Stock assessment of small tunas, Euthynnus alletteratus and Auxis rochei (Actinopterygii, Scombriformes, Scombridae), in central Mediterranean (GSA 13): Implications for fishery management using data-limited approaches

Résumé

Small tunas are economically important for Mediterranean fisheries. Yet they remain poorly assessed due to data limitations. This study provides the first quantitative stock assessment of little tunny, Euthynnus alletteratus (Rafinesque, 1810), and bullet tuna, Auxis rochei (Risso, 1810), in the central Mediterranean Sea (GSA 13) using two complementary data-limited methods. The Length-Based Bayesian Biomass (LBB) method was applied to length-frequency data collected in 2022 (408 E. alletteratus and 508 A. rochei ), and the CMSY++ method was applied to the official landings time series from 1996 to 2022. The results reveal contrasting stock statuses. Auxis rochei is critically overfished, with current biomass at only 8% of unexploited levels ( B / B 0 = 0.08) and extremely high fishing mortality ( F / M = 15). CMSY++ confirmed overfishing ( B / B MSY = 0.52; F / F MSY = 1.30). Length structure analysis revealed that 42% of the catch were immature juveniles (32.6 cm). In contrast, E. alletteratus is also overfished, albeit less severely ( B / B 0 = 0.25; F / M = 1.4). CMSY++ results ( B / B MSY = 0.75; F / F MSY = 1.39) and a low proportion of juveniles (2%) suggest a healthier demographic structure, although large spawners (>90 cm) are rare. Based on these findings, we recommend the following species-specific measures: for A. rochei , increase the minimum conservation reference sizes (MCRS) from 27 cm to the optimal 34 cm, and reduce fishing effort. For E. alletteratus , prohibit the landing of specimens measuring more than 80 cm, and increase the MCRS from 54 cm to 66 cm. This study demonstrates the feasibility of quantitatively assessing data-poor small tunas in the Mediterranean and provides essential baselines for their sustainable management.

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