PORTO ERCOLE, Italy – Stone fortresses and watchtowers that centuries ago stood guard against marauding pirates loom above pristine waters threatened by a modern peril: fuel trapped within the capsized Costa Concordia luxury liner.

A half-million gallons (2,400 tons) of heavy fuel oil is in danger of leaking out and polluting some of the Mediterranean’s most unspoiled sea, where dolphins chase playfully after sailboats and fishermen’s catches are so prized that wholesalers come from across Italy to scoop up cod, lobster, scampi, swordfish and other delicacies.

“Even the Caribbean has nothing on us,” said Francesco Arpino, a scuba instructor in the chic port of Porto Ercole, noting how the sleek granite sea bottom helps keep visibility crystal clear even 135 feet (40 meters) down.

Divers in these transparent waters marvel at an underwater world of sea horses and red coral, while on the surface sperm whales cut through the sea.

But worry is clouding this paradise, which includes a stretch of Tuscan coastline that has been the holiday haunt of soccer and screen stars, politicians and European royals.

Rough seas hindering divers’ search for bodies in the Concordia’s submerged section have also delayed the start of a pumping operation expected to last weeks to remove the fuel from the ship. Floating barriers aimed at containing any spillage now surround the vessel.

According to the Dutch salvage firm Smit, which has been contracted to remove the fuel, there are about a half million gallons (2,400 tons) of heavy fuel oil on board, as well as some 200 tons of diesel oil and smaller amounts of lubricants and other environmentally hazardous materials.

The ship lies dangerously close to a drop-off point on the sea bottom. Should strong waves nudge the vessel from its precarious perch, it could plunge some 90 feet (30 meters), further complicating the pumping operation and possibly rupturing fuel tanks. Italy’s environment minister has warned that if the tanks break, the thick black fuel would block sunlight vital for marine life in the seabed.

A week after the Concordia struck a reef off the island of Giglio, flipping on its side, its crippled 114,000-ton hull rests on seabed rich with an underwater prairie of sea grass vital to the ecosystem. Environmentalists warn the sheer weight of the wreckage has likely already damaged a variety of marine life, including endangered sea sponges, and crustaceans and mollusks, even before a drop of fuel leaks.

“The longer it stays there, the longer it impedes light from reaching the vegetation,” said Francesco Cinelli, an ecology professor at the University of Pisa in Tuscany.

The seabed is a flourishing home to Poseidon sea grass native to the Mediterranean, Cinelli told The Associated Press in a telephone interview.

“Sea grass … is to the sea what forests are to terra firma,” Cinelli said. They produce oxygen and serve as a refuge for organisms to reproduce or hide from predators.

The Tuscan archipelago’s seven islands are at the heart of Europe’s largest marine park, extending over some 150,000 acres (60,000 hectares) of sea.

They include the islands of Elba, where Napoleon lived in exile, and Montecristo, a setting for Alexandre Dumas’ novel “The Count of Monte Cristo,” where rare Mediterranean monk seals have been spotted near the coast.

Montecristo has a two-year waiting list of people hoping to be among the 1,000 people annually escorted ashore by forest rangers to admire the uninhabited island. Navigation, bathing and fishing are strictly prohibited up to a half mile (one kilometer) from Montecristo’s rocky, cove-dotted coast. A monastery established on the island in the 7th century was abandoned 900 years later after repeated pirate raids.

Come spring, Porto Ercole’s slips will be full, with yachts dropping anchor just outside the port. A steep hill provides a panoramic view of a sprawling seaside villa, once a holiday retreat of Dutch royals, and of the crescent-shaped island of Giannutri, with its ancient Roman ruins.

Alberto Teodori, who said he has been hired as a skipper for the yachts of Rome’s VIPs for 30 years, noted that the area thrives on tourism in the spring and summer and survives on fishing in the offseason.

If the Concordia’s fuel should pollute the sea, “Giglio will be dead for 10, 15 years,” Teodori fretted, as workers nearby shellacked the hull of an aging fishing boat.

The international ocean-advocacy group, Oceana, describes the national marine park as an “ecological diamond,” favored by divers for its great variety of species.

“If the pollution gets into the water, we are ruined,” said Raffaella Manno, who with her husband runs a portside counter selling fresh fish in Porto Santo Stefano, a nearby town where ferries and hydrofoils depart for Giglio.

She said fish from the archipelago’s waters are prized throughout Italy for their quality and variety.

“The water is clean and the reefs are rich” for fish to feed, she said, as trucks carrying oil-removal equipment waited to board ferries to Giglio. “The priciest markets in Italy come here to buy, from Milan, Turin, even Naples.”

Concordia’s captain, initially jailed and then placed under house arrest in his hometown near Naples, is suspected of having deliberately deviated from the ship’s route, to hug Giglio’s reef-studded coastline in order to perform a kind of “salute” to amuse passengers and islanders.

The maneuver is apparently a common practice by cruise ships, environmentalists lament.

“These salutes are an established practice by the big cruise ships,” said Francesco Emilio Borrelli, a Green party official from Naples. He said that the Greens have received reports of numerous such sightings by ships sailing by the Naples area islands of Capri, Ischia and Procida.

Even before the Concordia tragedy, environmentalists had railed against what they brand “sea monsters,” – massive cruise liners releasing huge amounts of greenhouse gases – sailing perilously close to the coast to thrill the passengers aboard.

“These virtual cities put at risk the richness of biodiversity, which we must never forget is at the foundation of our very survival on Earth,” said Marevivo, an Italian environmental group.

• Frances D’emilio, Associated Press