An “essential services” timetable will operate until at least 23 January with the Mallaig-Lochboisdale route suspended and reductions on nine other of CalMac’s 26 routes.
CalMac also warned of further changes “at very short notice”.
It follows ScotRail reducing trains from Tuesday in an attempt to provide a more reliable service as staff absences topped 400 for the first time.
CalMac said it could not operate its full timetable because of the number of staff testing positive for Covid, self isolating or waiting for test results.
It said 93 crew and 18 port staff were absent – 9 per cent of crews and 5 per cent of port staff, on top of a non-coronavirus absence rate of 6 per cent.
Managing director Robbie Drummond said: “Over the past week, the number of Covid-related absences amongst vessel crew has increased by 166 per cent, from 35 on Christmas Eve to 93 today.
"Covid absences amongst port staff have risen by one third over the same period.
"The loss of such large numbers of staff makes it necessary for us to take immediate action to try and preserve essential services to communities.
“We fully appreciate the difficulties these changes will cause for some of our customers, and we apologise for any disruption that may result.
"However, because this situation is changing rapidly, we must protect core services, which is why we are now having to introduce this temporary timetable.”
CalMac said all its services were also subject to “change, cancellation or interruption at very short notice” and sailings might be cancelled “right up to the point of departure”.
The Arran Ferry Action Group said it was “the worst possible situation at the worst possible time” for the island.
Chairman Sam Bourne said: “We acknowledge the huge difficulties faced by CalMac as they navigate the staffing challenges posed by the Covid pandemic and also recognise some of our fellow islands are suffering more than we are.
“This timetable runs over the period when our main vessel, MV Caledonian Isles, is planned to be away on refit.
"We would normally expect the two ‘old ladies’ of the fleet to provide relief cover with MV Isle of Arran and MV Hebridean Isles.
"This year we will have a single-vessel service, meaning a significant cut in capacity on our main route.
“We are already seeing cancellations and numerous disrupted plans.
"Allied with the continuing disruption caused by the poor condition of the berth in Ardrossan, it means January looks like a very challenging month for Arran’s residents, businesses and visitors.”
Timetable changes (other routes unchanged):
· Mallaig – Lochboisdale cancelled.
· Oban – Coll – Tiree
Sunday is being removed.
· Oban – Colonsay
Wednesday service removed.
· Kennacraig – Islay
Single vessel timetable MV Finlaggan as normal
· Wemyss Bay-Rothesay
Single vessel timetable
Published two vessel timetable 2-4 January then single vessel MV HebIsles until 22 January.
Half hourly timetable
Combined Tarbert – to Portavadie and Tarbert to Lochranza service