Size and age structure of the historical mediterranean bluefin tuna population inferred by vertebral analysis

Bluefin Tuna (Thunnus thynnus, BFT) is one of the most ecologically and commercially important large pelagic predators in the Mediterranean Sea and one of the most intensively exploited marine fish species globally. Despite this, fundamental knowledge gaps persist regarding the biological characteristics of Mediterranean BFT populations prior to the onset of industrial fishing in the second half of the 20th century. In this context, the Sella historical collection, comprising 1,854 specimens from multiple Mediterranean tuna traps (Central, Western and Eastern Mediterranean) dating from 1911 to 1941, represents an invaluable resource for filling these gaps. Fork length (FL) was estimated empirically from vertebral columns measurements or of individual vertebrae and age from growth rings on vertebral sections. The methodology was validated by comparing FL estimates with total length (TL) values from Sella's original field notes using published conversion equations. Although the spatio-temporal composition of the collection revealed significant imbalances in data availability, notable temporal variation in demographic structure was documented across the three fishing periods, with decade-specific von Bertalanffy growth models revealing differences in growth conditions pr demographic composition across fishing periods. Linear regression models confirmed significant positive FL-age relationships in all periods. Comparison of historical size-frequency distributions with modern ICCAT catch-at-size data (1950–2023) revealed potential long-term shifts in Mediterranean BFT population size structure, associated with the transition from fixed trap to industrial purse seine fisheries and the effects of intensive exploitation. This study demonstrates the irreplaceable scientific value of historical collections as source of pre-industrial biological baselines, contributing to addressing the shifting baseline syndrome in the long-term management of exploited fish populations.