Distribution and activity of pelagic fish – acoustic studies in the Baltic Sea

Fisheries agencies around the Baltic Sea use hydroacoustics to assess stock sizes of herring and sprat. These assessments rely on the assumption that the acoustic properties of Baltic clupeids are the same as North Sea herring. This may lead to biased results, as system-specific differences in salinity and fish fat content may influence acoustic target strength. The acoustic properties of the Baltic clupeids were explored and a new relationship between target strength and fish length was developed (paper I). The intercept of this new relationship is 3.4 dB higher than normally used by the agencies. Applying this new relationship would reduce the acoustic stock biomass estimates by