The seal blows air up under the ice

The only way to find out was to "follow" the seals under the ice. Davis's team did this by fitting $30,000 miniature cameras to the seals' backs.

More than 50 hours of film was recovered and showed that the seals often approach fish from below, so they stand out in silhouette against the overlying ice. It also showed the seals sneaking up to within a few centimetres of cod without frightening them off.

The evidence of the tapes suggests that seals do not rely on sonar to find their prey, as they made relatively little noise.

A data recorder was also fitted and logged time, depth, swimming speed and compass bearing every second. The seals start their dives by shooting down to depths of up to 73 metres at speeds of up to two metres per second. They then ascend and dive looking for prey.

The fish darts out away from the air bubbles

The video camera uses near infra-red light-emitting diodes for light. Seals and fish cannot see this light, so their behaviour should be unaffected. The camera takes 30 frames every second and one tape captures six hours of underwater action.

The cameras are glued to the seals' fur and would drop off when they moulted, but are removed after about four days. The seals are sedated while this happens.

Dr Davis said: "These observations highlight the broad range of insights that are possible with simultaneous recordings of video, audio, dive paths and effort."