Fish farms might seem a sensible alternative to over-fishing the world's oceans but a new report says they have a disastrous impact on both the environment and on stocks of wild fish.

Fish farming, also known as aquaculture, has been widely promoted as a smart way to feed the world's surging population.

But an international team of scientists, writing in the journal Nature, say it is contributing to the global collapse of the world's wild fish stocks.

This is because huge quantities of wild fish, especially anchovies and mackerel, are being scooped up and ground into meal to feed farmed fish.

The authors of the report say fish farming also harms the environment, by converting wetlands into artificial farms, and with the escape of farmed species that can interbreed with wild stocks and pass on weak or specially inbred genes.

This is particularly true, the researchers say, of farmed salmon species, which now account for up to two-fifths of salmon caught in the north Atlantic.

China praised

But the report singles out China for praise, for embracing a centuries-old sustainable technique of keeping different kinds of carp species in the same pond to create a balanced mini-ecosystem.

To make fish farming more sustainable worldwide, the authors recommend that farmed fish should be fed vegetable protein instead of fishmeal.

And they say tighter controls are needed to lessen the impact of farms on natural habitats. Otherwise, they say, fish farming is not the harmless alternative to ocean fishing that its advocates would claim.

Scotland has a large number of fish farms and Kevin Dunion, Director of Friends of the Earth, Scotland, said: "This important piece of work clearly shows that the intensive farming of fish such as salmon and trout is inherently unsustainable.

"It can take up to three tonnes of wild fish to produce a single tonne of farmed fish."