Differences in swimbladder volume between Baltic and Norwegian spring spawning herring: possible consequences for mean target strength
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Volume and shape of the swimbladder of Baltic and Norwegian spring spawning herring were measured, together with individual herring fat content. Swimbladder size was found to differ significantly between the two herring stocks. The larger swimbladder volume of Baltic herring is suggested to be associated with the fish’s low fat content, which may be linked to its specific energy budget and the low salinity of the Baltic Sea. A buoyancy model that considered the different salinity conditions and fat proportions was used to evaluate the observed differences in swimbladder volume. The swimbladder volume data were subsequently used to model the mean target strength (TS) as a function of depth and growth pattern. Backscattering of the swimbladder was modelled using the Modal-Based Deformed-Cylinder Model (MB-DCM), describing the swimbladder as a gas-filled elongated prolate spheroid. The fish body component was modelled as a fluid-filled ellipsoid using the Distorted Wave Born Approximation (DWBA). Modelling results support a different TS to size relationship for Baltic herring, with a stronger echo, due to the larger swimbladder. Depth and length dependent TS relationships based on the model results are suggested. Keywords: Baltic herring, swimbladder volume, depth dependence, target strength.