Aspects of the population dynamics of southern bluefin tuna with a focus on juvenile migration and mixing

The extent of mixing of juveniles is a fundamental aspect of southern bluefin tuna (SBT) ecology that continues to be debated. Juvenile SBT are found either side of the Indian Ocean, south of Africa and off southern Australia. By the 1980s it was generally accepted that some juveniles migrated directly from waters off Western Australia as far west as the south eastern Atlantic Ocean. Although little was known about the movements of these individuals, they were assumed to remain mostly separate from juveniles off southern Australia. However, the introduction of electronic archival tags in the 1990s has strongly influenced theories of juvenile migration of SBT. Some researchers have claimed the archival tag recoveries suggest the great majority of two- to four-year-old juveniles summer in the Great Australian Bight, while others argue that conventional and archival tag recoveries are better described by juveniles either side of the Indian Ocean remaining separate.I provide a synthesis of historical catch and archival tag recovery data to assess the relative support for the two alternative theories about juvenile spatial dynamics. I develop a generalised linear mixed modelling approach that allows more comprehensive assessment of evidence of incomplete mixing in tag recovery data. My analysis supports the hypothesis of juvenile subgroups remaining mostly separate either side of the Indian Ocean. Specifically, I describe evidence of a juvenile population consisting of contingents with distinct migratory behaviour. Recoveries of tags released from longline vessels provide evidence of fidelity to overwintering grounds. I argue the tag recovery data are consistent with contingents governed by the “entrainment” mechanism. An updated model of juvenile spatial dynamics reconciles archival tag recoveries with catch and conventional tag recovery data.The spatial dynamics I propose have important implications for the interpretation of existing datasets and for future research pr