3 years ago

Evidence of separate subgroups of juvenile southern bluefin tuna

Evidence of separate subgroups of juvenile southern bluefin tuna
Leesa A. Sidhu, Nokuthaba Sibanda, Ben O'Neill, Mark S. Chambers
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 favors 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 for the interpretation of existing datasets and future research priorities. We also provide the first evidence that the migration choices of juveniles that summer in the GAB are influenced by fidelity to winter feeding grounds and suggest this helps explain the collapse of the surface fishery off New South Wales in the 1980s. The manuscript considers a key uncertainty in the population dynamics of southern bluefin tuna—the migration patterns and mixing of juveniles either side of the southern Indian Ocean. Our analyses strongly support the hypothesis of separate juvenile subgroups or contingents either side of the Indian Ocean. We draw tentative conclusions about the nature of these contingents.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/ece3.3500

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