It was too dangerous for the Department of Transport's Dakota aircraft to fly their low-level dispersant-spraying sorties.
Heavy black fuel oil which powered the main engines, until now largely confined to the cove where the ship lies wrecked, has broken free and is spreading to mingle with the much more widely dispersed crude.
The ship's forward half is almost entirely covered in water, which may indicate that oil is leaking out of the forward and mid- ship tanks, to be replaced with seawater. However, a substantial proportion of the ship's 85,000 tonnes of crude oil is thought to be still on board.
A thin surface sheen of light oil has spread more than 16 miles north along the western coast, while crude oil mixed with water, staining it brown, has spread 10 miles north.
The sheen has surrounded cages on several of the islands' salmon farms, and the thicker oil is now threatening them.
Shetland Islands Council has built a short rock dam across the narrowest stretch between two islets, East and West Burra. This is to stop the tidal flow of water and slow the spread of oil.
John MacGregor, Secretary of State for Transport, visited Mainland yesterday amid criticism that he did not arrive sooner. 'We will do everything possible; clearly this is an appalling tragedy,' he said.
The Prime Minister was being kept informed, he added, but could give no information on whether John Major would visit the islands.
Mr MacGregor said a progress report from the Marine Accident Investigations Branch inquiry into the disaster would be ready for him within weeks but it would not be made public. Only the final version, which will take many months to prepare, will be published.
The captain and crew of the Braer are remaining in Shetland, closeted in a remote hotel, to allow the Department of Transport's accident investigators to question them and corroborate statements. Their chief inspector, Captain Peter Marriott, said: 'We will keep them here as long as we need to.'
Two compensation funds, the International Oil Pollution Compensation Fund and the protection and indemnity club to which the Braer's owner, B & H Ship Management, belongs, are opening a joint office in Lerwick, the Shetland capital, to receive compensation claims from islanders.
Dr Sian Pullen, the World Wide Fund for Nature's marine conservation officer, said: 'The oil the Braer was carrying contains high levels of aromatic compounds which are the most toxic compounds of crude oil. It is inevitable that this stuff will enter the marine food chains . . . the effects will be seen and felt for at least five years.'
Oil from the Braer has aready killed 'unusually large' numbers of fish, with serious consequences for seabirds now approaching their breeding season, Scottish Natural Heritage, the Government agency, said last night, writes David Nicholson-Lord.
Fish killed include sand eels, ling and wrasse. The area east of Sumburgh Head is an important spawning area for sand eels, which have a vital role in the food chain. Birds that feed on the sand eels and whose breeding season could be affected include puffins, kittiwakes and terns.
The agency said that by dusk on Thursday four oiled otters had been seen but not captured and one slightly oiled seal, still alive, had been found. Many marine algae and insects were likely to be killed, harming the 'rich biological diversity' of the Shetlands. Although dispersants have decreased in toxicity, it said they were still a danger to many organisms.
Magnus Magnusson, chairman of Scottish Natural Heritage, who is travelling to the Shetland Islands this weekend, said the oil spill was a 'tragedy for man and nature'.