Journal Article

Use of data storage tags to quantify vertical movements of cod: effects on acoustic measures

O. Heffernan, D. Righton and K. Michalsen

in ICES Journal of Marine Science

Published on behalf of ICES/CIEM

Volume 61, issue 7, pages 1062-1070
Published in print January 2004 | ISSN: 1054-3139
Published online January 2004 | e-ISSN: 1095-9289 | DOI: http://dx.doi.org/10.1016/j.icesjms.2004.07.003
Use of data storage tags to quantify vertical movements of cod: effects on acoustic measures

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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

Journal Article.  4984 words.  Illustrated.

Subjects: Environmental Science ; Marine and Estuarine Biology

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