Duncan (left) and Ramsay leaving court in Glasgow

John Peter Duncan and Jerry Ramsay of the Shetland-based Altaire exceeded their North Sea herring and mackerel quota by landing catches in Denmark.

They hid the extra 7,000 tonnes of black market fish from the authorities by altering the vessel's log books.

The pair appeared before the High Court in Glasgow where the Crown is seeking the return of their profits.

The law only provides for the two men, who live on mainland Shetland and have the reputation of being millionaires, to be fined.

Assets frozen

During their court appearance the men were handed a prosecutor's statement under the Proceeds of Crime Act and are being pursued to pay back the money plus interest.

They have also had their assets frozen pending the outcome of further court proceedings.

The Altaire fishing boat is the UK's largest trawler

Shetland fishermen catch half the allowable UK quota of North Sea herring and mackerel.

Duncan and Ramsay unloaded 17,085 tonnes of mackerel over 20 landings in Denmark, declaring to the Scottish Fisheries Protection Agency that they had landed just 10,031 tonnes.

Herring landings totalled over 1,012 tonnes, but they declared just 469 tonnes.

The figures relate to the period between March 2000 and March 2002.

Paul du Vivier of the Scottish Fisheries Protection Agency said: "Illegal landing of 7,000 tonnes of mackerel and 500 tonnes of herring amounts to theft to the disadvantage of the honest fisherman, who goes about his business lawfully and responsibly."