FISH farms off the north-west of Scotland could be the latest victims of red tide algal blooms which have recently killed more than half a million salmon on fish farms in Shetland.

The blooms, which suffocate the fish by removing oxygen from the water, have already been detected in Orkney and are now beginning to show up off the north-west coastline, where a high proportion of Scotland's fish farming is located.

They are being detected on satellite images acquired from Nasa by the Dundee satellite receiving station of the Natural Environment Research Council and processed at Plymouth Marine Laboratory.

The sheer extent of the red blooms makes it highly unlikely the salmon farms, which have been accused by some environmentalists of causing or exacerbating the blooms, are themselves implicated.

Dr Ron Stagg, deputy director of the Fisheries Research Service marine laboratory in Aberdeen said: ''Satellite imaging shows the Shetland blooms are associated with the continental shelf edge. The most logical explanation is an upwelling of nutrient-rich water coinciding with a period of calm weather which has allowed the light penetration necessary to stimulate the blooms.''

He acknowledged there was much debate about the role of fish-farm waste nutrients but pointed out that, in terms of the total nutrient flow in the ocean, this was a very small addition. The presence of the algae is likely to have another, knock-on effect, giving shellfish fishermen and scallop dredgers an equally serious headache.

Scientists say when the algae exhaust the available nutrients, they die and rain down on the seabed. As they decay, the chemicals released interact to produce toxins which in turn are taken up by shellfish, eventually leading to the concentrations which can cause illness in humans.

Mark Jones, a partner in the Inverness-based Fish Vet Group, said they had detected high levels of the algae gymnodinium and alexandrium on fish-farm sites in Orkney and the north-west Highlands from samples sent for analysis by fish farmers. However, there had been no reports yet of mass deaths of the scale seen in Shetland.

Mr Jones said: ''Mature fish have been affected and we are seeing small numbers dying while they are being handled.

''One of the problems in north-west Scotland is that the fish farm sites are relatively shallow. In Canada, where sites are often in deeper water, cages can be lowered beneath the blooms.''

Considerable differences of opinion continue over why the blooms occur.

The industry itself claims fish farmers would never pollute the very environment on whose absolute cleanliness they are so dependent.

A spokeswoman at Scottish Quality Salmon, the major industry body with many west coast members, pointed out yesterday that current thinking appeared to favour the idea the blooms were an oceanic phenomenon which occurred inshore in the right conditions.

She supported Mr Jones's view that warm water spinning off from the North Atlantic drift, the continuation of the Gulf Stream, was possibly creating the conditions for the blooms to thrive.

Rejecting any link between fish-farm nutrients and blooms, she pointed out they were occurring regularly in areas where there was no salmon farming.

Alistair Davidson, marine consultant for WWF Scotland, said that of 60 sea areas closed to shellfish fishing last year 57 were near salmon farms.

''We know these blooms are occurring more frequently and over larger areas,'' he said. ''We also know that changes in nutrient levels promote them.

''We are aware that they occur in many different ways and for different reasons, but if you examine what has changed the status of the water in recent history around the Scottish coasts, you have to look at the role of aquaculture.

''Until there is a proper inquiry into any possible links, there should be a moratorium on further expansion of the Scottish salmon farming industry.''