The International Fund for Animal Welfare project team will be joined by the Countryside Council for Wales in locating and assessing the numbers of harbour porpoises in the area.

Criticism: Dolphins and porpoises are vulnerable to fishing nets

The study begins amid fears that a number of porpoise colonies off the UK coast - particularly in the English Channel - are coming under threat.

Once a common sight in British estuaries, harbour porpoise numbers have been steadily declining since the 1940s, largely as a result of intensive fishing activities.

The RSPCA has recently criticised the numbers of porpoises dying in fishing nets.

The harbour porpoise - one of the smallest members of the cetacean (whale) family - is the only porpoise found in the north Atlantic and the west Wales coast holds one of the most important colonies.

The species has been placed under the Wildlife and Countryside Act 1981 and received its own Biodiversity Action Plan.

They are particularly difficult to spot in the wild, making the survey a considerable challenge.

This is the first dedicated acoustic survey for porpoises in Welsh waters

Chris Pierpoint, porpoise expert

Anna Moscrop of IFAW said: "Harbour porpoises are notoriously difficult to spot at sea in anything but calm conditions.

"When we pick up the clicks the team will endeavour to photograph or video any porpoises observed."

Porpoises give birth and breed in Welsh coastal waters and the young families are reported in Cardigan Bay area from mid-May onwards each year.

Locations such as Strumble Head near Fishguard, are well-known locally for attracting large aggregations of porpoises that come to feed in the strong tide races.

With the help of the IFAW team, porpoise expert Chris Pierpoint is conducting the survey, using local boats and volunteer observers.

During the first two days of the survey, many sightings and acoustic detections have already been made.

Cousins: The dolphin is better known than its smaller relative

"This is the first dedicated acoustic survey for porpoises in Welsh waters," said Mr Pierpoint.

"With IFAW's support we hope to demonstrate that the south west Wales coast is probably one of the most important areas for porpoises around Britain."

The last major census of whales, dolphins and porpoises in 1994 revealed that only 352,000 harbour porpoises lived in the whole of the North Sea area with up to 10,000 a year killed in fishing nets.

In the same survey, not a single porpoise was seen in the entire English Channel area and their numbers are considered to be under threat.