About 36 tonnes of oil is now thought to have leaked into the sea from a sunken chemical tanker which is lying about 30 miles (50km) off Guernsey.
The Ece is lying about 70m down
A decision on whether the Ece should be salvaged is being delayed until further surveys are carried out.
There are concerns about pollution from the 80 tonnes of fuel oil the vessel was carrying.
Experts predict the long-term impact will be small, but Simon Wormsley from the World Wildlife Fund disagrees.
He said: "If it leaks at a even slow rate, depending on the tides, it could end up on land or sensitive areas.
"There's a definite threat from that fuel oil."
Meanwhile, Guernsey's harbourmaster, Captain Peter Gill, says divers still have not reached the wreck.
He said a remote control vehicle was being sent down to examine it.
"They will look at it properly and from that they will be able to establish the likelihood of any further leakage," he said.
The Ece sank on Wednesday near the site of the collision, 56 miles (90km) west of Cap de La Hague.
The vessel had been badly damaged following a collision with a cargo ship off the north west coast of Guernsey early on Tuesday.
French divers had found a 5m (16ft 4in) hole below its waterline, although there was no leak of the phosphoric acid.
French authorities have imposed a one-mile exclusion zone with warning signs pointing out the wreckage to other vessels.
The 22 rescued crew members left Guernsey on Thursday and are on their way back home to the Philippines and Turkey.
The alarm had been raised at about 0220 GMT on Tuesday when the Ece collided with the freighter General Grot-Rowecki about 30 miles north-west of Guernsey.
The Maltese-registered carrier, carrying 26,000 tonnes of phosphorus, was only slightly damaged.
About 500 vessels every day pass through the Channel, one of the busiest shipping lanes in the world.