The suitability of mandible growth layers in the common minke whale (Balaenoptera acutorostrata) for age determination

Résumé

Ovaries from 82 female minke whales (30 from 1999 and 52 from 2001) caught in the North Atlantic were examined macroscopically and the number of corpora lutea, c. albicatia and c. artretica determined by two or three readers. From these whales and an additional 19 males (13 from 1999 and 6 from 2001), the number of GLGs in the buccal wall of the anterior part of both mandibles were counted. Mandible GLGs were counted by either examining digital images of haematoxylin stained 200-500mm segments, or from high-resolution X-ray images of 3mm thick unstained segments examined by two readers. The readers agreed completely when counting ovarian corpora lutea, but there was disagreement with the interpretation of c. albicantia and c. artretica in some ovaries. The average CV of the number of ovulations (nc.lutea + nc.albicantia) was 6%; when counting only c. albicantia the CV was 16.7%, and 64.9% when counting only c. artretica. The precision when counting mandible GLGs using the digital images was poor, with mean CV of 82%, compared to 41% using the X-ray images. There was poor agreement between the repeated readings of the X-ray images by each reader, as well as between the readers. Mean GLG count using either method did not correlate with the number of ovulations, and provided biologically unreasonable von Bertalanffy growth models. This study shows that there is some uncertainty when examining ovaries, although this is small compared to the variability and bias associated with counting mandible GLGs. New bone is deposited in the mandible in such a way that growth layers do not continuously accumulate, or cannot be distinguished using present technology and methods.