Why are more than 100 harbour porpoise gathering off Orkney?

The annual meeting of about 150 porpoise near Longhope, Orkney could be the largest in the UK, and it has been well documented for more than a hundred years.

Despite this, no-one knows why they come to Orkney or what they are doing in the weeks they are here.

After 10 years studying the animals, dive boat skipper Hazel Weaver is partnering with Orkney Marine Mammal Research Initiative, also known as OMMRI, on a study that could be the start of finally solving the mystery of what is known locally as Hoy's puffing pigs.