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ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on April 3, 2008
ICES Journal of Marine Science: Journal du Conseil 2008 65(4):612-622; doi:10.1093/icesjms/fsn032
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© 2008 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

School-based indicators of tuna population status

James T. Dell1 and Alistair J. Hobday2

1 School of Zoology, University of Tasmania, Hobart, Tasmania 7005, Australia
2 CSIRO Marine and Atmospheric Research, Castray Esplanade, Hobart, Tasmania 7000, Australia

Correspondence to J. T. Dell: tel: +61 03 62325182; fax: +61 62325187; e-mail: james.dell{at}csiro.au

Dell, J. T. and Hobday, A. J. 2008. School-based indicators of tuna population status. – ICES Journal of Marine Science, 65: 612–622.

Theory and limited observation suggest that fish schools consist of more individuals of similar size when populations are large than when they are small. The hypothesis that population size might be indicated by school structure is tested for southern bluefin tuna (SBT), a commercially important large pelagic species, which has undergone an estimated 60% reduction in juvenile biomass since 1960. Fish size data are used to determine whether there have been changes in schooling behaviour that can be used as simple indicators of abundance. During tagging studies, juvenile SBT are removed sequentially from a school, measured, tagged, and released. These sequential size measurements are used here to describe school composition from different years in two locations using simple school metrics (including mean fish size, variance in size, and mean difference in size between sequential fish). Trends were significant in most metrics over the 40-year period analysed, and were inversely related to independent estimates of population size. Simple school metrics are cost-efficient and easily interpreted by stakeholders. Monitoring population trends in near real time through school composition metrics may indicate further decline or recovery of SBT and, therefore, assist future management of tuna and other schooling species.

Keywords: population monitoring, population trends, schooling behaviour, size-based index, southern bluefin tuna, Thunnus maccoyii

Received 27 February 2007; accepted 3 February 2008; advance access publication 3 April 2008.


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