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The ability to determine the age of individual humpback whales Megaptera novaeangliae and estimate population age distributions is fundamental to assessments of status and long-term viability. Existing methods of ageing large whales rely either on limited longitudinal sighting studies of individual whales from their year of birth, or on post-mortem procedures to extract tissues suitable for determining age. Here we describe a potential method for ageing live free-ranging humpback whales using low-impact biopsy sampling techniques. Shallow outer-blubber samples were obtained from known-age whales from 2 distinct populations (North Atlantic, Gulf of Maine, n = 39; North Pacific, Southeast Alaska, n = 31), and analyzed for their fatty acid (FA) compositions. Multilinear FAage models were derived for these known-age whales, and serve as the basis from which the age of unknown-age whales can be estimated. Four FA-age models were developed; one for each humpback population analyzed separately, and an additional 2 by combining both populations into a single dataset and deriving models based on ‘exact’ and ‘exact’ plus ‘minimum’ known-age whales independently. Each of these empirical models was based on a linear combination of 2 FA ratios rather than individual FA compositions, and shown to be largely independent of sex, diet and nutritional status. Although the precision (σ) of these models was somewhat variable (ranging between 3.1 and 5.3 yr for the specific populations modeled), the results suggest that it may be possible to estimate the age of individual humpback whales from any population with better than decadal resolution using this approach.

Age determination of humpback whales Megaptera novaeangliae through blubber fatty acid compositions of biopsy samples