Difficulties, Solutions and Age Comparisons Between Dorsal Spines and Otoliths for Western Atlantic Bluefin Tuna and Implications for Management

Atlantic bluefin tuna (Thunnus thynnus) are currently managed as two separate stocks separated at 45W.  Assessment models utilize growth curves constructed from dorsal spines in the eastern Atlantic and a combination of otoliths and modal size data in the western Atlantic.  Interpreting these structures can be problematic due to low visual clarity of annuli (otoliths) or the deposition of multiple rings within a year and resorption of the nucleus (dorsal spines), which removes readable annuli from the first years of life.  While it is generally accepted that both structures represent the age of fish from their respective stocks, there has been relatively little work comparing estimates between these structures to assess potential bias in age assignment.   We sampled dorsal fin rays and otoliths from several hundred bluefin tuna collected in the Gulf of Maine between 2010 and 2012.  Here we present methodologies to improve visual clarity, interpretation of annuli, and generate age bias plots and growth curves to compare potential differences in age assignment between structures and readers.