A fishing boat from Peterhead on the north-east coast of mainland Scotland has been identified as the cause of an outbreak of Covid-19 in Orkney, as the local health chief said he was profoundly concerned that the virus was spreading rapidly across the island community.

The head of NHS Orkney, Michael Dickson, said: “We have five positive cases from a fishing vessel and more than nine contacts. Our understanding is they travelled from mainland Scotland, from Peterhead, before arriving in Orkney.”

Contact tracers are working to isolate those who may be affected following an incident management team meeting on Thursday afternoon.

Dickson assured the local population that health authorities were taking the outbreak “extremely seriously. “We believe there is a real risk to the people of Orkney, including those on the remote isles, and urge them to take care,” he said.

“Several individuals have developed Covid symptoms and then travelled to their homes across the isles and to mainland Scotland. I am asking that anyone in Orkney who shows symptoms of Covid to immediately self-isolate for at least 10 days and seek a test.

“While Orkney’s Covid figures did not change, we are aware of a significant number of symptomatic cases, all of whom have been tested. Some of these figures will be attributed to mainland board figures. This happens when a patient’s home address is registered off-island.”

At Thursday’s daily briefing, Nicola Sturgeon suggested that the fact the islands had gone so long without new cases might mean people “were not being as assiduous in complying with rules … I would say to all island communities [guidelines] are just as important as for those on the mainland.”

The first minister addressed two new coronavirus clusters, one in Orkney and another of eight cases in Glasgow, as well as the ongoing outbreak in Aberdeen, where 182 cases have been linked to the outbreak and 961 contacts identified.

She said clusters of this kind were inevitable and should not cause undue alarm, but that they were a stark reminder that the virus still posed a risk. “We should become more used to hearing about several different clusters because the virus hasn’t gone away … Covid is still present and it will spread really rapidly if it gets the opportunity,” she said.