Talks over the controversial mackerel quota wrangle, which has angered Scottish fishermen, have got under way in London.
Iceland and the Faroes have been warned they face sanctions over their quota agreements.
Fishermen blockaded a Faroese boat trying to land its mackerel catch in Peterhead in August due to the dispute.
The EU, Norway, Iceland and the Faroes are discussing the quotas, and Scottish fishing leaders hope for a resolution.
Iceland and the Faroe Islands unilaterally declared large catch quotas for the valuable fish earlier this year, which it has been claimed could damage its sustainability.
Traditionally Iceland landed very little mackerel, but fishermen there say stocks have been moving further north into their waters and this year Iceland set its own quota at 130,000 tonnes.
The Faroe Islands have also increased their quota from 25,000 to 85,000 tonnes.
Mackerel is the most valuable stock to the Scottish industry and fishermen argue the increases are unfair and unsustainable.
Last year it brought £135m into the economy.
The Scottish Fishermen's Federation (SFF) and the Scottish Pelagic Fishermen's Association (SPFA) said the talks would see them pressing for a fair deal which also protected the stock from over-fishing.
SPFA chief executive Ian Gatt said: "This is a defining moment for our fishermen and the implications from the talks could shape the future direction of our industry.
"At stake is the urgent need to find a resolution to the dispute.
"We previously had a long-term management plan in place for mackerel that has resulted in a healthy stock but this is now in jeopardy due to the irresponsible actions of Iceland and the Faroes."
Scottish Fisheries Secretary Richard Lochhead said: "We need to have a new deal in place for mackerel, which ensures a stock that Scotland and others have managed sustainably and successfully can continue to thrive.
"I am glad that all parties have finally come to the table and are stating their intention to participate in a constructive and positive negotiation."
The EU fisheries commissioner Maria Damanaki warned in Aberdeen earlier this month: "I am going to protect our interests."
Mackerel stocks in the North Atlantic have been carefully managed in recent years.
But Iceland and the Faroes say Europe is stubbornly protecting quotas by refusing meaningful negotiation.
As the fish has gravitated north in recent years, Iceland claims it is merely fishing within its own zones.
Fridrik Arngrimsson, chief executive of the Federation of Icelandic Fishing Vessel Owners, told the BBC they believed their catches were sustainable.
He said the aim was to reach a deal in the next three days but admitted he was not confident.
The Faroese fishing boat Jupiter had to leave port in August after being confronted by Peterhead fishermen angry over the mackerel quotas.
It was prevented from landing its £400,000 catch.