Errors in age estimates of North Atlantic minke whales when counting growth zones in bulla tympanica

Abstract

Age estimation of common minke whales (Balaenoptera acutorostrata) has always been difficult, and the accuracy of the current method of counting growth layer groups (GLGs) in the periosteal zone of the tympanic bulla has been questioned. To evaluate this method, two readers aged three sections from each of right and left bulla tympanica from 35 male and 57 female North Atlantic minke whales. A single age estimate with variance was calculated for each whale using General Linear Mixed Model Poisson based regression, and this estimate was compared with the number of ovulations and body length to evaluate the bias of the age determination method. The results showed a poor fit between age and number of ovulations with R2 = 0.0014. Bias was estimated to be a 37% underestimate of ‘true’ age assuming an ovulation rate of one per year and age at sexual maturity of eight years. Precision of the bulla age estimates was lower than those of Antarctic minke whales aged using the earplug method. The high bias reduces the applicability of the bulla method in routine age-determination with a management objective. Other age determination methods for the species should be improved or developed to ensure proper monitoring of demography and life history for the North Atlantic minke whale.