Herring (Clupea harengus membras L.) is one of the key species of the Baltic ecosystem. It is an important prey of many predatory fish, such as the Baltic cod and salmon, and it has a strong impact on the pelagic ecosystem as a predator of zooplankton and nektobenthos. Due to its wide salinity tolerance range, Baltic herring is able to live and reproduce in almost every part of the Baltic Sea despite of the varying environment (for information of the physical characteristics, visit the Baltic Sea Portal of the Finnish Environment Institute, Finnish Meteorological Institute, and Ministry of the Environment). Although the abundance and biomass of herring fluctuate from time to time even strongly, the stock has been large enough to be exploited commercially over decades. In Finland, the annual landings of herring represent over 80 % of the total fish catch by volume in the sea area and about 50 % of the economical value of the commercial fisheries.
The Archipelago Sea on the southwest coast of Finland is an island rich sea area characterised by complex coastline and shallow water body, mean depth being only ca 20 m. It has a large littoral area where vegetation mainly consists of macroalgae Fucus, Cladophora, Enteromorpha, Pilayella, Ectocarpus and vascular plants such as Potamogeton and Ranunculus, more rarely Zostera. Several rivers discharge fresh water to the area the largest being Aurajoki, Mynäjoki and Paimionjoki, which all have rather sheltered and productive estuaries.