Oral Presentation Australian Society for Fish Biology Conference 2017

Albany a turning point for juvenile southern bluefin tuna? (#94)

Mark Chambers 1 , Leesa A Sidhu 1 , Ben O'Neill 1 , Nokuthaba Sibanda 2
  1. School of Physical, Environmental and Mathematical Sciences, University of New South Wales at the Australian Defence Force Academy, Canberra, ACT, AUSTRALIA
  2. School of Mathematics and Statistics, Victoria University of Wellington, Wellington, New Zealand

Archival tagging studies of southern bluefin tuna (SBT, Thunnus maccoyii) have revealed that juveniles residing in the Great Australian Bight (GAB) over the austral summer undertake seasonal cyclic migrations to the southeast Indian Ocean and the Tasman Sea during winter. However, there remains disagreement about the extent of mixing between juvenile SBT regularly caught by longline fleets south of Africa and those observed in the GAB. Some researchers have argued that archival tag recoveries indicate most juveniles reside in the GAB over the austral summer. Others have suggested that recoveries of conventional and archival tags are better explained by a juvenile population consisting of separate groups on the eastern and western sides of the Indian Ocean with limited intermixing. We present analyses of catch and tag recovery data and re-examine archival tagging studies. The evidence provided strongly favours the hypothesis of separate juvenile subgroups, or contingents, with limited intermixing. We draw some tentative conclusions about the nature of the putative contingents and discuss some implications of these findings including their possible connection with the collapse of the surface fishery off New South Wales in the 1980s, repeated observations of dual spawning peaks in the tropical Indian Ocean and bimodal length frequency distributions of one-year-olds off Western Australia. We also provide the first evidence that the migration choices of juveniles that do summer in the GAB are influenced by fidelity to winter feeding grounds.