issue 234 - August 1992
Seas and oceans cover nearly 72 per cent of the earth's surface.
For air-breathing, land-dwelling creatures like ourselves the oceans have always
been a strange, foreign region of our planet - both fascinating and frightening.
Today the mystery and the allure of the sea is as strong as ever; yet the oceans
are seriously threatened by the poisons of industrial society.
Seas and Oceans
Although the two names are often used interchangeably they are different. Seas are smaller bodies of salt water that are either landlocked or connected to the oceans by a narrow channel.
. Inland seas include the Dead Sea, the Caspian Sea and the Sea of Azov. Partially landlocked seas include the Mediterranean, the Red Sea, the Baltic and the Black Sea. Other seas' include Hudson's Bay, the South China Sea, the North Sea and the Sea of Japan.1
Why is the ocean blue?
The ocean looks blue because of sunlight reflected off tiny particles in the water. The sea can also appear green (in the tropics for example) when blue mixes with yellow pigments in microscopic floating plants. The Black Sea appears black because it has little oxygen and lots of hydrogen sulphide. The Red Sea was named after the seasonal blooms of red algae that tint the surface water.2
Ridges, Trenches and Plates
The most extraordinary aspect of the ocean is its underwater topography. Much of what we know has been learned in the last 40 years and details are still sketchy. But we do know that the oceans, like the continents, sit on vast tectonic plates which are in slow but constant motion. These shifting plates have helped create the physical features of the ocean's floor - ridges, trenches, plains and undersea volcanoes.
. The world's largest mountain range is in the Atlantic Ocean; its tallest peak at 8,000 metres breaks the surface to form one of the Azores islands. This mid-Atlantic ridge forms a continuous chain of mountains more than 88,000 kilometres long.2
. Colliding tectonic plates also create plunging trenches of spectacular depth. The Mindanao Trench off the southern Philippines is 11 kilometres deep. The earth's tallest mountain, Mount Everest, could be placed there and still have 1.6 kilometres of water above it.3
About 90% of the world's marine fish breed along the coast. Around the world the habitats that nurture ocean life are threatened by human activities.
. Shore-hugging mangrove forests are rich breeding grounds for shrimps, fish, crabs and other marine animals. Mangroves are being cut all over the Third World. Between 1963 and 1977 nearly half of India's mangroves were destroyed; 50% of Ecuador's mangroves have been cut and the land used for commercial fish ponds.4
. Population pressures and industrial development have prompted Western countries to clear coastal wetlands. Italy has lost 95% of its swamps and the US has lost over half its coastal wetlands.5
. Coral reefs are one of the most productive marine eco-systems; one reef may support 3,000 different species. Reefs are being destroyed by sedimentation, blast fishing and coral mining. A study by the International Society for Reef Studies found seriously damaged reefs in more than 20 countries.6,7
The ocean is an intimate part of global weather systems, circulating both nutrients and heat through a complex system of ocean currents.
. Cold water is denser than warm water; the constant sinking of cold water near the poles generates deep steady currents flowing towards the equator while warm water flows towards the poles. Changes in the speed or direction of these ocean currents can significantly alter global weather.
. Cold water from the poles rises in so-called upwelling zones' in the tropics. Deep-sea currents take about 1,000 years to travel from the equator to the pole and back again.3
. The sun evaporates about a million million tons of sea water daily which eventually falls to earth as precipitation; the sea also releases up to 10 billion tons of salt into the atmosphere annually. These salt particles act as seed crystals around which raindrops can form.3
The global fishery is in serious trouble as a growing army of boats armed with sophisticated fish-finding gear pursue fewer and fewer fish.
. Fish account for more than 40% of the annual protein supply of two billion people in the Third World and developing countries produce 52 million tons yearly, over half the global production.4
. Almost a quarter of the total yearly catch is turned into animal feed. Chile processes 90% of its catch into fishmeal, most of which is exported to the West.8
. The total global catch continues to increase slightly though the catch per person has stalled. By the year 2025 a further 100 million tons of fish will be needed every year to feed a growing world population.9
. Most traditional food fishes are now over-exploited and stocks are falling. The North Atlantic cod fishery, once one of the world's richest fishing grounds, has been destroyed by overfishing and destructive fishing techniques.10
1 Times Atlas of the Oceans.
2 The Oceans, A Book of Questions and Answers, Don Groves, Wiley and Sons 1989.
3 The Mysterious Oceans, Jon Erickson, Tab Books 1988.
4 'The Ocean Blues', N Lenssen, from Worldwatch Reader, ed L Brown, Norton 1991.
5 World Resources 1992-93, World Resources Institute, Oxford University Press 1992.
6 Coral Reefs, Alan White, ICLARM 1990.
7 'Reefs on the Rocks', Greenpeace Magazine, July/Aug 1990.
8 'Full Nets and Empty Stomachs', Third World Guide 91/92.
9 Strategy for International Research on Living Aquatic Resources Management, ICLARM, 1992.
10 The Living Ocean, B Thorne-Miller & J Catena, Island Press/Friends of the Earth, 1991.
11 UN Food and Agricultural Organization Statistical Yearbook, 1991.