Maarten Den Heijer, 30, from Oban, Argyll, and Robert MacDonald, 45, from Appin, Argyll, died of oxygen starvation trying to rescue a colleague from a chamber in a barge at a salmon farm.
Scottish Sea Farms accepted it failed to assess the risks associated with working in confined spaces, including depleted oxygen, and a sheriff set a fine of £600,000. However, the Court of Criminal Appeal in Edinburgh ruled that was excessive, even for a company with annual gross profit of more than £11 million and cut the fine to £333,335.
The accident happened in May 2009 at a sea farm at Loch Creran, north of Oban.
The barge stored fish feed, kept on deck in containers with 11 separate chambers below deck.
A problem with the cabling of a hydraulic crane led to a sealed hatch of one chamer being opened. It was given 20 minutes to vent, thenCampbell Files, 42, another Sea Farms employee, and Arthur Raikes, of Cumbernauld engineering firm Logan Inglis, went down. As soon as Mr Files reached the bottom, about 10ft below deck, he lapsed in and out of consciousness. Mr Raikes climbed out and raised the alarm.
Mr MacDonald reached his colleague and sat him up, and gave a thumbs up sign to people on deck, but he then sat down and his head slumped.
Mr Den Heijer went in but he, too, was overcome in seconds.
The emergency services were called and firefighters in breathing apparatus lifted the three men out. Only Mr Files survived.