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Marine — biodiversity under pressure

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Article Published 17 Mar 2010 Last modified 21 Mar 2023

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Topics: Biodiversity — Ecosystems

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The province of Canakkale lies on both sides of the Dardanelles, connecting the Sea of Marmara to the Aegean Sea: its shores touch both Europe and Asia. It is here that Homer described the mythical wooden horse of Troy in his Illiad, and 130 000 soldiers died at Gallipoli during World War I. Today, the Canakkale marina hosts many colourful yachts, making a stopover in this historically and mythologically rich area.

Just a few kilometres along the coast in Behramkale, we meet Saim Erol. He is one of the few active fishermen left in this small fishing village founded on the site of famous Temple of Athena and with breathtaking views of the Gulf of Edremit. 'Yesterday I set more than 700 metres of nets. All I caught was four red mullets. Not even worth the diesel I used!' says Saim, who has been fishing in these waters for more than 20 years.

The fact that there are fewer fish to catch and more boats chasing them is a sore point. Looking at his six-metre boat and then at the larger vessel out in the sea, he adds, 'I knew everything about this coast, where to fish and when. But things have changed. What I knew seems no longer valid. The sea has changed.'

Over the past 20 years, as the area has turned into a tourist hot spot, most of the fishermen have given up and now earn their living taking tourists to remote beaches only accessible by boat. 'At least that gives them some money to put aside for the winter,' says Hasan Ali Özden, retired teacher and amateur fisherman. 'About five miles to the west, the fishermen in Sivrice are luckier. Once in a while they hit the migration path of swordfish. And that’s good money. But it has been many years since a year of plenty.'

The triple impacts of climate change, invasive alien species and acidification

Fisheries are heavily dependent on healthy marine ecosystems but climate change is altering how things work. Professor Nuran Ünsal from Istanbul University points to alterations in migration patterns and their impacts on fish stocks. Migratory species with high economic value, such as Atlantic bonito, blue fish or mackerel, migrate south to the Mediterranean in the autumn and north to the Black Sea in the spring, where they breed. Year after year, however, steadily fewer fish have been migrating through the Turkish Straits.

'Changes in water temperatures and seasonal winds, crucial for the necessary currents, have disrupted their migration patterns,' says Professor Ünsal. 'Such species need a very specific setting with the right water temperature and amount of food, as well as enough time to breed.'

'Twenty years ago, they migrated south in September. With warmer water temperatures in the Black Sea now, they do not need to migrate south until mid-October or early November. This means they stay shorter in the Mediterranean and as a result they are fewer and smaller when they return north.'

Fish in warmer water are caught in a bind: as they adapt, their metabolism speeds up. They grow more quickly, although often to a smaller adult body size, and they need more food and more oxygen to support their higher metabolism. At the same time, as the water temperature increases, the amount of oxygen it contains decreases. Many fish experience what is called an 'oxygen squeeze': their need goes up and supply goes down.

Climate change is also altering the salinity and acidity of sea water and the way it forms layers. The impacts could be catastrophic. They include the collapse of coral reefs, the spread of invasive species and diseases, the loss of top predators and ultimately the whole structure of the marine food chain.

Invasive species In the late-1980s, the anchovy stocks in the Black Sea collapsed due to a combination of factors. These included overfishing; nutrient enrichment (especially from the Danube river); warmer water temperatures due to climate change; and the invasion of a new species to the region, Mnemiopsis leidyi, a combjelly originally from the north‑western Atlantic. Introduced in the Black Sea, most likely via ballast waters of cargo ships, Mnemiopsis leidyi feeds on fish larvae as well as organisms that would otherwise feed the anchovy. In the 1990s, another combjelly species Beroe ovata from north-western Atlantic and preying almost exclusively on Mnemiopsis leidyi, was also accidentally introduced in the Black Sea ecosystem. The introduction of this predator for Mnemiopsis leidyi, cooler temperatures from 1991 to 1993 and a decrease in nutrient flows, alongside reduced fishing during the collapse, lessened some of the pressures on the anchovy stocks. Since, the Black Sea ecosystem has shown some signs of recovery. A similar ecosystem shift has also been observed in the Baltic Sea. Overfishing and climate change has altered the Baltic fish community from a cod-dominated one to one dominated by herring and sprat. Whether introduced deliberately or accidentally, invasive alien species can cause havoc to people, ecosystems and native plant and animal species. The problem of invasive species is expected to worsen in the coming century due to climate change, increasing trade and tourism.

Blue carbon: the acid test

The Earth’s oceans are an immense, 'blue' carbon sink (or store of carbon dioxide). In fact, they are the largest store of carbon on the planet, with the terrestrial counterpart, including forests, a distant second. These natural sinks have functioned effectively for millennia, buffering the planet from the abrupt climatic changes due to greenhouse gases. But today, the carbon dioxide is increasing in the atmosphere faster than the land and oceans can absorb it.

Enhanced uptake of carbon dioxide from the atmosphere has increased the average acidity of the ocean. By 2100 the ocean is likely to be more acidic than at any time during the last 20 million years. Acidification is driving a decline in the amount of carbonate ion, which is needed to make aragonite and calcite — two forms of calcium carbonate that many marine organisms use to build their shells and skeletal material.

In Europe, researchers have begun to observe changes in the shells and skeletons of the microscopic organisms that form the beginning of the marine food chain. The falling rate of calcification is likely to have an immediate negative effect on their ability to survive and on the wide number of species that feed on them.

Corals are particularly at risk because they use calcification to create their skeletons, which make up what we see as coral reefs. Coral reefs are also home to as many as two million marine species and the source of a quarter of the global fish catch in developing countries around the world. The consequences of acidification go well beyond direct effects on calcification by marine organisms. More acidic water can have a major impact on respiration in some non-calcifying species such as squid (11). While the full consequences of ocean acidification have not yet been fully determined, it has been estimated that up to seven per cent of these 'blue carbon sinks' are being lost annually — seven times the rate of loss of 50 years ago.

Just like forests on land, marine ecosystems have a crucial role to play in the fight against climate change. Losing either would be catastrophic but we still do not fully understand just how quickly life under the surface of the oceans is likely to change.

Chasing the few fish left in our seas

Overfishing is the main culprit for the lack of fish in our seas. In Europe the picture looks very bleak: almost nine out of ten commercial stocks in the north-east Atlantic, Baltic and Mediterranean Seas are overfished. About one-third of those are so heavily overfished that the stock risks losing its reproductive capacity.

In the last decade alone, total landings in the European Union have declined by one-third⁽¹²⁾ and aquaculture in Europe has not been able to compensate. Global fish consumption per person has more than doubled since 1973, with Europeans consuming on average 21 kg of fishery products annually, slightly more than the global average of 17 kg, but below the USA, Chinese and Canadian consumption levels of around 25 kg. There is a wide variation within the EU, ranging from 4 kg per person in Romania to 57 kg in Portugal.

To meet Europe's demand for fish, about two-thirds of the fish are imported⁽¹³⁾. Europeans therefore have an impact on fish stocks and aquaculture production all around the world. Today, consumers, processers and retailers are increasingly concerned about overfishing and often require guarantees that the fish they consume and sell originate from well-managed and sustainable fisheries. But such assurances are difficult to give for most fish stocks in European waters.

In Europe, current re-evaluation of the Common Fisheries Policy⁽¹⁴⁾ is taking a fresh look at fisheries from a broader maritime and environmental perspective⁽¹⁵⁾. There will be a far greater emphasis on the ecological sustainability of fisheries outside Europe and the need to manage and exploit natural resources responsibly without jeopardising their future. It will be important to see just how this new approach to securing Europe’s fisheries will fit into the existing international regime and the proposed regular process for assessing the global marine environment.

Towards a global assessment of the marine environment In 2002, the World Summit on Sustainable Development's Johannesburg Plan of Implementation contained specific targets for fisheries management, including restoring fish stocks to maximum sustainable yield by 2015. It also identified the need to establish a 'regular process' under the United Nations for global reporting and assessment of the state of the marine environment, including socio‑economic aspects, both current and foreseeable, and building on existing regional assessments. This important step recognised the need for concerted international efforts to protect and manage the global commons sustainably. It marked the beginning of a concrete, action-oriented process to ensure that countries sign up for sustained, long-term and targeted efforts. The United Nations General Assembly endorsed the proposal in 2005 ⁽¹⁶⁾ and in 2009 recognised the work of the Group of Experts on the scientific basis of the global assessment. As with all international processes, however, implementing the Regular Process for Global Reporting and Assessment will take some years ⁽¹⁷⁾.

11. see also "Our Endangered Oceans", Dr. Richard Moss, WWF

12. Eurostat, European Commission, Commission working document 'Reflections on further reform of the Common Fisheries Policy'

13. European Commission Statistics

14. The EU treaties establish fisheries management as one of the exclusive competences of the Community. This is because fish move across national jurisdictions and fishermen have followed them long before Exclusive Economic Zones were introduced and the Common Fisheries Policy born. In 2009, the CEC published a green paper outlining the changes needed to address some of the most critical problems facing European fisheries. Reform of the Common Fisheries Policy, Brussels, 22.4.2009. COM(2009)163 final

15. Directive 2008/56/EC of the European Parliament and of the Council of 17 June 2008 establishing the framework for Community action in the field of marine environmental policy (Marine Strategy Framework Directive) (OJ L 164, 25.6.2008)

16. Resolution 60/30 of the General Assembly on Oceans and the Law of the Sea

17. Resolution 61 of the General Assembly on Oceans and the Law of the Sea