Seals' footage of fishy activity

A team of American marine biologists has formed an unlikely alliance in its exploration of the dark waters under the frozen Antarctic sea ice. A film crew of 15 Weddell seals, which was initially the subject of the study, has provided footage that revealed unexpected secrets of Antarctic fish behaviour.

Lee Fuiman, of the University of Texas, Austin; Randall Davis, of Texas A&M University, Galveston; and Terrie Williams, of the University of California, Santa Cruz, set out to find out how the Weddell seals managed to hold their breath to catch prey diving under the huge pressure of the frozen Antarctic water. Low-light sensitive video cameras, lit by light invisible to the seals, were strapped to their backs. This allowed black-and-white recordings to be made in complete darkness.

The seals dived as deep as 400m providing footage of a world never seen before.

In particular, the researchers realised the videotapes gave them significant insight into the behaviour of the seals' prey. They noticed that silverfish migrated daily from depths of 250m by night to 345m by day, triggered by changes in lighting, despite little change in intensity during Antarctica's months of constant sunlight compounded by 3m of ice and 2m of snow on top of the water.

They also discovered that toothfish, previously thought to be a deep-water species, were often seen in shallower waters. The toothfish is now commercially fished and the seal camera will give useful information to prevent over-fishing.

Dr Fuiman explained that previous knowledge of the habits of these two fish had been gained by the inexact method of trawling in ice-free areas.

The findings will be published in the March edition of Marine Biology .

Meanwhile, the team has yet to begin analysing the data on the seals. They are trying to match speed, oxygen consumption and heart rate with the diving activities.