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ICES Journal of Marine Science: Journal du Conseil 2004 61(7):1062-1070; doi:10.1016/j.icesjms.2004.07.003
© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
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Use of data storage tags to quantify vertical movements of cod: effects on acoustic measures

O. Heffernana,*, D. Rightona and K. Michalsenb

a Centre for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory Pakefield Road, Lowestoft, Suffolk NR33 0HT, England, UK
b Institute of Marine Research PO Box 1870 Nordnes, NO-5817 Bergen, Norway

*Correspondence to O. Heffernan. e-mail: olheffernan{at}hotmail.com.

Depth data from archival tagging studies of cod (Gadus morhua) were used in three different analyses with the aim of testing basic assumptions of cod behaviour. Examination of post-release depth profiles from cod tagged in the Barents, North, and Irish Seas revealed that some cod underwent a post-release period of adaptation to increasing depth as they readjusted their buoyancy to its pre-tagging level. This depth adaptation behaviour was characterized by gradually increasing mean depth, and enabled the calculation of neutral-buoyancy compliant descent rates, which were less than 1 m h–1. Estimated rates of vertical movement were shown to be highly dependent upon the frequency at which depth was sampled. Maximum estimated rates of ascent and descent from sampling intervals of 10 or 15 min were inconsistent with the maintenance of neutral buoyancy, but estimates from sampling intervals greater than 1 h were not. Calculation of tilt angles using depth data sampled at 10-s intervals showed that cod were often tilted more than 5° relative to the horizontal, and that this effect was more pronounced at night. These findings suggest that basic assumptions regarding cod physiology and behaviour require revision if the accuracy and precision of acoustic methods are to be improved.

Keywords: acoustic survey methodologies, cod, data storage tags, tilt angle, target strength, vertical movement

Received 18 March 2003; accepted 30 April 2004.


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