Standardization of otolith-based ageing protocols for atlantic bluefin tuna

Experts engaged in otolith-based age determination for Atlantic bluefin tuna convened at a workshop to evaluate ageing errors associated with preparation and viewing protocols and interpretation of annuli. Images of serial sections of the same otoliths, viewed under different light (reflected vs. transmitted) were interpreted by eight experienced and two naïve readers. Significant bias was associated with the sectioning plane but not with the type of lighting. Annulus interpretations were most uncertain for the first 6 annuli. Interpretation of the last opaque zone, near the edge of the sectioned otolith, also varied substantially. A precision trial on otoliths contributed from three laboratories resulted in an average ageing error of 5.8±2.8%. Estimated size-at-age estimates conformed well to the Restrepo et al. (2010) growth model regardless of the level of reader experience. The group tasked itself to (1) develop an annotated reader set of 200 images of otoliths; (2) measure annulus dimensions to improve interpretations for younger ages; and (3) conduct marginal increment analysis to further evaluate the timing of annulus formation and inform interpretations of opaque/translucent zones. RÉSUMÉ