Officials describe the landing of illegal catches worth more than £37m as a 'crime against the marine environment'

Trawlermen from Shetland have admitted illegally catching more than £37m worth of mackerel and herring during a long-running plot to sidestep the strict quotas introduced to protect against overfishing.

Six trawler skippers today pleaded guilty at the high court in Edinburgh to falsifying records to avoid declaring £15.25m worth of catches between 2002 and 2005. Fourteen men have now pleaded guilty to illegally landing herring and mackerel in Lerwick over that period, a once widespread practice known as "black landings", after a long and complex investigation by fisheries officials, prosecutors and police. In what is believed to be the largest case of its kind in the UK, the skippers admitted making a total of 524 undeclared landings worth £37,212,271 between 2002 and 2005, during three separate court hearings this year.

The skippers, who are expected to be sentenced next year, face substantial confiscation orders and unlimited fines and prosecutors are to be allowed to seize money and assets. A prominent fish processing company in Lerwick, Shetland Catch Ltd, has also pleaded guilty to helping all 14 men make the undeclared landings. Prosecutors said they expected further, related cases to come to court.

Scott Pattison, director of operations at the Crown Office, said: "The ramifications of overfishing on such a scale are extremely serious, due to the potentially devastating impact for the marine environment and the fishing industry itself."

The skippers were accused of "knowingly or recklessly" logging false information about the size of their catches, despite having strictly controlled quotas. Prosecutors said in all these cases "there was a false declaration of the quantity of fish landed. This was the principal method of deception used by the skipper accused throughout the relevant period."

Cephas Ralph, the head of compliance at Marine Scotland, the government fisheries agency, said these offences undermined efforts to make sure fishing was sustainable. "Illegal fishing is a crime committed against the marine environment and the many honest fishermen who abide by the regulations and fish responsibly," he said.

The "black landings" practice, once rife at many Scottish ports, has now largely died out after tighter controls at ports were introduced and skippers signed up to voluntary conservation deals to protect fish stocks.

As a result, Scotland's mackerel and herring fisheries, now the country's two most valuable fisheries, together valued at £198m last year, have recently won coveted sustainability labels from the Marine Stewardship Council.

The six skippers who pleaded guilty today were: Laurence Anderson Irvine, 64; Gary Williamson, 51; William Andrew Williamson; 63; George Andrew Henry, 59; John William Stewart, 55; and Colin Andrew Leask, 37.