It is the largest oil spill in decades, but the disaster has unfolded outside the glare of international attention that big spills have previously attracted.

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ZHOUSHAN, China — A fiery collision that sank an Iranian tanker in the East China Sea a month ago has resulted in an environmental threat that experts say is unlike any before: an almost invisible type of petroleum has begun to contaminate some of the most important fishing grounds in Asia, from China to Japan and beyond.

It is the largest oil spill in decades, but the disaster has unfolded outside the glare of international attention that big spills have previously attracted. That is because of its remote location on the high seas and also the type of petroleum involved: condensate, a toxic, liquid byproduct of natural-gas production.

Unlike the crude oil in better-known disasters like the Exxon Valdez and the Deepwater Horizon, condensate does not clump into black globules that can be easily spotted or produce heart-wrenching images of animals mired in muck. There’s no visible slick that can be pumped out. Experts say the only real solution is to let it evaporate or dissolve. Absorbed into the water, it will remain toxic for a time, though it will also disperse more quickly into the ocean than crude oil.

Experts say there has never been so large a spill of condensate; up to 111,000 metric tons has poured into the ocean. It has almost certainly already invaded an ecosystem that includes some of the world’s most bountiful fisheries off Zhoushan, the archipelago that rises where the Yangtze River flows into the East China Sea.

The area produced 5 million tons of seafood of up to four dozen species for China alone last year, according to Greenpeace, including crab, squid, yellow croaker, mackerel and a local favorite, hairtail. If projections are correct, the toxins could soon make their way into equally abundant Japanese fisheries.

Exposure to condensate is extremely unhealthy to humans and potentially fatal. The effects of eating fish contaminated with it remain essentially untested, but experts strongly advise against doing so.

“This is an oil spill of a type we haven’t seen before,” said Paul Johnston, a scientist at Greenpeace Research Laboratories at the University of Exeter in England. “Working out the impact is actually a huge task — probably next to impossible.”

For China, the disaster has become a test of its ambitions as a global and regional steward of the seas, especially at a time when it is reinforcing its territorial claims, including disputed territories with Japan in these waters. Given its proximity, China has taken the lead in investigating the disaster and monitoring the spill, but it has faced some criticism for what some see as a slow and inadequate response thus far.

Officials in Beijing announced Feb. 1 that samples of fish taken within 4 to 5 nautical miles of the sunken ship contained traces of petroleum hydrocarbons, suggesting possible condensate contamination; they pledged to expand the range of testing to 90 miles, and closely monitor fish coming into markets.

The threat of contamination has raised anxiety in the ports that cling to the rugged coastlines of Zhoushan’s islands, though such fears are usually expressed with quiet resignation lest one offend the government.

“The quality will go down because of the oil in the water,” Hai Tao, a fish wholesaler at the International Aquatic Product City in Putuo, a district on Zhoushan’s biggest island, said as he watched a ship unload hundreds of crates of mantis shrimp, a delicacy headed to restaurants across China.

The spill began on the evening of Jan. 6, when the Sanchi, a Panamanian-flagged, Iranian-owned tanker, collided with a cargo ship in waters roughly 160 nautical miles east of Shanghai. The Sanchi exploded and burned for more than a week before sinking. All 32 crew members are presumed dead.

Katya Popova, a senior research scientist at the National Oceanography Center in England, said there had not been a sufficiently coordinated international operation, and that was worsening the scale of the disaster.

The lack of visible devastation has almost certainly dampened public reaction that might have galvanized a more vigorous response.

In Beijing, officials have been eager to demonstrate that the government was doing everything possible first to respond to the disaster and then to protect the health of its economically and politically sensitive fishing industry, which employs 14 million people.

They have issued regular statements and held briefings, showing video of efforts to clean up the condensate and to monitor the sunken wreck, which was located at a depth of 115 meters, or about 377 feet. It is believed to still be leaking condensate and other fuels.

Han Xu, deputy director of the fisheries administration bureau of the Ministry of Agriculture, told reporters at a news conference in Beijing late last month that the accident had “a certain impact on the density of fishery resources” in the area, but that the government did not yet know the extent of the threat.

“At present, the investigation and monitoring are still ongoing and we are awaiting results of investigations into pollution and successive fishery resource investigations,” he said.

In the meantime, authorities have ordered a ban on fishing in the areas affected.

The size of the area affected by the disaster has expanded and contracted. At one point in January, there were three different spills spotted on the surface, covering an area that measured more than 128 square miles. Complicating the calculations is uncertainty about the amount of condensate that ended up in the water.

China’s Ministry of Transportation initially played down the possibility of a spill, then said 136,000 metric tons had been lost. Later, it revised the figure downward to 111,000 tons — still enough to make it the worst tanker spill at sea since 1991.

Some of the condensate may have burned off in the fires, sparing the sea, but contaminating the air. Officials said they were testing air samples in the provinces around Shanghai.

The spill is already drifting east toward Japan, but winds and currents can be unpredictable. The contamination could even reach waters as far off as Tokyo.

The Japanese Coast Guard has announced that black globules had been found on at least nine islands along the chain between Okinawa and the main Japanese islands. Those would not be from the condensate, though they could be other oil from the Sanchi wreck.

In any case, the discoveries suggested the condensate may have already reached Japan’s third-most-important fishery, teeming with bonito and yellowfin tuna.

The cause of the disaster remains a mystery. The Sanchi was nearing the end of its voyage to South Korea through one of the most heavily traversed parts of the world’s oceans when it collided with the CF Crystal, a bulk carrier flagged in Hong Kong that was delivering grain to China from the United States.

As the Sanchi erupted into flames, the Crystal managed to make harbor — and is now in one of Zhoushan’s many ports.

At least five Chinese Coast Guard ships, aided by fishing boats, led the rescue efforts and the long struggle to extinguish the blaze that consumed the tanker for eight days before it sank Jan. 14. Japan and South Korea each sent one ship, and the U.S. Navy sent a P-8A Poseidon aircraft from Kadena Air Base on Okinawa.

A Chinese emergency team in flame-resistant suits at one point boarded the burning ship, recovering the bodies of two crewmen and the “black box” data recorder before the intensity of the heat drove them off. One other body was pulled from the sea.

On the Shengsi islands, the part of the Zhoushan archipelago that was closest to the accident, the spill could threaten an industry already strained by polluted runoff from the Yangtze and by overfishing.

At one village nestled in a harbor, three boats unloaded their final catches before the start this week of the Lunar New Year holidays. An astounding variety of fish were sorted dockside into plastic trays. Wu Zhihong, who with her husband owns one of the trawlers, said the catch over the last year had been an improvement over the year before.

Wu expressed hope that the damage from the spill would be limited, absorbed into a wider, forgiving ocean.

“The sea is very big,” she said amid a cacophony of fishmongers who descended on the pier to bargain over the catch.