Age Structure of Atlantic Bluefin Tuna from the North Carolina Winter Fishery

Improved stock assessments for Atlantic bluefin tuna consider mixing and differential stock production but rely heavily on age structure, which is currently inferred from length data. Age structure is particularly important for this moderately long-lived species because population resilience is linked to periodic production of strong year-classes. For the diminished western stock of Atlantic bluefin tuna, length data showed evidence of a strong year-class occurring in 2003, following >20 years of poor recruitment. Age structure was estimated utilizing otoliths collected over three years (2011-2013) from a representative sample (N=337; >30% sampling rate) from the North Carolina winter recreational fishery. The North Carolina sample presented the opportunity to collect a range of sizes that should include the abundant 2003 year class. Age estimates were determined using standardized methods developed by an international team of bluefin tuna scientists. Ageing methods included protocols related to section type, image enhancement, annotation, consistent interpretation of the first annulus, and calibration based on a reference set. Ages ranged from 4 to 18 years, with 99.1% of individuals between 4 and 11 years, and confirmed a strong 2003 year-class along with two others. Mean sizes-at-age were similar to those of the currently accepted ICCAT growth model.