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Baltic Sea - pelagic food web

Main Contributors:

Henrik Österblom, Johanna Yletyinen

Other Contributors:

Jonas Hentati-Sundberg, Thorsten Blenckner

Summary

The Baltic Sea is a semi-enclosed, brackish sea located in Northern Europe. The regime shift described for the Central Baltic Sea involves a drastic change from a cod- to a sprat-dominated ecosystem in a marine food web. Through the biomass decrease of a high trophic level, a commercially high valued and favored table fish was replaced by a low trophic level and low commercial value fish. The prerequisite for the change in the system was most probably loss of resilience, which was caused by poor cod recruitment conditions and too high fishing pressure. Anthropogenic eutrophication and infrequent inflows of saline water from the North Sea contributed to the changed deep water conditions in the Central Baltic, resulting in anoxia and low salinity lowering the cod reproduction rates. Since cod (Gadus morhua) is the main predator of sprat (Sprattus sprattus), the cod decrease caused a trophic cascade as the sprat stock dramatically increased. 

Type of regime shift

  • Unknown

Ecosystem type

  • Marine & coastal

Land uses

  • Fisheries

Spatial scale of the case study

  • Sub-continental/regional (e.g. southern Africa, Amazon basin)

Continent or Ocean

  • Europe

Region

  • Northern Europe

Countries

  • Lithuania
  • Poland
  • Russia
  • Sweden
  • Denmark
  • Estonia
  • Finland
  • Germany
  • Latvia

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Drivers

Key direct drivers

  • Harvest and resource consumption
  • External inputs (eg fertilizers)
  • Species introduction or removal

Impacts

Ecosystem type

  • Marine & coastal

Key Ecosystem Processes

  • Nutrient cycling
  • Water cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Fisheries

Regulating services

  • Water purification

Cultural services

  • Recreation
  • Aesthetic values

Human Well-being

  • Food and nutrition
  • Health (eg toxins, disease)
  • Livelihoods and economic activity
  • Cultural, aesthetic and recreational values

Key Attributes

Spatial scale of RS

  • Sub-continental/regional

Time scale of RS

  • Years
  • Decades

Reversibility

  • Unknown

Evidence

  • Models
  • Contemporary observations
  • Experiments

Confidence: Existence of RS

  • Contested – Reasonable evidence both for and against the existence of RS

Confidence: Mechanism underlying RS

  • Contested – Multiple proposed mechanisms, reasonable evidence both for and against different mechanisms

Alternate regimes

The cod-dominated regime

The cod-dominated regime was characterized by abundant cod stocks, not only in the southern part of the Baltic Sea, but also further north and closer to shore. Cod predation regulated the number of sprat and herring. Pseudocalanus spp. dominated the zooplankton community. This regime was observed primarily during the early- to mid-1980s. 

Sprat-dominated regime

The sprat-dominated regime came into effect in 1989. A lower cod stock, an increased number of sprat and favorable conditions for sprat recruitment characterize the regime. In zooplankton community, Acartia spp. and Temora longicornis are more abundant than Psedocalanus spp. It is suggested that sprat's high predation on zooplankton lead to algal growth and declining oxygen content in deep waters. The high fishing pressure on cod is preventing the recovery of the stock and allowing the sprat to control the zooplankton. 

Drivers and causes of the regime shift

The key drivers for the Central Baltic cod-dominated regime to shift to a sprat-dominated regime are suggested to be poor cod recruitment conditions and too high fishing pressure caused by eutrophication, altered deep water conditions, infrequent inflows of North Sea saline waters and international fishing policies.

Climate functions as an indirect driver through changes in the physical environment and altered food supply for early stages. Decreased salinity, increased temperature and high sprat-predation pressure might have caused the regime subshift from Pseudocalanus spp. to Acartia spp. and Temora longicornis. Low salinity and oxygen conditions decrease the amount of the main prey of cod larvae. Higher temperatures support sprat recruitment by improving egg survival and food supply as Acartia spp. and Temora longicornis increase. 

How the regime shift worked

Cod is the main predator of sprat. It has been suggested that the large cod stock was able to control the sprat stock in the cod-dominated regime. Anthropogenic eutrophication and infrequent inflows of saline water from the North Sea contributed to increased anoxia and lowered salinity in the Central Baltic. Due to low salinity and oxygen content in the spawning areas, cod reproduction rates have remained low since the early 1980s. Deep water anoxia is a chronic stress for the survival of the cod eggs. Baltic cod needs salinity for the eggs to float in the water. If the required salinity is in anoxic conditions, eggs die.

Anoxia is one of the effects of the Baltic Sea eutrophication, but it can be temporarily reduced by inflows of North Sea saline waters, which replenish the deep-water layers of the Baltic Sea with saline and well-oxygenated water. Major Baltic Inflows decreased frequently in the early 1980s and there was a stagnation period until the early 1990s. Although the cod stock decreased due to cod recruitment decline caused by degraded environmental conditions, the fishing effort remained high. Unfavorable cod recruitment conditions and high fishing pressure caused a remarkable drop in cod biomass. As the Central Baltic Sea is characterized by simple food web with low species diversity, cod decrease allowed its' prey sprat to increase. Cod fishing further reduced the stock, making cod even more sensitive to adverse environmental conditions. International fishing quotas have been set to limit the cod fishing but they have not always been followed. The quotas have consistently been greater than the scientific recommendations.

Studies suggest that the large increase of the sprat stock has changed the system from mainly bottom-up to top-down trophic control. A trophic cascade occurred as the sprat stock dramatically increased. High sprat abundance changed the quantity and quality of zooplankton, which feeds on phytoplankton. A sub-shift in zooplankton species can be observed to have happened until the end of the 1980s: a shift in dominance occurred from Pseudocalanus spp., the main food supply for cod larvae, to the main food supply of sprat larvae and adults, Acartia spp. and Temora longicornis. Salinity, temperature, predation of sprat, herring and mysids and climate in general regulate the zooplankton species as they all have their specific abiotic preferences. Through trophic cascades large sprat stock also lead to higher phytoplankton biomass and algal blooms during summer. Algae growth promotes further anoxia. A sprat-dominated food web has been suggested to stabilize the cod stock by sprat predating on cod eggs and larvae, and through sprat competing with young cod for zooplankton resources. Changed abundance of sprat may have even been reflected to the top levels of the Baltic food web: the condition of Common Murre (Uria aalge) chicks seem to increase and decrease in relation to sprat stock. This way human exploitation combined to natural changes caused variations in several trophic levels, which altered the ecosystem functioning as the interaction between species changed.

 

Impacts on ecosystem services and human well-being

The main ecosystem services associated with the cod-dominated regime were commercially viable, high value cod stocks, recreational value of large cod stocks and a sustainable small-scale fishing sector favoring regional development. The regime was possibly related to high water quality stimulating tourism development. Cod provide the Baltic Sea region with provisioning services: Cod is commercially the most important species in the Baltic Sea for a large number of fishermen. In the sprat-dominated region, the economic value of the catch for human consumption decreases due to the relative composition of the fish species. Being a predator at the top of the food chain, cod provides the Baltic Sea with regulating ecosystem services: cod reduces sprat, which prey on zooplankton and early life stages of cod. The high summer levels of phytoplankton could partly be a result of the sprat predation-induced decrease in total zooplankton biomass. The decay of increased algae leads to oxygen deficit and thereby causes damage to the biodiversity. The regulating role of cod may also be reflected to the top levels of the Baltic food web, the sea birds. The sprat-dominated regime is dominated by low value fish used for reduction fisheries and international fish meal and fish oil markets. These large-scale fisheries are mainly based outside the Baltic Sea region, which makes the regime less advantageous for the local development. The sprat-dominated regime may be associated with cascading effects on water quality as the low biomass of summer zooplankton may increase the probability for cyanobacterial blooms.

Low cod biomass makes industrial fishing and businesses associated with recreational fishing less profitable. Increased occurrence of algal blooms makes the Baltic Sea less attractive for tourists and recreation. From the socioeconomic perspective, tourist industry is more important than fishing industry in for example Sweden. The Baltic Sea environment and its cultural fishing surroundings are important tourist attractions. The toxic blooms and low water quality are nuisance for coastal property owners, local enterprises, bathers and others seeking for creation on the coasts. 

Management options

The goal for managing the regime shift is to maintain the resilience of the Baltic Sea ecosystem by targeting eutrophication and recovery of the cod stock. The management actions to reduce nutrient pollution began in the 1970s, when Helsinki Commission (HELCOM) adopted several recommendations for all sectors (industry, agriculture, wastewater treatment). Since the 1980s, HELCOM has worked with the 50% reduction targets for nutrients and discharges. Several countries have recently agreed to substantially reduce fertilizer use in the region (Baltic Sea Action Plan). In 1990s, it became obvious that the important Baltic Sea fish stocks were in a poor state and the fisheries were not under satisfactory control. The International Baltic Sea Fishery Commission (IBSFC) reacted with several resolutions to reduce fishing mortality and improve selectivity for cod. The main initiatives were to improve control and enforcement to assure adherence to TACs, set TACs based on target fishing mortalities, introduce technical measures to improve selectivity, and seasonal closures of areas to protect the spawning stock. However, the overexploitation of cod resources continued: the scientific advice was disregarded when setting quotas and the quotas restricting exploitation were not enforced.

In 1999, a Long-Term Management Strategy for Cod Stocks in the Baltic Sea was made including e.g. efforts to maintain a minimum spawning-stock biomass. The Recovery Plan for Baltic Sea Cod was developed in 2001 requiring a.o. seasonal closures and ban on fishing on the cod spawning grounds. Still, the problems remained and in 2004 it was acknowledged that the state of the cod stock had not improved, requiring determined action based on scientific advice. The EU agreed a plan for the Baltic Sea cod in 2007. In 2009 and 2010, The European Council Regulation for the Baltic TAC involved reductions in the number of fishing days per year. The cod fisheries are regulated by a seasonal closure to protect spawning fish. A year-round area closure for all fisheries in specific parts of the Bornholm Deep, the Gotland Basin and the Gdansk Deep was introduced in 2005.

High-grading has been prohibited since 2010 in all Baltic fisheries. In 2003, the minimum landing size was raised from 35 cm to 38 cm to protect the juvenile cods, which have not had a chance to reproduce yet. Additionally, fishing nets with exit windows have been introduced. To combat illegal fishing EU Commission has made inspections on randomly selected vessels and concluded that illegal fishing is substantial and practiced by many Baltic Sea countries. 

Key References

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  10. Möllmann C, Diekmann R, Müller-Karulis B, Kornilovs G, Plikshs M, Axe P. 2009. Reorganization of a large marine ecosystem due to atmospheric and anthropogenic pressure: a discontinuous regime shift in the Central Baltic Sea. Glob Change Biol 15, 1377–1393.
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Citation

Henrik Österblom, Johanna Yletyinen, Jonas Hentati-Sundberg, Thorsten Blenckner. Baltic Sea - pelagic food web. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-03-19 12:58:03 GMT.
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