And if, as is often the case, these smaller critters are the main food for fish, then the overall effect is to nurture more marine life. Imants Priede, at the University of Aberdeen, UK, says there is growing evidence that -- while far from universal -- the phenomenon is real. "On continental shelves such as the southern North Sea, where shifting sands and gravels are stirred up by strong tidal currents, even trawling more than once a year may be beneficial for some fish stocks," he says.

Trawling clearly does remove fish, such as large crustaceans and shellfish, says Daniel van Denderen of the Wageningen Institute for Marine Resources and Ecosystem Studies in the Netherlands. But this is a lucky break for smaller species lurking in the sandy seabed, such as worms. They survive and, with fewer rivals, flourish.

Trawlers attach large nets to a metal beam dragged across the seabed to catch bottom-dwelling fish. Greenpeace likens it to "driving a huge bulldozer through a forest", leaving a flat, featureless desert on the seabed.

Trawling the seabed for fish is an environmental disaster; it wrecks ecosystems, destroys fish stocks and leaves behind a marine desert. But there is growing evidence that the effect is sometimes very different, with trawling increasing fish stocks from the North Sea to the California coast. A new modelling study may for the first time have demonstrated why.