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' This paper not to be cited without prior reference to the author

International Council for the C.M. 1984/B:18 l

Exploration of the Sea Pelagic Fish Committee l

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IN SITU OBSERVATIONS OF SFJIMRLADDER

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COMPRESSION IN KERRING by

Egil Ona

Institute of Marine Research 5011 Rergen, Norway

ABSTRACT

The hydrostatic function of the swimbladder in herring is experimentally investigated through in situ diver-assisted

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measurements of swimbladder volume on net-caught and caged fish. In agreement with morpho-physiological evidence, the gas volume of the swimbladder was found to change with pressure according to Boyle's law. The volume remained uncompensated at depth, and additional reductions through gas diffusion were

observed with time. Besides being depth dependent, the swim-

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bladder volume showed a strong correlation with the condition

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factor and fat content of the fish. The observations are i

highly pertinent to the acoustic assessment methods used on this and related species, as they indicate that both depth and seasonal variations in the acoustic scattering properties of

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herring are likely.

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