Consumers in Japan, China and South Korea should be wary of buying seafood until governments in the region have monitored and released details about the toxic impact of the Sanchi oil spill, scientists have warned.

The worst oil ship disaster in decades has so far produced two visible plumes covering almost 100 square kilometres on the surface of the East China Sea, but maritime disaster experts say this is just the tip of the iceberg and millions of fish are likely to have been contaminated by carcinogens.

The Iranian tanker was carrying about 136,000 tonnes (a million barrels) of light oil when it collided with a freighter on 6 June. All 32 crew are thought to have died in the blast, which sent billows of smoke into the sky and fire across the nearby ocean. The wreck then reportedly drifted from Chinese waters into the Japanese exclusive economic zone, where it finally sank on Monday.

The extent of the pollution is unclear because light oil does not form a black slick (which is more likely to have come from the heavier oil from the ship’s fuel tanks). It is less visible and more soluble, which means it has a greater impact below the surface.

The State Oceanic Administration said in its statement late on Wednesday it had collected water samples from 19 spill sites, and found water from five sites had petroleum substances at levels exceeding standards.

Edward Overton, an environmental science professor at Louisiana State University downplayed concerns. “A light distillate or oil will readily evaporate and leave minimal residues,” he said. “Much of this oil is burning so will not cause significant environmental damage.”

Other scientists fear dangerous amounts may still be leaking from the sunken wreck – where the oil cannot evaporate or burn – and poisoning fisheries of supply mackerel, croaker, cuttlefish, herring, shrimp and crabs.

China and Japan have been criticised for being slow to assess the environmental impact, prompting speculation that their maritime border dispute may have complicated the response.

Chinese scientists have said they lack monitoring buoys – which have previously been the subject of suspicion between the two sides.

Richard Steiner, a marine scientist and retired University of Alaska professor who helped the United Nations to draw up guidelines for oil spill assessments, urged Beijing and Tokyo to begin an impact assessment two days after the initial collision. Current markers and satellite buoys would have indicated the direction of the toxic plume which could have travelled more than 200 miles with just a one-knot per hour current since 6 January.

He was pleased China is planning to use remotely operated underwater vehicles to inspect the wreckage, but said such actions were tardy.

“I find fault with both governments for not acting immediately,” he said and urged both nations to close fisheries until consumers can be sure that seafood from the area is safe.

Based on simulations, the National Oceanography Centre in Southampton predicts the plume could reach Japan within a month.

Other reports have suggested it could also reach South Korea’s Jeju Island. Although it will be steadily dispersed, oceanographers urged shoppers to be cautious until it is clear the leak has stopped.

“Until there is a clear steer on monitoring, I wouldn’t touch any fish that might have passed through this area because it might be contaminated,” said Simon Boxall of the University of Southampton. “China and Japan need to sit down together, recognise they have a problem and put public safety first.”

Steiner echoed this, saying international cooperation was crucial when dealing with previous spills and would be necessary to minimise future impacts. He said all the countries in the region should put in place protocols on maritime traffic, disaster response and environmental assessments “Japan, China and South Korea need to cooperate to prevent this happening again.”



