Departure behaviour of juvenile southern bluefin tuna (Thunnus maccoyii) from southern Western Australia temperate waters in relation to the Leeuwin Current KO FUJIOKA, 1, * ALISTAIR J. HOBDAY, 2 RYO KAWABE, 3 KAZUSHI MIYASHITA, 4 YOSHIMI TAKAO, 5 OSAMU SAKAI 1 AND TOMOYUKI ITOH 1 1 National Research Institute of Far Seas Fisheries, Fisheries Research Agency, Shimizu, Shizuoka 424-8633, Japan 2 Wealth from Oceans Flagship, CSIRO Marine and Atmospheric Research, GPO Box 1538, Hobart, Tas. 7001, Australia 3 Graduate School of Fisheries Science and Environmental Studies, Institute for East China Sea Research, Nagasaki University, Taira-machi, Nagasaki 851-2213, Japan 4 Field Science Center for Northern Biosphere, Hokkaido University, Minato, Hakodate 041-8611, Japan 5 National Research Institute of Fisheries Engineering, Fisheries Research Agency, Hasaki, Ibaraki 314-0408, Japan ABSTRACT The arrival of the warm tropical Leeuwin Current (LC) into southern Western Australia (SWA) may influence the movement timing and foraging habitat of juvenile southern bluefin tuna (SBT). Seasonal and interannual changes in the strength of the LC lead to thermal differences and potential changes in food availability between tropical and temperate waters in SWA. This phenomenon could influence the habitat utilization of SBT in these summer grounds. Move- ment characteristics determined from SBT tagged with acoustic transmitters (N = 244) using cross-shelf lines of automated receivers for three summer-autumn sea- sons (2004 ⁄ 2005, 2005 ⁄ 2006, 2006 ⁄ 2007) in SWA revealed interannual variability. Each year, the east- ward movements of tagged fish increased as tempera- tures increased, and fish left the region when temperatures exceeded 20.0°C, a temperature indica- tive of the leading edge of the LC in SWA waters. Interannual fluctuations in the timing of movements were detected. When the LC was narrow and restricted to the shelf edge in 2004 ⁄ 2005, the distri- bution of SBT in shelf waters did not change before or after LC intrusion. In contrast, long distance eastward movements frequently occurred when the LC intru- sion was spread wide over the continental shelf in 2005 ⁄ 2006 and 2006 ⁄ 2007. This suggests that, off SWA, juvenile SBT move quickly out of local foraging habitats defined by cool sub-tropical temperate waters ahead of the tropical LC intrusion, despite these wa- ters not being physiologically limiting. These results suggest that the behavioural response of SBT may be related to changes in prey availability as a result of changes in oceanographic conditions. Key words: automated acoustic telemetry, continental shelf, frontal zone, habitat shift, horizontal movement, Leeuwin Current, population distribution INTRODUCTION Southern bluefin tuna (SBT, Thunnus maccoyii) is a commercially important species, and stock trends have attracted attention over the years due to the low population levels (CCSBT, 2009a). The rate of recovery of this stock is low because it takes 10–12 yr to reach sexual maturity (Farley and Davis, 1998; Davis et al., 2001). Management of SBT stocks is supported by indices derived from recruitment moni- toring surveys of juvenile SBT to estimate trends in abundance. These surveys involve estimating the abundances of 1–2-yr-old fish via both acoustic and trolling surveys and 2–4-yr-old fish from aerial surveys (Itoh and Tsuji, 2004; CCSBT, 2009b). It is important to understand movement characteristics and variabil- ity in order to time recruitment monitoring surveys appropriately and correct for bias in population indices (Hobday et al., 2009). Between October and February, adult SBT spawn in the northeast Indian Ocean (Farley and Davis, 1998) and post-larval fishes are dispersed southwards along the coast of Western Australia. Juvenile SBT (mainly 1-yr-old fish) are found in SWA shelf waters from mid austral spring to early autumn *Correspondence. e-mail: fuji88@affrc.go.jp Received 26 April 2011 Revised version accepted 18 December 2011 FISHERIES OCEANOGRAPHY Fish. Oceanogr. Ó 2012 Blackwell Publishing Ltd. doi:10.1111/j.1365-2419.2012.00620.x 1