In situ observations of swimbladder compression in herring
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The hydrostatic function of the swimbladder in herring is experimentally investigated through in situ diver-assisted 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 swimbladder volume showed a strong correlation with the condition factor and fat content of the fish. The observations are 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 herring are likely.
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