ge-class distributions generated by each reader. Until accuracy can be improved, application of this ageing technique should be limited to s ample sizes of at least 6-7 individuals within age classes of greater than or equal to 1 year.