Imagine the international outrage if the now-famous Somali pirates - in a fit of pique over an attempted hostage rescue, perhaps - decided to blow up their latest haul, the Saudi supertanker

Sirius Star, and burnt the $100 million of crude oil she is carrying. Now imagine that they managed to hijack 500 supertankers of the same size, and burnt them all. Hard to believe, but this is the damage that is inflicted every year by the world's fishing fleets.

Every year $50 billion goes up in smoke because of the world's inability to control the sea. According to a recent World Bank report condemning the staggering inefficiency of heavily subsidised fishing fleets, the money is squandered as fishermen vie with each other to chase the ever-diminishing stocks of fish, building bigger and bigger boats in the process.

If world fish stocks were rebuilt, today's catch could be achieved with about half the current global fishing effort. But a measured approach is impossible when no one owns the sea. It is a classic tragedy of the commons: if one country leaves the fish alone, another will reap the benefits. The result is a rush that will only stop when the fish have disappeared.

The legendary freedom of the seas carries much of the blame. Up to the Second World War control extended only as far as a cannon could fire - three miles. Between 1958 and 1960 it was expanded to 12 miles and in 1982 the International Law of the Sea conference was adopted and extended national jurisdictions to 200 miles out, or to the edge of the continental shelf. More than half of the ocean remains outside these Exclusive Economic Zones (EEZs).

Just as the law finds it hard to get a grip on the ocean, human perceptions have always been skewed by our inability to visualise its vast dimensions. From a beach we can only see a few miles out, and from the surface we can only see a few feet down. Like children staring at the stars, we're filled with a sense of awe but blind to the true scale of what's before us.

For most of our history the oceans seemed infinite in size and productivity. But because of the advent of industrial fishing the seas now produce an estimated 5 per cent of what they did in their prime. Global marine catches have remained stagnant at around 85 million tonnes a year for more than a decade, but are only made possible by advanced fishing technology and more days spent at sea by bigger ships. These developments mask the collapsing marine environment, a situation aggravated by worsening pollution and habitat loss. The World Bank insists that $50 billion of waste is a conservative estimate, excluding even huge losses from illegal fishing.

Our blindness to what goes on beyond our coast remains: shipping creates more than twice the carbon emissions generated by all the world's aircraft - but isn't even mentioned in the Kyoto accord.

The oceans hold enormous potential for protein, power and buffering the impacts of human activity. With human populations likely to reach 10 billion by 2050, it is essential that the world's biggest biosphere (around 95 per cent of the world's living space) is properly managed. Today one billion of us rely on fish for our main source of protein. An estimated quarter of the world's oil lies beneath the melting Arctic. Untold genetic riches lie in the sea's diversity. And the power of tides and waves has the potential to supply much of the world's energy.

We need to start getting to grips with the scale of the sea, in our minds as well as our courts. More than 95 per cent of the world's fish are caught within EEZs, and these waters certainly can be protected. Fencing off areas of ocean shows dramatic results. Research shows that if more than 30 per cent of a coastline is protected from all forms of fishing, marine life rebounds with astonishing speed. Reserves off George's Bank, established in 1994, are now so fertile that by 2003 three quarters of all haddock caught in US waters were being caught within 5km of them. Defending entire ecosystems in this way brings more than just fish to our dishes: biodiversity provides resilience to the effects of climate change; biomass filters man-made waste.

The area of ocean that lies beyond national control is shrinking as nations extend their claims, but the deep seas will remain a frontier for the foreseeable future: the last International Convention on the Law of the Sea took 27 years of discussion to implement.

Technology can help enforcement in national seas, extending the arm of the law far beyond the cannonball-range and into the farthest corners of the ocean. Vessel management systems - black boxes that track ships via satellite - have proved they can keep fishermen out of protected areas. When the value of what the sea can deliver is taken into account, the cost of defending it shrinks in comparison.

Defending the oceans means changing our relationship with them. We should start in our own back yard. The haphazard freedom of Britain's seas and their communal use have left them in disarray, making development of offshore wind farms as complicated as the designation of a fishing-free area.

With our attention focused now on dramatic events in the most lawless corner of the high seas, we have an opportunity to change this. The draft Marine Bill is expected to get its first reading shortly, and if given the necessary teeth the resulting Marine Management Organisation could bring order to our greatest asset, the ocean.

Frank Pope is ocean correspondent

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