More research into dolphins has been called for

It is also feared that of dolphins are under threat then other mammals like whales could also be at risk.

Mr Archer said: "The only way forward is for experts to investigate what is going wrong.

"We need to be able to eliminate any possible threats. For example we think that dolphins get caught in nets being set illegally in the sea by salmon poachers."

The dolphin colony is a popular tourist attraction in the Moray Firth area and is thought to contribute substantially to the local economy.

Boats carrying sightseers operate from a number of ports, including Inverness, Cromarty and Nairn.

Mr Archer said he the noise of boats' engines disrupts essential sound communication between mothers and their calves.

The animals do not seem worried by the vessels, but Mr Archer says any communication breakdown between adults and their offspring can lead to the calves becoming confused and damaged by the propellers of any circulating leisure boats.

The Moray Firth pod lives in colder waters than any others, as this is the most northerly colony on Earth.

Britain's only other resident dolphins live in the water of Cardigan Bay, off West Wales, although they are visitors to inshore waters round several parts of the British Isles.