Although the ship's cargo is largely harmless to fish because it floats on the surface, the chemical dispersants to be employed against the oil slick produce an oily emulsion that can kill fish as it sinks to the sea bed.
A senior government scientist said that although the chemicals used to disperse oil were less toxic than they were in the 1960s, they were still dangerous to fish. 'Fish as a whole don't suffer from oil pollution, except those close to the shoreline. The problem is excessive use of chemical dispersants. The combination of dispersant and oil is more toxic than oil alone,' he said.
Jim Moncrieff, chief executive of the Shetlands Salmon Farmers Association, said: 'If dispersants are used on oil, that would be far worse to salmon farming than the oil itself.'
The Shetlands account for more than a quarter of the UK's production of farmed salmon, and last year the isles produced 10,000 tons of salmon valued at more than pounds 35m, Mr Moncrieff said. 'And those prices were depressed,' he added.
The nearest salmon farm is less than 13 miles (21km) from the leaking tanker, and fish farmers are 'greatly concerned' about the impact a clean-up of a large spill would have on their businesses.
'It's potentially quite devastating, although it all depends on where the oil goes,' Mr Moncrieff said. Because the tanker ran aground at the southernmost tip of the Shetlands, fish farmers fear that oil could end up polluting both east and west coasts at tides and prevailing winds could carry any slick further afield.
Putting booms around the cages where salmon are kept could keep drifting oil out, but if it has been dispersed with chemicals, the resulting globules will sink and can be washed into the fish farms.
Commercial fishing could also be affected by the slick, as oily emulsions produced after a clean-up can wipe out very young fish feeding close to the shore. Fish larvae - which hatch in the early summer - are particularly prone to the effects of oil dispersants. Herring, sand eels, cod, and haddock are all at risk. The spill could add to the considerable problems already faced by fishermen working in areas of depleted stocks.Reuse content