4. The Benguela Current Large Marine Ecosystem (BCLME)

A highly productive coastal upwelling system

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Figure showing Namiban fishery catches, with a marked decline from around 1980.
Namibian fisheries catches 1950-2000.   Source: M.O'Toole, BCLME.
Namibian fisheries catches 1950-2000.

The Beguela Current LME is the strongest coastal upwelling system in the world. It is highly productive and supports a rich fishery with catches of rock lobster, cods, hakes and haddock, sardines and anchovies of over a million tons per year.

Sardines and anchovies spawn on the Agulhas Bank, and their eggs and larvae drift with the westward current back into the Benguela system.

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Dolphins chasing sardines
Dolphins feeding on sardines, which are clumped together in a so-called 'bait ball'. Source: University of Cape Town.
Dolphins feeding on sardines

These small fishes are a rich source of food for larger fish species, marine mammals and birds. Sardine and anchovy are commercially important, but have declined in recent years.

Over-fishing, habitat modification through coastal development, and a high pollution risk associated with ongoing seabed mining and off-shore oil productiona are all potential threats to the ecosystem, unless they are carefully managed.

International collaboration

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Colourful fishing boats in Hout Bay near Cape Town
Fishing boats in Hout Bay near Cape Town
Fishing boats near Cape Town

The Benguela Current ecosystem stretches from the tip of southern Africa to the front with the Guinea current off northern Angola.

To meet the challenges of sustainable management of the Benguela system, South Africa, Namibia and Angloa have joined in the Benguela Commission to deal with environ-mental issues that cross national boundaries. See Links

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Diamond mining off the coast of Namibia
Dredging for diamonds off the coast of Namibia. Source: Bonaparte Diamond Mines.
Dredging for diamonds

Transboundary issues include the migration of valuable fish stocks across national boundaries, the introduction of invasive alien species via the ballast water of ships moving through the region, and pollutants or harmful algal blooms that can drift with winds and currents from the waters of one country into another.

The countries collaborate to

  • develop fishery regulations based on sound knowledge of the ecosystem,
  • develop a viable mariculture policy,
  • analyze the socioeconomic consequences of different harvesting methods,
  • solve conflicts between fisheries and coastal and offshore diamond, gold, oil and gas production.
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Sea surface temperature map of southern Africa with the warm Agulhas Current and the cold Benguela Current.
Sea surface temperature map of southern Africa 3 February 2008 (southern summer) with the warm Agulhas Current and the cold Benguela Current.
Source: NOC. Produced with Bilko software from Ostia SST data and NOAA-NGDC Coastline Extractor
Sea surface temperature 3 Feb.2008

Harmful algal blooms and sulphur eruptions

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Lobster walk out of the sea during a period of oxygen depletion.
Rock lobsters escaping the sea during a low oxygen event. Source: University of Cape Town.
Rock lobster walk-out

High phytoplankton productivity in the nutrient rich upwelling waters is the basis for the rich life in the Benguela. But the favourable conditions can also lead to harmful algal blooms. The blooms occur naturally as a result of the high nutrient content in the upwelling water, but pollution from sewage and land run-off may increase the risk.

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Lobster walk out of the sea during a period of oxygen depletion.
Sulphur eruptions off the coast of Namibia. Sourc: NASA
Sulphur eruptions off the coast of Namibia

When a dense bloom is over, the phytoplankton die and sink. As they decompose they use all the oxygen in the water. Bottom dwelling animals suffocate if they cannot escape.

The lack of oxygen in sediments under the strongest upwelling areas means that decomposition often produces hydrogen sulphide (which is toxic and smells like rotten eggs), and colours the sediments black.

Sometimes the sulphurous gases escape from the sediment and rise towards the sea surface in huge eruptions that are visible from space (left). These toxic eruptions can kill billions of fish. See Links

Concerns about global warming

Both the harmful blooms and the sulphur eruptions may increase with global warming. There is also concern that global warming may disrupt the balance of upwelling, sheltered areas and mixing between warm and cold currents that is so favorable to the anchovies and sardines.

Periodic El Niño like events already cause disruption of fish, bird and mammal migrations, and seriously reduce fishery recruitment. These ‘Benguela Niños’ may become more frequent as the world warms.