Winds can change seal pup migration path

In 1892, U.S. Revenue Marine Capt. C.L. Hooper provided information to the U.S. Senate about the seal fur trade. After traveling to the Aleutian Islands and speaking to the indigenous people there, he said, he learned that “seals travel with a fair wind.”

Centuries later, scientists are finding truth to this statement. At the American Geophysical Union’s Ocean Sciences Meeting in Portland Oregon, last month, scientists presented preliminary research that showed strong winds may push northern fur seal (Callorhinus ursinus) pups on their first migration in different directions. Roughly half of the world’s northern fur seals breed on islands in the Bering Sea during the summer before migrating to the North Pacific Ocean to forage for food during the fall.

“The number of seal pups born on the islands is a proxy for overall how many northern fur seals there are,” said Noel Pelland, a physical oceanographer and National Research Council postdoctoral associate at the National Oceanic and Atmospheric Administration’s Alaska Fisheries Science Center, who led the research. “And the number of pups has gone down for mostly unexplained reasons since the 1970s.”

To understand more about where the pups go on their long fall migration, Pelland and his colleagues satellite-tagged more than 150 four-month-old pups. They then looked at climate data to determine if it influenced their movements. “We don’t fully understand why the population has gone down but it’s inhibited by the fact that we don’t have a strong idea of ecological and climatic factors that influence the northern fur seal.”

Using climate data from 1997 to 2015, they found that strong winds were an influential factor in seal pup migration. During the autumn, there are more storms, Pelland said, and more intense winds.

The researchers found that during years when strong winds blew from the west, the pups ended up farther east in the Gulf of Alaska. But in years where the winds were weaker and came from the north, the pups ended up farther south, near the Aleutian Islands. “It sort of looked like the pups were blown to those places,” he said.

Pelland also found that the pups were moving downwind and somewhat to the right. He said this movement is likely related to the nature of the currents driven by the winds, which are also somewhat to the right, due to the rotation of the earth.

Pelland hopes this information can be used in the future to determine if it matters where the pups end up and if their survival is impacted. He also hopes to look at historical data and compare that to climate data to determine if there has been an impact on pup survival. The next step will be forecasting their movements.

“This is one of the most historically studied mammal species because they were important for the fur trade,” Pelland said. “Projects like this one are on one of the most well-studied mammal species, but even so, there are these huge outstanding questions we have about them.”