The world's fisheries are in a far worse state than anyone thought. Great predatory fish such as shark, marlin, swordfish and tuna that once filled the seas are much scarcer than they once were, according to a new assessment. And worse, stocks that appear to be flourishing may already have been stripped bare without anyone noticing.

Many of the world's important fisheries have already collapsed. The most notorious recent example is the cod fishery in the Atlantic off Newfoundland. Now a decade-long trawl through the archives by marine ecologists has revealed just how fast commercial fishing fleets clear the sea of large fish.

Populations of these species plummet as soon as big fishing boats arrive, the researchers found (Nature, vol 423, p 280). They fall by about 80 per cent within the first 10 or 15 years, but eventually stabilise at around 10 per cent of the original numbers. In other words, the world's oceans once held 10 times as many large, predatory fish as they do today.