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The basking shark needs to eat huge quantities of plankton and small invertebrates to survive. (Courtesy Atlas of Oceans)

State of our oceans

  • BY John Farndon |
  • April 27, 2011

Just as the ocean is revealing its mysteries, we're finding out how delicate its many ecosystems are.

DISMAYED BY THE IMPACT of a corporation dam on migrating shad, the young American author and philosopher Henry David Thoreau wrote in 1839, "Who hears the fishes when they cry?" Today, this seems a remarkably prophetic observation.

It is indeed difficult to be sure what is going on in the oceans. Both their huge scale and their inaccessibility means that scientists can sometimes only guess at the problems, and their findings are constantly disputed, especially by those with vested interests. It has only been in the last few decades that the ocean bed has been fully mapped, and just a tiny percentage has been fully explored. So we only know a small portion of what goes on in a healthy ocean. It is that much harder to be sure how it is being damaged. Sometimes it is only practical to take small samples - a study of a single species, or even a small population, or a small location - to find out what is happening.

Yet gradually the picture is beginning to snap into focus. It is becoming more and more clear that the human impact on the oceans is escalating. What is more it is happening rapidly. Indeed, it is happening at such a speed that it has caught everyone by surprise.

Human impact on oceans

In February 2008, an international team of scientists led by Dr Benjamin Halpern of the National Center for Ecological Analysis and Synthesis in Santa Barbara, USA, developed the first detailed global map of human impacts on the seas, using a sophisticated model to handle huge amounts of data. The team divided the world's oceans into square kilometre sections and combined data for each on 17 different human impacts to oceans, including fishing, coastal development, fertilizer runoff, and pollution from shipping traffic.

Their map (below, courtesy of Ben Halpern/NCEAS) showed that just 4 per cent of the world's oceans are now entirely undamaged by human activity. Climate change, fishing, pollution, and other human factors have taken their toll in some way on all the other 96 per cent of the world's oceans. Forty-one per cent of the oceans are seriously damaged. Even the scientists working on the map were shocked to find that virtually nowhere seems to have escaped - and they believe that soon even the small area of pristine waters near the poles will be affected as climate change melts the polar ice caps. 

The result was shocking because previously the true scale of the problems had been masked by focusing only on single problems and small areas. As Benjamin said: "for the first time we have produced a global map of all of these different activities layered on top of each other so that we can get this big picture of the overall impact that humans are having rather than just single impacts."


Survey of ocean life

One of the ironies is that marine life is coming under threat just as we are beginning to learn how astonishingly rich it is. In 2010, the results of the extraordinary 10-year Census of Marine Life were announced. Thousands of previously unknown species have been discovered, and it is absolutely certain there are many more yet to be discovered.

The oceans are so vast that the Census can give only an impression of what is out there. Yet it has shown that the variety of life in the oceans is much, much, greater than scientists ever imagined.

Many of the human threats to the oceans centre on what we take out and what we put in. Over the last half century, hundreds of millions of tons of fish and other creatures have been taken from the oceans. Meanwhile, hundreds of millions of tons of waste have been dumped into them.

Threats to ocean life

The introduction of fishing quotas and much publicised campaigns to avoid eating endangered cod or bluefin tuna have failed to halt the pace at which every fish that is vaguely edible is scooped from the ocean by industrial trawlers. Well over 90 million tonnes of fish are taken from the sea every year, and that rate of catch is simply too much. The world's fish population is large, but not limitless, and more and more populations are being fished to exhaustion.

Since the 1950s, the numbers of bluefin tuna, Atlantic cod, American eels, and some sharks have been reduced by 95 per cent. As one population is exhausted, the fishing industry simply moves on to another. But they cannot move on continuously, or wild fish of any kind will be off the menu forever, and the biodiversity of the ocean will be restricted with unpredictable consequences.

Moreover, an incredibly wasteful quarter of the tonnage of marine creatures taken from the sea never reaches the dining table - or even the fish meal factory. This is 'useless' by-catch caught up in the nets then discarded. Every year, hundreds of millions of fish, marine mammals such as dolphins and seals, and sea turtles and seabirds perish in this way.

The ocean'smost endangered places

There are dangers to marine life all around the world, on both a global and local scale, but four kinds of marine zone stand out as being particularly vulnerable: coral reefs, coasts, continental shelves, and the great open ocean.

1. Reefs
Corals are particularly sensitive to changes. Disease, coral bleaching, acidification, and ocean warming have all begun to take their toll. Well over half of the world's live coral has been lost since the 1950s. In the Caribbean and the Indo-Pacific, there are places where more than 80 per cent has gone. There are now scores of corals on the International Union for Conservation of Nature's (IUCN) list of critically endangered species. Reef fish stocks are down, too, with large predators rare, and a sharp decline in sponges and sea turtles.

2. Coasts
Coastal shallows, wetlands, and estuaries are all vulnerable, too. They are marginal zones, not just because they are around the sea's margins but because any small shift in conditions can have a dramatic effect. A slight rise in sea levels or a small change in the quality of the water can have a huge impact on marine life in these delicately balanced ecosystems. Two-thirds of wetlands and 90 per cent of mangroves have been removed in the last half century.

3. Continental shelves
Close to the shore and fairly shallow, the continental shelves are exposed to a wide range of human activity. They are also the most biologically productive areas of the ocean because nutrients are brought readily to the surface in their shallow waters. It is not surprising to discover that, when the NOAA decided to designate 64 regions around the world as Large Marine Ecosystems (LME) for conservation purposes, all of them are on continental shelves or on the outer margins of ocean current systems.

The 64 LMEs yield 80 per cent of the world's annual fishery catch. But they are foci of coastal ocean pollution and nutrient over-enrichment, habitat degradation (such as the loss of seagrass beds, corals, and mangrove swamps), overfishing, biodiversity loss, and climate change effects.

4. Open ocean
The open ocean is so vast and so remote that it is hard to imagine human activities having much effect. Yet fishing has taken a huge toll on populations of large hunting species such as tuna, sharks, and billfish. Global warming is also beginning to interfere with the slow rise of nutrients that is so vital to the vast mass of plankton that fills the ocean's upper layers and provides the basis for all other life in the open ocean. Now fisheries, driven from the shallows by overfishing, are beginning to chase prey deep in the open ocean, and the slow-growing, much scarcer deep-water fish, such as orange roughies, are now becoming as endangered as their cousins in the shallows.


This is an edited extract from Atlas of Oceans by John Farndon, available from Australian Geographic.



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