Hundreds of years ago, Brazil's southern Atlantic waters were sailed by pirates in search of plunder. Now these remote depths, dubbed the "Blue Amazon", hold richer treasures for scientists.

Nowadays it is the Brazilian navy that patrols the seas off the country's eastern coast to protect its oil platforms and mineral reserves in a region teeming with turtles and whales.

And hundreds of miles over the water on the small island of Trindade, naturalists are working to recover a paradise that was ruined by British settlers in the 18th century.

The Brazilian navy patrol ship APA sets sail from Rio de Janeiro for Trindade, which lies 1,167 kilometers from Vitoria in southeastern Brazil.

Dozens of fixed and floating oil platforms line the horizon.

Brazil drills 90 percent of its oil from beneath the sea bed in this ocean region of 3.6 million square kilometers.

But the waters also conceal other seabed resources: gold, phosphorous, manganese and lime deposits which have numerous uses in construction and agriculture.

Among the organisms in the ocean are "calcareous algae", useful to farmers, says Cesar de Melo, an agronomist at Lavras Federal University.

He says that Brazil imports 80 percent of its fertilizer, while one of nature's best fertilizers is teeming in the nearby ocean. De Melo has spent 20 years studying its benefits.

Using the algae as fertilizer, he said, "adds minerals to the land and makes it sustainable, increasing crop production and quality and resistance to pests and plant diseases."

- Back from extinction -

On the way back from Trindade the ship will patrol the oil-drilling areas, expelling any unauthorized vessels that stray into the security zone.

But first it must take supplies and personnel to Trindade, home to a few scientists and about 30 navy personnel.

When the British astronomer Edmond Halley stepped off his boat here in 1700 with a herd of sheep and goats, he set Trindade on the path to devastation.

The animals bred uncontrollably, eating away the island's vegetation and the eggs of the island's green turtles and disrupting its water courses.

Brazil won control of the island in the late 19th century. A strategic spot in the mid-Atlantic, it was the scene of a sea battle between British and German ships at the outbreak of World War I.

In the 1990s, Brazil finally decided to cull hundreds of rampant goats, sheep and pigs to save the island's native species.

The last goat was shot down in 2005. Now little by little, scientists say, the rocky island is regaining its natural diversity.

No tourists come to the island and inhabitants are banned from bathing in the sea on most of the beaches or even walking on them alone because of fierce tidal waves that crash onto the shores.

"On my first mission here in 1994, I found the island devastated," said Ruy Valka Alves, a botanist from Brazil's National Museum, who has visited Trindade about 20 times.

"Today I believe the vegetation is clearly regenerating. We have tested it through field work and satellite images," he added.

"The water has risen in some of the streams. If we let nature run its course, we are having notable results, even without intervening further."

The island's shores abound in coral, fish, turtles and an endangered species of crab.

Various species of seabird are coming to breed again on Trindade as the trees grow back.

"The island is enormously rich. There are more species to be discovered," said Anabele Stefania Gomes, a botanist who is studying plants on the island.

"There are species that had been extinct here and are now managing to return."