Diet of young southern bluefin tuna Thunnus maccoyii in the southwestern coastal waters of Australia in summer

The diet of juvenile (predominantly age 1) southern bluefin tuna Thunnus maccoyii (SBT, N = 720), caught over 11 years of the recruitment monitoring survey off southern Western Australia during summer, consisted overwhelmingly of teleosts (97.4% by volume). Pilchard Sardinops sagax (27.4%V), blue mackerel Scomber australasicus (16.7%V), and jack mackerel Trachurus declivis (14.2%V) were the major taxa, with pilchard more abundant in coastal waters and jack mackerel more frequently encountered in fish caught closer to the shelf-edge. Prey size varied from 5 to 240 mm, with 67% of ingested items measuring between 30 and 50 mm. Pilchard dominated the prey size category 130–190 mm (84% by number), but the overall contribution of this species to the diet of juvenile SBT was much lower than previously reported. Future research in relation to the feeding ecology of juvenile SBT should focus on the biology and ecology of the young lifestages of the main prey species in this area and on prey distribution and dynamics as a key factor linking environmental change and SBT distribution.