Aquaculture for all

Resuscitating the Baltic Sea Out of Anoxia

Cod Welfare Sustainability +4 more

Anoxic or "dead" zones have become markedly distinct in the Baltic Sea over the years. The continuous formation of these oxygen-deprived areas is proving to be more and more detrimental to aquatic organisms, writes Rachel Ralte, reporting for TheFishSite.

A report by the European Environment Agency (EEA), EEA Signals 2009: Key Environmental Issues Facing Europe, outlines the key issues faced by Europe's aquacultural sector.

What happened two decades ago

About 20 years ago, a group of lobster fishermen in Denmark found an abundance of lobsters that were either already dead or on the brink of death. Later, it was discovered that the main reason behind this was that the lobsters were suffocating. Researchers at Denmark's National Environmental Research Institute observed that a vast area on the bottom of the Kattegat Sea was oxygen-deficient. This unusual phenomenon was caused by what is referred to as anoxia or a lack of oxygen.

Now it has been found that major parts of the Baltic Sea are affected by anoxic or dead zones, and as a result, aquatic species are gradually becoming extinct.

What's happening now

"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."

Bornholm, a Danish island at the entrance of the Baltic Sea, has, for centuries, depended largely on the abundance of fish found within the fisheries in the area. During the 1970s, about half of the region's income was derived from cod. However, by 1990, several changes have occured and the fishermen's future is not so bright.

According to the EEA's report: "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." So complex is the Baltic catastrophe that it is posing never-ending challenges to policy makers in the marine world.

Regulations: the roles they play

The fishermen at Bornholm, not unlike their European counterparts, are under legal obligation to observe restrictions under the Common Fisheries Policy. The main rules that apply are how many fish can be caught, of which kind and where.

The International Council for the Exploration of the Sea (ICES) provides invaluable data and scientific advice. What is important to assess is the health of the most fished commercial species, the report states. More particularly, the number of fish of a certain age in an area is of utmost importance. It is imperative that young fish be preserved for two to five years when they are mature - the more mature fish, the more eggs are spawned.

Now regulations on total allowable catches (TACs) have been framed by EU member states.

Understanding anoxic zones, understanding the situation

To understand the current Bornholm fisheries situation, it is necessary to understand anoxic areas and how they are formed. The 1960s was a good period - a period when agriculture and urbanisation was undergoing development and artificial fertilisers were used. This led to a dramatic rise in nutrient inputs and pollution in the Baltic Sea. This was then followed by increased growth in phytoplankton and fish production (as an abundant supply of phytoplankton means more fish food). However, anoxia-related problems were taking place in the deepest waters.

According to the report, when water near the seabed reaches an anoxic point, hydrogen sulphide, which is toxic to almost every life form, is formed. It is most probable that the lack of oxygen and the hydrogen sulphide combined led to the death of the Norway lobsters back in 1986.

At present, the anoxic areas are so vast that they have resulted in a reduction in spawning areas in the Central Eastern Baltic, reducing the spawning success rate of cod.

The 1980s: good times for cod fisheries

The 1980s were marked as the best period for cod fisheries. From 1978 to 1983, the survival rate was known to be at its best.

Fishing pressure was reduced by the late '70s. Climatic conditions were also favourable, thereby bringing high salinity in water from the North Sea. The reports says, "Saltwater 'intrusions' into the Baltic are important in terms of maintaining salinity and oxygen levels."

When did things go wrong?

Ever since the mid-'80s, the inflow of saline water from the North Sea has declined, leading to poor conditions for egg survival. There's also the problem of oxygen-content depletion.

Illegal fishing is only adding to this problem. According to the report, estimates show that an additional 30 per cent is landed illegally in this part of the Baltic Sea.

Another problem is climate change. It affects both the temperature and salt balance in the Baltic. The report says that 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. Increased rains and a reduced inflow of high salinity water has decreased salinity in the Baltic. These two factors, temperature and salinity, are both climate-driven.

Figure 1 shows estimates of the extent of hypoxia (oxygen content less than 2 ml/l) and anoxia (oxygen content = nil). Over time, there has been a steady increase in the area affected by hydrogen sulphide in the East and West Gotland Basins, and the outer Gulf of Finland. Water from the Gulf of Finland does not enter into the Gulf of Bothnia. As a result, despite its depth, it remains well oxygenated, even during autumn.

Hoping for a brighter future

In an attempt to alleviate the Baltic Sea situation, countries within the region have formed an agreement, a "Baltic Sea Action Plan". Integration of agricultural, fisheries and regional policies are under development. An important policy that is being framed is the Marine Strategy Framework Directive, a policy where bordering countries should achieve a 'good environmental status' for the Baltic Sea by 2020, including a requirement that fish communities are brought back to 'a good state'.

The European Commission is also forming a strategy which, according to the EEA report, "will lead to an action plan defining the key players, the financial instruments to be deployed, as well as a work schedule." Sweden will play a key role in this strategy. In fact, the country has made it one of its top priorities for the latter half of 2009.

Several other measures are also being taken by the EU. If all policies are implemented and followed, the Baltic Sea should regain its former sheen and economic wealth.

Further Reading

- You can view report EEA Signals 2009: Key Environmental Issues Facing Europe by clicking here.

February 2009
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