Grieg Seafood Shetland – which has 219 employees – operates 17 active seawater licences, one freshwater facility and one harvesting plant.
However, the company said the Covid-19 pandemic had “severely” hit all of its markets.
It aims to sell its Shetland operation “when the timing is right”.
Grieg Seafood has previously stated it would cease operations at its five farms on Skye after the current harvest was completed.
The company said its decision on the future of the Skye sites had been expedited by high fish mortality rates at three of the farms.
Greig Seafood said “abnormal levels” of jellyfish between late July and early September had been the main cause for the deaths of 627,000 fish.
Andreas Kvame, chief executive officer of Grieg Seafood ASA, said: “We have seen steady improvements in fish health and survival over the last years on our farms in Shetland. I am both thankful for, and impressed by, the efforts of employees.
“At the same time, the Covid-19 pandemic has severely impacted all of our markets. In light of these circumstances, we have to prioritise resources and investments and make some tough decisions.”
He said the company’s Norwegian and Canadian operations had the “largest potential for sustainable growth”.
‘Regret this brings uncertainty’
Mr Kvame added: “While Grieg Seafood ASA expects the process to be completed during 2021, there is no defined timescale for the review.
“The company does not know what the final outcome will be, but it aims to sell its Shetland operations when the timing is right.”
He said it was not known how a potential sale could impact employees.
“We regret that this process brings uncertainty for our employees and also to the local communities in Shetland”, he said. “Whatever the outcome of the review will be, I am confident that our Shetland farms and operations will continue to contribute significantly to Shetland’s economy in the future.”
Operations will continue as normal until the review is completed.