(Releads with government confirmation, adds captain quotes)

By Sonya Dowsett and Emma Pinedo

MADRID, Oct 25 (Reuters) - A Spanish fishing boat found seven bodies and one survivor in an open-topped wooden boat adrift off Mauritania, with dozens more passengers thought to be missing, Spain said on Thursday.

According to the survivor, the boat began its journey with around 50 people on board and had been adrift for around 20 days, the Development Ministry said in a statement. The vessel was half-sunk when found, the captain of the fishing boat said.

"There was a foul stench and I think those people had been dead for around six or seven days, maybe more," the captain told national radio from the fishing boat, his voice trembling with emotion. He said it was unlikely others had survived.

Thousands of illegal immigrants in rickety wooden boats attempt the journey from Africa to Spain's Canary Islands every year. Many die trying to reach Europe in search of work.

The captain said the survivor told him the boat had run out of petrol far from the coast after leaving the Mauritanian fishing port of Nouadhibou. He believed the other passengers had been swept off board.

The survivor, in a very weak state, and the seven corpses were transferred from the fishing boat "Tiburon Tercero" to a Spanish rescue boat at 1230 GMT on Thursday and will be taken to the Canary Islands, the Development Ministry said.

The Spanish are conducting the rescue operation, even though the vessel was found in waters assigned to Senegalese rescue, the Ministry said.

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