Scottish fishing boats are under fire for trawling seas far from home for catches of tuna, shark, swordfish, mackerel and sardines.

The Sunday Herald can reveal that at least five vessels registered in Scotland have been licensed to fish in the Indian Ocean and off the northwest African coast.

Along with boats from elsewhere in Europe, they are facing criticisms that they are plundering foreign seas, damaging local fishing industries and threatening fish stocks.

As fish stocks in European waters have declined, big fishing businesses have increasingly searched further afield for more lucrative and less depleted waters. Scottish fishermen, already in straitened circumstances, are also keen to exploit foreign waters to keep operating.

According to a study for conservation group WWF, one-third of the world's oceans are heavily fished, 10 times more than in the 1950s.

More than 700 vessels from European Union countries are fishing seas outwith the EU, mostly from Spain, France and Portugal. But they also include nine vessels registered in the UK, five of them in Scotland.

"It will surprise many people to learn that Scottish-registered boats, along with others from across Europe, are catching fish as far away as Morocco and the Indian Ocean," said Dr Richard Dixon, director of WWF Scotland.

"Most of the fish stocks in these areas have little or no protection from over-fishing. We are demanding changes to the rules governing fishing to ensure vessels that fish abroad follow the same rules and respect the same sustainability principles as those operating within EU waters."

Four Scottish boats have been licensed to fish in the Indian Ocean under EU agreements with Madagascar and Mozambique. The Mar de Bens, the Mar de Creta, the Blue Gate and the Tahume can fish for tuna, sharks and swordfish.

A deal between Morocco and the EU to allow European boats the right to fish off Africa has caused a fierce political row. There was concern that EU boats could not be prevented from overfishing in foreign waters.

The £30 million-a-year agreement with Morocco was suspended last month after it was voted down by the European Parliament. But the European Commission is trying to renegotiate the deal.

Scottish boat The Krossfjord, registered at Banff, fished for mackerel and sardines off the western Sahara before the deal's suspension. The area was annexed by Morocco in 1976, and the Sahrawi people have been fighting for independence. The EU fishing deal "intensifies the Moroccan repression of the Sahrawi people," claims Aminatua Haidar, a Sahrawi human rights activist., a group that campaigns against secret subsidies, is preparing a report on "under-the-radar" EU fishing agreements. "We're concerned that there are few safeguards to ensure European vessels do not further plunder unsustainable fish stocks," said the group's Jack Thurston.

But fishing off northwest Africa was defended by Ian Gatt, chief executive of the Scottish Pelagic Fishermen's Association. He said the Krossfjord had invested in a factory in Dhakla, Western Sahara, which employs 300 people.

He added: "If the EU stops fishing there, its place will be taken by countries with no sustainability scruples like China and Russia."

The Scottish Government says EU agreements help countries develop sustainable fishing industries.