The Marine Conservation Society said historic litter often washes up at this time of year due to the seasonal storms – and that eroding coastal landfill sites are releasing a lot of “retro” rubbish too.
Spokesperson Catherine Gemmell said: “Litter, especially plastic litter, never disappears and can last a very long time in the marine environment and also travel across oceans.”
Meanwhile John Berry, of the Scottish Islands Federation and Greener Orkney, said he was not surprised by the increase in litter on Sanday.
“We’ve got the really clean beaches [in Orkney] but we’ve got some really dirty beaches as well,” he added.
“I’m really not surprised to see that, with a slightly different weather pattern, that we’re getting a lot of the old legacy material.”
He added: “But we will clean it up in spring. And next year, it will be back. So we will do the exercise again.”