A POD of whales spotted swimming in the river Clyde has been connected to a mysterious spike in sightings of the rare deep-water animals across Northern Europe.

Marine wildlife experts from the British Divers Marine Life Rescue were called out to Victoria Harbour in Greenock over the weekend after the group of northern bottlenose whales was seen in the estuary.

The pod was captured on film by Scott Cromar, who shared this video yesterday:

The northern bottlenose is one of the deepest-diving mammals known to man. They can reach depths of up around 2400m. 

It is dangerous for the animals to be in water as shallow as the Clyde estuary which, despite being the deepest coastal waters in the British Isles, is only up to 164m deep.

The whales, which rely on squid and fish for their fresh water needs, may die of dehydration as they are unable to feed in such shallow waters.

The sightings in the Clyde follow on from various sightings of rare beaked whales across Northern Europe. 

READ MORE: Whale and dolphin watch project calls on Scots to help

There have been beached northern bottlenoses in Ireland and the Faroe Islands, and more spotted in the Netherlands' Oosterschelde estuary, all in the past two weeks.

A Sowerby's beaked whale was found dead on the Norfolk coast on Saturday. Two others of that species have been found dead in Belgium and the Netherlands over the past ten days.

It is not understood what has caused the whales to enter such shallow waters, but The Guardian reported that the sudden frequency of the events, which are normally so rare, may indicate a common cause.

It may be that something is disrupting the animals' navigational sense. This could be the recent storms, or military sonar.