A specialist aircraft has joined the search for a dozen large fishfarm cages full of salmon that broke free from their moorings in Shetland at Christmas.

The plane conducted a sweep of up to 500 square miles but has found no trace of the 100ft wide cages which were swept out to sea from the Shetland island of Unst.

Shetland Coastguard had appealed for help to tow them to shore as they could present a hazard to navigation around the south coast of Unst.

They had been anchored together in a grid pattern and were due to be harvested.

The Sumburgh-based search and rescue helicopter located them last week about 30 miles east of the island.

A Shetland Coastguard spokesman said: "We had the anti-pollution spotter aircraft up yesterday.

"As an exercise it searched an area between 300 and 500 square miles and didn't see a thing. Local opinion is divided. Some think they have sunk, some they have split up and others that they are still out there but we are looking in the wrong place."

He said the loss had been discovered on Boxing Day after the fishfarm workers had returned having had Christmas Day off.

The cages were full of salmon and industry sources say if the fish had died their weight would sink the cages.

Mark Warrington, managing director of Lakeland Unst Ltd owners of the fishfarm said: "We are working closely with both the Maritime and Coastguard Agency and Marine Scotland and are taking advice from them. We are doing our utmost to recover the cages, but the bad weather is hampering our efforts."