It comes as the First Minister announced that all adults will have access to twice-weekly lateral flow tests from Monday April 26, the date on which it is now confirmed that mainland Scotland will enter level three restrictions.
From that date non-essential shops will reopen, pubs and restaurants will be able to open in a limited way, and travel will be allowed to the rest of the UK.
Scotland’s islands, most of which are already in level three because of lower rates of the virus, will remain at that level.
This is to allow travel from mainland Scotland, Ms Sturgeon said, but it presents a risk to island communities as Covid-19 rates there are generally lower.
Scots visiting the islands from the mainland will be asked to take one lateral flow test three days before leaving, and a second on the day of departure.
The tests will not be mandatory, with Ms Sturgeon citing “ethical” concerns.
“We are keeping Island communities, at the same level as the rest of the country, so that we can allow travel between Island communities, and the rest of Scotland," she told a Covid-19 media briefing on Tuesday.
"But we know that does create a risk for our island communities. Many of them currently have very, very low rates of Covid, and they will be welcoming visitors from other parts of Scotland – which I know many of them are looking forward to – where Covid will still be circulating at a higher level, and more widely.
"So if you are planning to travel to an island, we do encourage you to take two lateral flow tests before you depart.
"The second test should be on the day that you're planning to travel, the first should be three days before you travel.
“And that means if you test positive, you have a chance to take a PCR test, which is more sensitive, to confirm the result before making a final decision on cancelling.
Chief Medical Officer Dr Gregor Smith said: “This is an opportunity for people to make sure, as they travel to the islands, that they’re not bringing any infection with them.”