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ICES Journal of Marine Science: Journal du Conseil 2003 60(4):885-898; doi:10.1016/S1054-3139(03)00070-5
© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
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Acoustic observations of the swimming behavior of the euphausiid Euphausia pacifica Hansen

Alex De Robertisa,*, Chad Schellb and Jules S Jaffeb

a Scripps Institution of Oceanography La Jolla, CA 92039-0208, USA
b Marine Physical Laboratory, Scripps Institution of Oceanography La Jolla, CA 92039-0238, USA

*Correspondence to A. De Robertis; current address: Northwest Fisheries Science Center, Hatfield Marine Science Center, Newport, OR 97365, USA; tel: +1 541 867 0411; fax: +1 541 867 0389. e-mail: alex.derobertis{at}noaa.gov.

A high-resolution, 445 kHz, multi-beam sonar and new data-processing techniques were used to characterize the swimming behavior of the euphausiid Euphausia pacifica Hansen, in Saanich Inlet, British Columbia. The instrument was deployed when the euphausiids were at depth during the day and during dusk periods of population ascent. Three-dimensional swimming paths of individual euphausiids were reconstructed by linking successive, acoustically determined positions. Median swimming speeds were 1.8 cm s–1 during the day and 2.2–3.5 cm s–1 during dusk periods of vertical ascent. The presence of a fish at distances of 20–300 cm did not affect the swimming speed or turning of the euphausiids, suggesting that they did not respond to the presence of a potential predator at these distances. Euphausiids moved primarily obliquely in the vertical plane, with few individuals moving directly up or down, even during periods of vertical migration at dusk. We hypothesize that oblique swimming trajectories of euphausiids may reduce vulnerability to visual predators by allowing them to maintain bioluminescent counter-illumination during vertical migration.

Keywords: acoustic tracking, bioluminescence, euphausiid, krill, swimming behavior

Received 10 October 2002; accepted 12 January 2003.


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