The Rail, Maritime and Transport union (RMT) has announced three strike dates, starting next Friday, in a dispute over job cuts and new flexible working arrangements.
Serco, the company that succeeded publicly owned Northlink as the operator of Northern Isles ferry services in July, is shedding 36 jobs and introducing contracts allowing it to vary the size of its crews between the busy summer period and winter.
However, it said compulsory redundancies had been avoided.
The dates were announced as it emerged the staffing changes that triggered the dispute had been approved by Government agency Transport Scotland when it ran a contest to operate the service earlier this year.
RMT members will stage 24-hour strikes on three successive Fridays – December 14, December 21 and December 28 – on Orkney to Scrabster and Shetland to Aberdeen services.
The Aberdeen to Orkney to Shetland route will be affected two days later, on the Sundays of December 16, December 23 and December 30.
Bob Crow, the RMT's general secretary, said: "Serco are fully aware we will not accept attacks on our members' jobs and conditions and what is now an all-out attempt to bulldoze through cuts to core staffing numbers and the casualisation of these skilled shipping grades with ad-hoc agency arrangements and de-facto zero hours contracts."
The RMT said 35 of the 36 jobs being axed were among catering staff, many of whom live in the Northern Isles.
Stuart Garrett, managing director of Serco Northlink, said he was "dismayed and surprised" by the action, given compulsory redundancies had been avoided and seafaring staff were in line for a 4.25% pay rise.
He added: "If the RMT insists on proceeding with this industrial action we will be looking to maintain as many services as possible to minimise disruption to the public, particularly given the difficulties this action could cause to people's travel plans over the festive period."
In a response to a question from Labour's Siobhan McMahon, MSP for Central Scotland, Transport Minister Keith Brown said Serco had planned for the workforce changes when it bid for the contract.
He added: "It is correctly a matter for the operator, as the employer, and not the Scottish Government, to discuss current staffing levels with the staff concerned and with their trade unions."
However, Labour's Shadow Infrastructure Secretary Richard Baker said: "The shedding of experienced workers from this lifeline ferry service raises further concerns over the decision to privatise the Northern Isles ferry services."