Fewer than half of passengers travelling on CalMac ferries are now complying with regulations to wear face coverings on board, the company said on Wednesday.
Scottish Government rules state that on public transport people must by law wear a face covering, unless exempt.
And CalMac is now urging passengers to adhere to the law, saying ferry services are experiencing regular disruption due to crew members testing positive.
The firm says passenger compliance with the wearing of face coverings has now dropped to less than 50%.
Robbie Drummond, managing director of CalMac, said: “Please wear a face covering when travelling with CalMac, either on board or inside one of our port offices, unless exempt. This helps to protect everyone, including other passengers and crew members.
“Vehicle traffic is heading back towards normal levels and on some routes higher than the records set in 2019, such as on the Oban-Colonsay route.
“During August 1-27 we carried 163,500 cars which is only 3% down on 2019 and 6400 commercial vehicles which is an increase of 4% on 2019.”
Services to and from Mull and Arran were cancelled recently after crew members tested positive, while port staff from Kennacraig and Mull had to be drafted into Oban for the same reason.
When a staff member tests positive, close contacts must also be withdrawn from service until a negative test is returned, in line with Scottish Government rules on isolation.
Every case identified on board a ferry also results in that vessel having to be removed to undergo a deep clean by a specialist cleaning company.
Mr Drummond added: “The service can be severely disrupted if a crew member tests positive. Sailings are having to be cancelled due to a lack of crew when they require to be tested, and time must also be spent having vessels cleaned.
“Enforcing the law is a matter for the police but please respect CalMac crew and wear a face covering when asked to do so. You will be helping to protect people’s health as well as lifeline ferry services.”
Covid cases have been rising in recent weeks, with the peak higher than at any point during the pandemic and showing little signs of slowing.
Figures went above 7000 new cases in a single day for the first time last month, according to Scottish Government figures.
Meanwhile, a total of 10,612 people have died in Scotland with confirmed or suspected coronavirus, according to the latest weekly statistics published by the National Records of Scotland on Wednesday.
Almost all coronavirus restrictions – including social distancing and limits on the size of social gatherings – were lifted across Scotland on August 9.
But a number of mitigation measures remain in place including the wearing of face coverings in public places.