Ecosystem overviews

Baltic sea ecosystem overview

Our Ecosystem Overviews use risk-based methods to identify the main human pressures and explain how these affect key ecosystem components in each ICES ecoregion
​​​​​​​​​The Baltic Sea is one of the largest brackish water bodies in the world, covering 420 000 km2. It is a semi-enclosed shallow
sea with an average depth of 60 m, where one third of the area is less than 30 m deep. This ecoregion has many islands and a long and diverse coastline, especially in the northern areas. It is characterized by strong temperature and salinity gradients, from relatively warmer and saline waters in the southwestern part to cold and almost freshwater in the northernmost parts. ​

Many species and habitats of the Baltic Sea are not in good condition, according to recent assessments. This affects foodweb functionality, reduces the resilience and resistance against further environmental changes, and diminishes prospects for socioeconomic benefits, including fishing opportunities.

Key Signals

  • The overall loading of nutrients has decreased ​markedly due to improved management, but annual nutrient inputs still exceed regionally agreed goals in the central Baltic Sea, the Archipelago Sea, and the Gulf of Finland. Nutrient concentrations in the water​ column and sequestered in the sediments remain relatively high, and phosphorus is increasing in some areas. As examples of the consequences of this nutrient load, blue-green algal blooms are common in offshore areas and there is excessive filamentous algal growth in many coastal areas.
  • The extent of deep-water areas with poor or no oxygen, caused by a combination of eutrophication and a reduced frequency of inflows of saline and oxygen-rich water from the North Sea, remains high.
  • Climate-driven changes in water temperature (including changes in ice cover) and salinity will have an increasing influence on the ecosystem’s structure and function.
  • Contaminant levels remain elevated, and the overall contamination status has been at the same level for the past two decades, but many potential contaminants are not monitored. Some of the main contaminants have been reduced (e.g. DDT, dioxins, and PCBs).
  • The rate of observed introduction of non-indigenous species has more than doubled in the 21st century.
  • Overall fishing effort fell by approximately 50% from 2004 to 2012. Discarding still exists, even though it is largely illegal.
    • The spawning-stock biomass (SSB) of most pelagic stocks has increased and is above or close to the biomass reference points. An exception is the western Baltic herring stock. Both sprat and herring are experiencing overfishing (fished at greater than FMSY).
    • Both Baltic cod stocks are exploited above FMSY. The SSB of western Baltic cod has been below the limit reference point for at least ten years. The SSB of eastern Baltic cod is decreasing, with the value for 2018 being the lowest observed in the time-series. Its size structure and condition factor have deteriorated markedly without signs of improvement.
    • The status of European eel continues to be critical.

  • Disturbance of seabed habitats due to physical abrasion from mobile bottom-contacting fishing gears occurs mostly in the southern Baltic Sea and may reduce benthic diversity and biomass depending on the substrate type.
  • Structural shifts in the open-sea foodweb (including phytoplankton and zooplankton communities) of the central Baltic Sea occurred in the late 1980s and early 1990s. These were attributed to changes in abiotic conditions, such as increasing water temperature and hypoxia, and decreasing salinity, in combination with overfishing of eastern Baltic cod, in particular, during years characterized by low reproductive success of cod. Since then, the open-sea system has been dominated by small pelagic fish, such as sprat.
  • Changes in coastal fish communities over the past decades have been linked to increasing water temperatures, decreasing salinities, and eutrophication. Increasing abundances of fish from the carp family (Cyprinidae) and decreases in piscivorous fish have been seen in many coastal areas during the past decade.
  • In general, those seabird species eating sprat and herring have increased in number, while several that feed on the benthos are decreasing, possibly partly caused by bycatch in static net fisheries.
  • Grey seal populations have had a high growth rate over the past few decades following the cessation of hunting in the 1980s, but this has levelled off in recent years. The growth rate of the southern Baltic harbour seal population has also been high.
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Baltic sea ecosystem overview

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