Length estimation of Atlantic bluefin tuna (Thunnus thynnus) using vertebrae

Atlantic bluefin tuna (Thunnus thynnus; BFT) is a large (up to 3.3 m in length) pelagic predator which has been exploited throughout the eastern Atlantic and Mediterranean since prehistoric times, as attested by its archaeological remains. One key insight derivable from these remains is body size, which can indicate past fishing abilities, the impact of fishing, and past migration behaviour. Despite this, there exists no reliable method to estimate the size of BFT found in archaeological sites. Here, thirteen modern Thunnus spp. skeletons were studied to provide power regression equations that estimate body length from vertebra dimensions. In modern specimens, the majority of BFT vertebrae can be differentiated by their morphological features and thus individual regression equations can be applied for each rank (position in vertebral column). In an archaeological context, poor preservation may limit one’s ability to identify rank, hence ‘types’ of vertebrae were defined, which enable length estimates when rank cannot be determined. At least one vertebra dimension; height, width, or length correlated highly with body length when vertebrae were ranked (R2 >0.97) or identified to types (R2 >0.98). Whether using rank or type, length estimates appear accurate to approximately ±10%. Finally, the method was applied to a sample of Roman-era BFT vertebrae to demonstrate its potential. It is acknowledged that further studies with larger sample sizes would provide more precision in BFT length estimates.