The ground-breaking underwater acoustic device, known as POD, sits on the seabed and picks up the clicks that porpoises and their dolphin cousins emit, as a form of sonar, to detect fish.
Scientists working with The Wildlife Trusts hope it will provide information about porpoises which is essential for the mammals' protection.
The harbour porpoise, Phocoena phocoena, is becoming a seriously threatened species around the coasts of Britain. Once a familiar sight, they have virtually vanished from the English Channel and the southern North Sea.
Accidental catches in commercial fishing gear are responsible for many deaths, with the Celtic Shelf hake fishery off southern Ireland alone thought to be responsible for killing 2,000 porpoises a year.
Accurate information about porpoise concentrations has so far been unavailable as they are difficult to observe. But the detector, the idea of Dr Nick Tregenza of the Cornwall Wildlife Trust, can pick up the activity of porpoises up to 500 metres away. It consists of a hydrophone attached to a small battery-powered computer inside a plastic block, which sits on the seabed for nearly four days at a time, at depths of up to 150m.
This week it was being used by the West Wales Wildlife Trust, which has a good concentration of porpoises in Cardigan Bay.
"We've already discovered that they hunt mainly at night," said Mick Baines, a marine biologist working with the trust. "We need to understand the ecology of the porpoise so we can manage threats such as entanglements in fishing nets. If we can identify high-risk areas we can manage the fishery."