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ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on June 11, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(5):963-972; doi:10.1093/icesjms/fsm075
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© 2007 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Diel variation in the vertical distribution and schooling behaviour of sardine (Sardina pilchardus) off Portugal

Juan Zwolinski1,, Alexandre Morais1, Vitor Marques1, Yorgos Stratoudakis1 and Paul G. Fernandes2

1 Instituto de Investigação das Pescas e do MAR (INIAP-IPIMAR), Av. Brasília s/n, 1449–006 Lisboa, Portugal
2 FRS Marine Laboratory Aberdeen, PO Box 101, Victoria Road, Aberdeen AB11 9DB, UK

Correspondence to J. Zwolinski: tel: +351 213027000; fax: +351 213025948; e-mail: juan{at}ipimar.pt

Zwolinski, J., Morais, A., Marques, V., Stratoudakis, Y., and Fernandes, P. G. 2007. Diel variation in the vertical distribution and schooling behaviour of sardine (Sardina pilchardus) off Portugal. – ICES Journal of Marine Science, 64: 963–972.

Diel patterns in the schooling behaviour and vertical distribution of pelagic fish schools were studied by examining their echotraces from repeated acoustic survey transects at three inshore sites off the Portuguese coast. At two sites, sardine was the dominant pelagic species, and echotrace characteristics of fish schools were similar to those reported in the literature. At the third site, where there was a multispecies pelagic assemblage that included sardine, there was more variability in several of the school descriptors. At all sites, fish schools expanded after sunset, enlarging their cross-sectional area along the horizontal plane and reducing their mean internal acoustic density, while maintaining their overall mean abundance. Downward migration was rapid (within 1 h) after sunset and simultaneous with school expansion. School-like aggregations with total backscattering similar to daytime schools were present throughout the night, although the proportion of small schools and scattered fish appeared to increase at that time. At dawn, sardine rose back up the water column and rapidly reformed into the typical daytime schools. This pattern of diel vertical migration is opposite to that described for most clupeoids worldwide. The implications of this behaviour on abundance estimation by acoustic monitoring surveys for small pelagic fish are discussed.

Keywords: acoustic surveys, clupeoids, diel cycles, schools, vertical migrations

Received 21 August 2006; accepted 9 April 2007; advance access publication 11 June 2007.


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