Fish out of water — marine management in a changing climate

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Article Published 23 Mar 2009 Last modified 16 Dec 2016, 03:02 PM

If left alone for two years, the cod population in the Baltic would recover.

Henrik Sparholt, ICES Advisory Programme Professional Officer

Observations of dissolved oxygen in the water in combination with the dead lobsters told researchers at the National Environmental Research Institute in Denmark that an unusually large area on the bottom of the southern Kattegat was devoid of oxygen. The strange events were caused by 'anoxia' or lack of oxygen on the sea bed that night. Scientists believe the lobsters were suffocating!
Twenty-two years later, large parts of the Baltic are affected by anoxic areas or 'dead zones'.

Collapse of the Bornholm fisheries

Bornholm, an idyllic Danish island situated at the entrance of the Baltic Sea more or less between Sweden, Germany and Poland, is well known for its smoked herring. For centuries the abundance of fish was the cornerstone of the local economy.

In the 1970s about half of the fisheries income came from cod. By the end of the 1980s cod fisheries had increased to 80 % of the total value. Many fishermen imagined a bright future and invested in new vessels. However, by 1990 the catch was on a steep decline. It has never recovered. This collapse put huge financial pressure on the local community.

The scale and rapidity of the collapse of cod stocks in the Baltic has meant that a lot of energy has gone into understanding what caused the boom and subsequent collapse. The region has become an international case study with lessons for other regions. The Baltic story is not a simple one — indeed the complexity of the situation illustrates the challenge facing policy makers in the marine environment.

Fishing for data

Bornholm fishermen, just like their counterparts around Europe, are legally bound to tight restrictions under the Common Fisheries Policy that establishes how many fish of which kind can be caught where.

The International Council for the Exploration of the Sea provides the scientific advice on the biologically safe levels. Fisheries survey data, fish catch statistics and environmental monitoring of oceanographic conditions provide invaluable data in terms of assessing the health of the most fished commercial species. In particular, the number of fish of a certain age in an area is important. The more young fish that survive in a year, the more fish can be expected to be caught two to five years later when the fish are mature. And the more mature fish that are available, the more eggs that are spawned.

Following the scientific advice, decisions on total allowable catches (TACs) are made by EU Member States. These decisions often reflect priorities other than the protection of stocks. In 2006, approximately 45 % of the assessed fish stocks in Europe's Seas were fished outside safe biological limits. These fishing levels were agreed at the ministerial level.

Fish breathe oxygen dissolved in water

Particularly since the 1960s increased use of artificial fertilizers in agriculture as well as urbanization has led to a dramatic rise in nutrient inputs — pollution — into the Baltic Sea. This has lead to increased phytoplankton growth and fish production (more phytoplankton means more food for fish). However, it has also resulted in increased problems with anoxia in the deepest waters of the sea.

When water near the seabed becomes anoxic, hydrogen sulphide is released from the sea floor into the water. Hydrogen sulphide is toxic to most life forms, and it was probably a combination of hydrogen sulphide and lack of oxygen that killed the Norway lobsters in the Kattegat that night back in 1986.

The anoxic areas in the Baltic Sea are now so large that they have led to a reduction in the size of potential spawning areas in the Central Eastern Baltic. This reduces the spawning success of cod.

Why were the early 1980s such good years for cod fisheries?

The high survival rate of cod eggs and larvae from 1978–1983 is explained by four factors. The primary explanation is that fishing pressure was reduced in the late 1970s. Secondly, climatic conditions brought inflows of high salinity water from the North Sea. The Baltic was actually a freshwater lake until sea levels rose about 8 000 years ago, allowing the North Sea to flow into the lake. Saltwater 'intrusions' into the Baltic are still important in terms of maintaining salinity and oxygen levels.

These inflows led to higher oxygen concentrations in the cod spawning areas and hence to high egg survival and thus more juvenile fish. Thirdly, there was an abundance of copepod larvae (pseudocalanus acuspes), the major food source for cod, and finally, there was a shortage of predators such as sprat and seals. Sprat prey on cod eggs and seals prey on cod.

And what went wrong?

Since the mid-1980s there have been fewer major inflows from the North Sea, leading to poor conditions for egg survival, and fewer juvenile fish. The reduced salinity has also led to reduced abundance of copepods, a staple food for larvae. Although the limit for biologically safe levels for fishing was reduced in the following years, the politically agreed catch (TAC's) has normally exceeded this level ( Figure 1). Illegal fishing adds to the problem. It has been estimated that an additional 30 % is landed illegally in this part of the Baltic Sea. In the summer of 2007 the illegal landings by the Polish fishing fleet were so extensive that Polish fisheries were stopped by the European Commission in the second half of 2007.

Graph Figure 1: Scientifically recommended catch levels (based on ICES advice), agreed total allowable catch (TAC) and actual catch in the fishing areas around Bornholm, in the years 1989–2007

And then climate change!

Climate change is affecting both the temperature and the salt balance of the Baltic. Temperature rise in the deep water will increase the metabolic demand for oxygen and reduce solubility of oxygen in the water. In turn, this will contribute to the wider geographic spread of anoxia. Salinity in the Baltic has decreased steadily since the mid 1980s due to increased rain and reduced flows from the North Sea into the Baltic Sea.

Both factors are driven by climate. Quite a small reduction in the salinity is already tipping the balance and changing the composition of the Baltic habitat. Of the three major fished species, cod, herring, and sprat, cod is particularly sensitive to reduced salinity because salinity affects both their reproductive capacity and the availability of the preferred food for cod larvae.

Projections for the future ocean climate of the Baltic are for continuing increases in rainfall and decreases in inflows from the North Sea. This means that stocks of cod and other marine fish are likely to decline further unless fishing pressure is reduced.

Climate change will alter the Baltic Sea and its ability to support exploitable cod populations. Management will need to accommodate these changes if the stock is to stay at a commercially relevant level.
Professor Brian MacKenzie, DTU-Aqua, Technical University of Denmark

Hope for the future

In response to the complex and severe environmental problems in the Baltic Sea, the countries in the region have agreed a 'Baltic Sea Action Plan' to develop national actions towards integrating agricultural, fisheries and regional policies. This plan, adopted in November 2007, is an important basis for more effective implementation of EU policy in the area.

This includes the new Marine Strategy Framework Directive, according to which bordering countries should achieve a 'good environmental status' of the Baltic Sea by 2020, including a requirement that fish communities are brought back to 'a good state'.

In addition, the European Commission is developing a Baltic Sea Regional strategy which will lead to an action plan defining the key players, the financial instruments to be deployed, as well as a work schedule. This strategy's adoption by Member States will constitute one of the priorities of the Swedish EU Presidency, in the second half of 2009. Sweden has identified the Baltic Sea environment as one of its top priorities.

The Common Fisheries Policy (CFP) was designed to regulate fishing activities from an environmental, economic and social point of view. However, many of the most commercially valuable fish species in Europe have been heavily over fished and their populations are now below safe biological numbers. The nature of the legislation makes it costly and difficult to successfully prosecute Member States who over-fish.

The obvious lack of success in sustainable management of many of its fish stocks has led marine experts to call for major revisions to the policy, which is clearly the product of compromise between countries. The marine environment should be treated as an ecosystem rather than as sectors to be exploited.

The EU Commissioner for Fisheries and Marine Affairs, Joe Borg has even said that the CFP 'does not encourage responsibility by fishermen or politicians' and launched an immediate review of the policy in September 2008, four years ahead of schedule.

Graph Figure 2: Estimates of the extent of hypoxia and anoxia in Autumn 2007.



Diaz, R. J. and Rosenberg, R., 2008. Spreading Dead Zones and Consequences for Marine Ecosystems. Science, vol. 321, pp. 926–929.
Mackenzie, B. R.; Gislason, H.; Mollmann, C.; Koster, F. W., 2007. Impact of 21st century climate change on the Baltic Sea fish community and fisheries. Global Change Biology, vol. 13, 7, pp. 1 348–1 367.
Sparholt, H.; Bertelsen, M.; Lassen, H., 2008. A meta-analysis of the status of ICES fish stocks during the past half century. ICES Journal of Marine Science, Vol. 64, 4, pp. 707–713.


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