Salmon served at the COP26 climate conference was from a farm given two government environmental enforcement notices over parasites.
Loch Duart Ltd was contracted to supply farmed salmon which was given pride of place on the "sustainable menu" at the summit taking place in Glasgow this week.
The Government has claimed "sustainability" is "at the heart of catering" for the conference.
But Loch Duart's flagship farm has topped a league table for the worst performers for the control of sea lice parasites during 2021.
It's the only farm to have received an enforcement notice from the Scottish Government for failure to control the parasites - and was handed two, in June and September this year.
Sea lice occur naturally, and live on the skin of fish. While they are not harmful to humans, they cause lesions on fish that can make fish unsellable.
The company said sea lice thrive in warmer waters, and added: "We are passionate about caring for our fish, despite the environmental changes creating conditions which contributed to our recent raised lice levels on one of our sites."
Salmon farms have been identified as a major source of sea lice finding their way into wild areas of the Scottish coastline, with the parasites spreading to fish in waters outside the farm.
Marine Scotland announced that weekly reporting of sea lice would become mandatory for fish farms in March this year (2021).
Campaigners argue the industry is “fundamentally unsustainable”, and said serving the product at a climate conference “shows bad judgement.”
But a spokesman for Loch Duart said the firm was "proud to be on the menu at COP26", and that the firm is "passionate about caring for our fish."
They blamed climate change, in part, for rising levels of sea lice at one of their sites.
He added: "Loch Duart employs a strategy of zero tolerance on sea lice and continually works on improving natural approaches to looking after our fish and the environment."
Andrew Graham-Stewart, Director of Salmon and Trout Conservation Scotland said: "For COP26 to serve up farmed salmon, the product of what is, in many people’s opinion, a fundamentally unsustainable industry, shows bad judgement, but to source it from a company with such poor environmental credentials is inexcusable."
The conference's "sustainable menu" listed Loch Duart smoked salmon & fennel with aromatic cress and beetroot dressing for £10 a plate, with a carbon footprint of 0.4kg CO2e.
Loch Duart, which previously called itself “the sustainable salmon company”, told the advertising regulator in 2019 that it had stopped using the word “sustainable” in branding and on promotional materials.
It followed a complaint to the Advertising Standards Agency, which argued the claim was inaccurate.
The Loch Duart spokesman added: "One of our sites experienced elevated levels of lice, triggering the standard Marine Scotland response process, expected in a highly regulated sector.
"All our other sites continued to be successfully managed through our natural approaches to welfare, despite the challenges of rising water temperatures.
"To have raised levels of this naturally occurring parasite is devastating for the team, who go above and beyond in their care for their salmon.
“While self-reporting for sea lice is standard in Scottish salmon farming, Loch Duart is the only Scottish salmon farm to welcome fisheries trust representatives to verify our sea lice auditing, providing completely independent oversight.
“Loch Duart always chooses welfare and quality over profit, even if this means bringing fish out of the water earlier than planned, as we did in this case, to avoid harm to the salmon, or the need to resort to medicinal options."
