Elephant seals dive for science

Jonathan AMOS

SCIENTIFIC RESEARCH: Elephant seals on South Georgia have been recruited to the cause of science.

Equipped with computerised tags stuck to their heads, the animals have been collecting remarkable new information on conditions in the Southern Ocean.

As the animals swim for thousands of km and dive down to 2,000m, their tags record details of temperature, depth and the salinity of the water.

When the seals pop up to breathe, the computers transmit the information to scientists in Scotland via satellite.

"These animals are opening an interesting new window on the ocean," said Mike Fedak, from the Sea Mammal Research Unit (SMRU), Gatty Marine Laboratory, University of St Andrews.

"They can go to places in the ocean that we very often can't go to, and can sample parts of the ocean where we can't afford to or logistically are not able to."

Climate understanding

SMRU has been running its Southern Elephant Seals as Oceanographic Samplers (SEaOS) project for just over two years. It is a collaborative effort with researchers in France, Australia and the US.

And it is being run in parallel with other projects using sea lions, tuna, and even sharks to gather ocean data.

The elephant seals' information has enabled researchers to study how changes in salinity and temperature affect the movement of water at different depths.

This has provided new insights into the habits and habitat both of the elephant seal and its prey species, squid.

But it has also significantly improved our understanding of the processes of heat exchange within the Southern Ocean and between this region and the rest of the world.

The region plays a critical role in the Great Ocean Conveyor - the mass movement of water around the globe that helps redistribute the energy in our climate system.

New dynamic

The South Georgia seals have helped trace the positions of ocean "fronts" in unprecedented detail.

"These are like the weather fronts you have in meteorology only they are in the ocean," explained SMRU's Lars Boehme.

These fronts show you where cold water is coming from the deep ocean up to the surface, or where warm water and nutrient-rich water is brought down into the deep ocean," he told the BBC News website.

"Knowing where these fronts are helps us to understand what happens to this global conveyor in this part of the Southern Ocean."

Sampling from ships using towed sensors had given a snapshot of the fronts' positions on a 10-year timeframe. Now, thanks to the seals and the 21,000 "profiles" they have collected over two years, the SMRU team has improved the resolution dramatically.

Ocean partnership

"Now, we are able to use just half a year of data to produce a snapshot of where these fronts are," Boehme said. "We also show the high dynamics of these fronts within half a year - they move around." But what about the "giant oceanographers" themselves; what's in it for the elephant seals?

It is a good deal, says marine biologist Martin Biuw; it is one which could aid the animals' conservation.

On the biological side of the project, we want to see from the physical oceanography and the environment something that might help us explain why elephant seal populations have gone through such different trends," the SMRU researcher said.

South Georgia's population at 400,000 is the biggest group and has been relatively stable since the end of large-scale hunting in the 1950s.

But the groups centred on the islands of Macquarie and Kerguelen have not fared so well; and in the case of Macquarie may still be in decline. Why this is so may emerge from an analysis of the data gathered by the seals.

Their tracked journeys have thrown new light on their wandering, where they go to feed and, amazingly, how they appear to use the frontal systems to navigate and find resources.

Fur fall

Much of this is speculative at the moment but what is not in doubt is the seals' diving prowess.

Adults will spend perhaps less than 10% of their time at the surface, preferring instead to swim below water for 30 minutes to an hour.

They must have some way to reduce their metabolic rate when they are diving, shutting down most of their systems and resting as they go," said Biuw.

"So, they would go to sleep in transit and then they wake up down at 1,500m and hope there is squid there for them to eat."

The scientists stress the animals are not bothered by the data loggers carried on their heads. The boxes are attached with an epoxy glue and simply fall off after about a year during the moulting season.

The SMRU team has been explaining the results of its project this week at the 2006 Ocean Sciences Meeting of the American Geophysical Union in Honolulu, Hawaii. BBC