Wednesday 24 August 2005

B2
1400-1515 hours

301
Behavioural plasticity of migrating juvenile southern bluefin tuna in relation
to regional oceanography of the southern Indian Ocean
Bestley, Sophie^1,2, Gunn, John¹, Hindell, Mark², Meyers, Gary¹, Patterson, Toby^1,2
1 CSIRO Marine Research, Tasmania, Australia
2 University of Tasmania, School of Zoology, Australia
Author email: sophie.bestley@csiro.au
The development of miniature electronic tags for application to marine animals has revolutionized our understanding of how many top pelagic predators utilize their ocean environment. In this paper we analyze nearly 5000 days of data collected throughout 1998-2000 by fourteen juvenile southern bluefin tuna during their annual long-distance migration from southern Australian waters into the southern Indian Ocean (35-148E, 27-47S) and return. Using the high resolution depth and temperature data collected by archival tags we classify and describe the vertical thermal environment occupied by migrating bluefin using an objective clustering algorithm. Eight thermal groups are identified based on daily vertical temperature profiles. These groups incorporate the temperature range 5-23°C and comparison with Levitus (1998) climatology and existing literature shows the algorithm distinguishes groups that may be associated as Great Australia Bight water, stratified subtropical waters, deeply mixed winter mode waters, and subantarctic front water. We use generalized linear models to test whether observed differences in the depth distribution of the migrant population are related to the local thermal profile. Between thermal groups we find substantial differences in the time bluefin spend at different depths and a bootstrap analysis shows these results to be robust across individual fish. These results show that juvenile bluefin do not occupy a consistent vertical niche: their highly-developed thermoregulation system allows them to employ a number of strategies and give flexible responses to their changing environment. Interestingly, preliminary analyses suggest that feeding frequency remains consistent between thermal groups, and neither depth distribution nor feeding frequency appears to change as fish change from migrating to residential behaviours. Our findings provide support for the hypothesis that the migrating population consistently targets different prey in different oceanographic environments, and further show that their diving and feeding behaviour is not limited during the periods of rapid travel.

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