ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on August 31, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(7):1293-1301; doi:10.1093/icesjms/fsm119
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Life under pressure: insights from electronic data-storage tags into cod swimbladder function
1 Centre for Environment, Fisheries and Aquaculture Science, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK
2 Department of Biology, University of Bergen, PO Box 7800, N-5020 Bergen, Norway
3 Institute of Marine Research, PO Box 1870 Nordnes, N-5817 Bergen, Norway
4 Marine Research Institute, Skúlagata 4, PO Box 1390, 121 Reykjavík, Iceland
5 Swedish Board of Fisheries, Institute of Marine Research, PO Box 4, S-453 21 Lysekil, Sweden
6 Marine Laboratory, Fisheries Research Services, 375 Victoria Road, 101, AB11 9DB Aberdeen, UK
7 Danish Institute for Fisheries Research, Charlottenlund Castle, 2920 Charlottenlund, Denmark
Correspondence to J. van der Kooij: tel: +44 1502 524416; fax: +44 1502 513865; e-mail: jeroen.vanderkooij{at}cefas.co.uk
van der Kooij, J., Righton, D., Strand, E., Michalsen, K., Thorsteinsson, V., Svedäng, H., Neat, F. C., and Neuenfeldt, S. 2007. Life under pressure: insights from electronic data-storage tags into cod swimbladder function. – ICES Journal of Marine Science. 64: 1293–1301.The behavioural response of cod (Gadus morhua) to sudden pressure reductions was investigated in a large electronic-tagging experiment using data collected from 141 cod tagged in five different areas of the Northeast Atlantic. More than 40% of cod exhibited a characteristic equilibration behaviour after a rapid pressure reduction caused either by capture before tagging, or by pressure reduction during a rapid ascent from the seabed, or when migrating to deeper water. The equilibration allowed the cod to regain demersal residence. The rate of descent averaged 10 m d–1 (ranging from 2 to 23 m d–1) over periods of less than a day to 1 month. Descent rates for cod on the Icelandic shelf were inversely related to fish length, i.e. smaller fish descended more rapidly, findings consistent with results achieved in the past under laboratory conditions. Modelling of swimbladder volume during equilibration suggested that cod were negatively buoyant for most of the time. The results imply that swimbladder functionality is retained after the probable barotrauma that would follow a large and rapid ascent, and that rates of gas exchange into the swimbladder may be naturally variable. These findings have implications for assumptions on discard mortality, the interpretation of cod behaviour, and its impact on biomass estimates obtained from acoustic surveys.
Keywords: behaviour, buoyancy, cod, discard, physoclists
Received 21 February 2007; accepted 3 June 2007; advance access publication 31 August 2007.