Sea World Scientists Use Satellite : Method of Tracking Harbor Seals a Success

Scientists at Sea World’s research institute in San Diego have succeeded in tracking a harbor seal with a satellite method that previously has worked only with much larger marine mammals.

The achievement overcomes difficulties that have kept such studies lagging far behind similar studies on land animals, said Doug DeMaster, a marine mammal specialist at the federal Southwest Fisheries Center in La Jolla, which will be using the technology this summer for a longer study of harbor seals.

“It’s a big deal in that for small species of marine mammals like seals, we really need to know the population we’re dealing with. And that requires long-term movement data,” DeMaster said. “In the past, the only way to get that was to put tags on the animal and hope we see it again, even though it might swim 200 or 300 miles away, or put (UHF or VHF) transmitters on it and have to fly around to find it--which is very expensive.”

Signals Sent to Satellite

Satellite tracking removes those uncertainties by using a transmitter attached to the animal that sends signals to a satellite to record the animal’s position. In recent years, scientists have reported success tracking whales and larger seals with the method.

Being able to do it with harbor seals could provide answers on how many seals there are and whether the Channel Islands seals are separate from seals seen closer to shore, DeMaster said. This is important to know because shark fishermen’s gill nets kill as many as 1,000 harbor seals a year around the Channel Islands, he said.

DeMaster estimated that there are 3,000 to 5,000 harbor seals off Southern California, and 20,000 statewide.

In their study on the 200-pound harbor seal, Sea World scientists feared that the 25-pound transmitter pack would disturb the animal’s behavior, said Brent Stewart of Sea World Research Institute’s Hubbs Marine Research Center, one of the biologists who worked on the study.

After they used captive seals at the park to determine that the seals were unaffected by the weight, they captured a female harbor seal April 1 on San Nicolas Island, about 70 miles west of Long Beach, and glued a transmitter and dive recorder to her fur, then let her go. They recovered the $3,500 transmitter by relocating the seal on the island April 20.

Dives Were Recorded

During the study, every time the swimming seal surfaced, the transmitter sent a signal to one of two polar-orbiting satellites. Meanwhile, the depth recorder kept track of her dives to provide a cross-check with the satellite’s location data.

Analysis indicated that satellite tracking determined the animal’s location to within half a mile, Stewart said.

With the success of this project, Stewart and colleague Steve Leatherwood will leave for Greenland next week to try the same method with ringed seals, in cooperation with Danish researchers.

The harbor seal study was financed by Sea World, private donations and a Danish government team that wants to test the technique on ringed seals, Stewart said. The biggest costs were equipment, at about $7,000, and the researchers’ salaries. The third scientist in the project was Pamela Yochem.