The Shetland Catch factory in Lerwick was one of the destinations for illegal landings of herring and mackerel worth more than £6.5 million made by four north-east fishing skippers.
The four, all shareholders in the Fraserburgh-based family business Klondyke Fishing Company, breached European regulations to land the black fish between January 2002 and February 2005.
They were eventually trapped by Operation Trawler which has already seen a string of successful prosecutions in Scotland and millions of pounds levied in fines and confiscations.
Thirteen Shetland fishermen were fined a total of £470,000 in February for their own black fish landings.
The processing plant, which has admitted helping skippers defy fishing quotas to land illegal catches worth an estimated £47.5 million, will find out next month what penalty it will face.
The four boats in the latest successful prosecution were operated by men from Fraserburgh, nearby St Coombs and Aberdeen. They were Andrew Tait, 50, William Tait, 50, Robert Tait, 45, and Peter Tait, 44.
After today’s hearing at the High Court in Edinburgh, Lindsay Miller, head of the serious and organised crime division at the Crown Office, said: “The legislation is there to protect the marine environment for the good of all and to safeguard the future of the fishing industry.
“Those who disregard it for their own financial gain will be brought to justice and made to pay for their crimes.”
In court the four skippers were served with papers which launch attempts by prosecutors to claw back their illegal profits.
Until the question of confiscation has been settled they cannot be fined – and the rules they have broken do not carry a possible jail sentence.