Investigation of the target strength-to-length relationship of Baltic herring (Clupea harengus) for use in biomass estimation

EU environmental protection policy, based on a sustainable development approach, requires fast and non-expensive methods of marine ecosystem monitoring. Acoustic techniques, which satisfy these requirements, are widely used as a reliable method of fish stock assessment. Systematic acoustic surveys are conducted in the Baltic Sea in order to estimate the abundance of ecologically and commercially important fish populations such as herring (Clupea harengus) and sprat (Sprattus sprattus). An important requirement for acoustic abundance estimation is the species specific target strength (TS)-to-fish length relationship to convert acoustic energy into numbers of fish. TS is a measure of a fishpsilas potential to reflect sound and one of the major challenges in the Baltic Sea is the development of a reliable TS-length relationship for Baltic herring. Currently, a TS relationship originally determined for North Sea herring is used for the assessment of Baltic herring. In situ TS measurements of Baltic herring conducted in different parts of the Baltic Sea during different seasons demonstrated that Baltic herring have a higher TS than herring living in the north-east Atlantic such as Norwegian spring-spawning or North Sea herring. Additionally, there is a strong variability in Baltic herring TS in different regions and seasons (up to 8 dB). To obtain an accurate TS-length relationship the impact of the biological difference between different herring stocks on the variability of the measured Baltic herring TS should be understood. The presented paper deals with these two important issues. A detailed comparison between the acoustic backscattering properties of Baltic and Norwegian spring-spawning herring was made. The effect of different physical, environmental and biological (depth, acoustic frequency, salinity, fish orientation, fat content and condition) factors on the backscattering of the Baltic herring stock is shown. It is suggested that a different TS-to-fish length relationship should be used for Baltic herring than the one that is currently applied.