Workers at the Skretting Ireland plant were told of the company's decision at a meeting this morning.
Skretting's Managing Director Anthony Murphy said it was forced to close the plant because continued production in Westport had become uneconomic.
He said operating costs in Westport were almost double that of any of the company's other plants.
The production of fish feed for the Irish market will in future be done at its plants in Invergordon in Scotland and Longridge in England.
SIPTU organisers have been meeting staff today and said they were shocked by the decision.
Michael Kilcoyne said the closure would be a serious blow to the locality, which already has a seriously high unemployment rate.
