Historical landing statistics, size, sex and microsatellite analyses of Bluefin tuna (Thunnus thynnus thynnus) in the Central-Southern Mediterranean Sea

Abstract

Bluefin tuna has been caught in the Maltese Islands, in the centre of the Mediterranean Sea, with traps since 1748 which reached regular usage around 1948. However this fishing method was replaced by bluefin tuna catches with long-line, initially as a by-catch in the swordfish fisheries prior to fine-tuning the gear for bluefin tuna in 1995, when the Japanese bought bluefin tuna caught by Maltese fishermen. Total bluefin tuna long-line fisheries landings in the Maltese Islands have followed an overall decline from a peak of 353,014kg in 1995 to 227,008kg in 2006. Close to 400 bluefin tuna specimens caught south of the Maltese Islands, were sampled during six years to study variations in sizes, sex ratios, biogeographic characteristics and molecular genetics using four microsatellite loci. This study results indicate decreasing average bluefin tuna catch body size and an increasing sex bias with females composing the greater percentage of specimens caught. A significant genetic heterogeneity, FST value of 0.018 (P=0.015), was found among the bluefin tuna sampled in the spawning and fishing region south of the Maltese Islands.