THE great seabird colonies of Shetland are in danger of being wiped out by a collapse in sandeel populations in the northern North Sea.

Ornithologists describe the sight of starving kittiwake chicks on the breeding cliffs pursuing any adult bird that comes within range, screaming for food that is never forthcoming.

Arctic terns, puffins, shags, and even hardy guillemots, which can fly many miles and dive 100 metres in pursuit of sandeels, are struggling to survive. Many arctic terns and kittiwakes are simply choosing not to lay eggs because they are so hungry.

For once, scientists say, fishermen are not to blame. The Danish sandeel fishery, which has a much-criticised total allowable catch of one million tons, has succeeded in catching only a third of its quota and the tiny, heavily-regulated Shetland sandeel fishery gave up months ago when the sandeels failed to arrive in the spring.

The ubiquitous, silver sandeels, which swim in gigantic shoals of millions of individuals and form a vital link in the ocean food chain for dozens of species from cod, haddock and Atlantic salmon to seabirds, have simply not appeared. Over a decade ago the shortage of sandeels prompted the government to impose restrictions on the sandeel fishery, but this appears to have made little difference in the long term.

Martin Heubeck, Aberdeen University's Shetland ornithologist, said the food problem is almost certainly due to changes in sea temperatures, ocean currents and availability of

plankton.

''There is no real understanding of what the problem is and there is no co-ordinated programme of research into it,'' he added, ''but seabird collapses are an indicator that all is not well with the seas.''

Euan Dunn, fishery policy officer at the Royal Society for the Protection of Birds, said the dearth of sandeels is general across the North Sea, including vital seabird areas off the Firth of Forth. He pointed out that this is the third year in succession that Danish industrial fishing fleets, which take sandeels to feed its pig industry and for food for salmon farms, have failed to get near their quotas, which ''emphasises the absurdity of the EC giving them a huge total allowable catch of a million tons''.

Many of the Shetland seabirds have, he said, been in relentless decline for three years and now the adults themselves are not surviving. The plight of the guillemots is causing great concern.

Pete Ellis, Shetland RSPB officer, said: ''For guillemots to be struggling things must be really bad. They can fly for tens of kilometres and dive 100 metres. They should be able to get fish in almost any circumstances, but this year they can't.'' Mr Ellis added: ''In the absence of any other ideas as to why this is happening, we look to climate change. We do not think it is the fishery because that is pretty highly controlled and even the fishermen are catching very few sandeels.''

Mr Heubeck believes that in the 25 years of monitoring seabirds 2003 is probably the poorest season yet. The past three years had been very poor, but this year was disastrous, he added.

''In the past quite a high proportion of kittiwakes laid eggs but the chicks died. This year many birds didn't even bother to breed.

''Only half of the kittiwakes that started to build nests went on to lay, and hardly any of them hatched chicks.''