Too few fish, too many nutrients
Eight neighbouring countries discuss the state of the Baltic Sea in Palanga, Lithuania. The "Our Baltic Conference" will focus on overfishing, munitions on the seabed and damage caused by advancing climate change and over-fertilisation. The discussion about fishing quotas for 2024 could become explosive.
The ecological status of the Baltic Sea is poor - very poor indeed. Representatives from eight EU countries are discussing this in the Lithuanian coastal town of Palanga.
This can first be seen in the fish stocks. "The cod and herring stocks in both the eastern and western Baltic Sea are in a catastrophic state," states marine biologist Christian Möllmann, Head of the Department of Marine Ecosystem Dynamics and Management at the Institute of Marine Ecosystem and Fisheries Sciences at the University of Hamburg, to the Science Media Centre Germany (SMC). Both fish stocks have now "largely collapsed". The EU Commission has therefore severely limited the permitted catch quotas for the two fish species and even banned the targeted fishing of cod completely. However, many experts believe that the bans do not go far enough, as the fish can still be caught as by-catch. According to Möllmann, other measures are needed for the stocks to recover - for example, he suggests banning fishing with bottom trawls and setting up strict protection zones.
Not everyone agrees. Christopher Zimmermann, Head of the Institute of Baltic Sea Fisheries at the Thünen Institute, believes that fishing is not the decisive factor. "The EU Commission is proposing lower catches for many stocks than recommended by science - for reasons that are hard to understand," he criticised to the SMC. The catch quotas for cod and herring have been reduced by 97 per cent in recent years, and the small by-catch quantities that are still permitted are not being exhausted by the fisheries anyway. Despite this, the stocks are not recovering. In his opinion, the problem lies elsewhere.
Overfishing is not the only problem
"According to the Helsinki Commission, the most important stressor for the Baltic Sea ecosystem is over-fertilisation," says Möllmann. The international Helsinki Commission monitors the protection of the Baltic Sea. Too much nitrogen and phosphorus - nutrients that allow algae to grow - enters the seas, particularly from agriculture. As a result, the algae consume a lot of oxygen, creating oxygen-depleted zones in which fish and other marine animals that depend on the element can no longer live. "The Baltic Sea suffers in particular from the lack of oxygen in the deep water below a depth of 80 to 100 metres," says Maren Voss, Head of the Marine Nitrogen Cycle working group at the Leibniz Institute for Baltic Sea Research Warnemünde. The oxygen deficiency is a direct consequence of the high nutrient inputs from agriculture.
In addition, the entire ecosystem is suffering from the constant warming of the oceans. As climate change progresses, not only is the average water temperature rising, but marine heatwaves are also occurring more frequently, as experts have now documented. The North Sea and Baltic Sea are not exempt from this - on the contrary. "The Baltic Sea is warming faster than the ocean because it lies in an almost completely enclosed basin and the water volume is only renewed very slowly," explains marine scientist Maren Voss. Between 1982 and 2006, the Baltic Sea warmed by 1.35 degrees, according to a research group from her institute - that is seven times the global average. Even if the year 2023 was an exception here.
Today, several 10,000 tonnes of bombs and munitions filled with mustard gas, clark and adamsite lie in the Bornholm Deep in Denmark and the Gotland Deep in Sweden
While new rules and laws will not directly stop the warming of the Baltic Sea, they could indirectly help to relieve the ecosystem. This is because the individual stress factors have a combined effect on marine life; relieving the pressure in each of the areas can therefore have a positive effect.
New on the agenda this year is likely to be the question of how to deal with munitions remnants on the seabed. According to estimates by German authorities, the Allies dumped around 300,000 tonnes of munitions in the German Baltic Sea after the end of the Second World War. There are also unexploded bombs, mines and wrecks. According to researchers, there are also other weapons lying dormant underwater: "Today, several 10,000 tonnes of bombs and munitions filled with mustard gas, clark and adamsite lie in the Bornholm Deep in Denmark and the Gotland Deep in Sweden," says Matthias Brenner from the Alfred Wegener Institute in Bremerhaven. He is involved in the CONMAR research project, which is looking at munitions in the Baltic Sea. As the munitions are slowly corroding, he says, more and more explosives such as TNT are getting into the water - with harmful effects on marine organisms. He sees the conference as an opportunity to identify international cooperation in this area - so that the weapons can one day be recovered safely.
Spectrum of Science
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Cover image: © vivoo / stock.adobe.com (detail) You can't tell by looking at the Baltic Sea: But its ecological condition is now very poor.
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