INTRO: Spotted seals and harbor seals look so much alike that when it comes to telling them apart, even biologists end up scratching their heads. In places like Bristol Bay, where the two species coexist, scientists are turning to DNA for answers. As Doug Schneider reports in this week's Arctic Science Journeys Radio, being able to tell the difference may help researchers learn more about seals and about how these marine mammals might respond to climate change.
STORY: Alaska's Bristol Bay is a place on the edge of two worlds. To the north is the Bering Strait and the icy High Arctic. To the south are the somewhat warmer waters of the North Pacific Ocean. Being on the edge makes Bristol Bay a gathering point for two normally separated species. One is the harbor seal, the plump, ubiquitous marine mammal that prefers ice-free areas; and the spotted seal, a species at home along the ice edge.
Now you might think two seal species that prefer very different habitats would also look quite different. Yet, it turns out that distinguishing between these two seals in the wild is almost impossible.
Robert Small is a marine mammal biologist with the Alaska Department of Fish and Game. He's been studying the bay's seals for four years. Even so, he admits that even he can't tell the two species apart.
SMALL: "No, I couldn't. If you have a live spotted seal and a live harbor seal side by side, you could make a guess."
Normally, spotted seals prefer the leading edge of sea ice as it advances and retreats with the changing seasons. But in Bristol Bay, spotted seals stick around even after the ice retreats, preferring to hang out with harbor seals and feed on returning salmon. Small and other scientists want to figure out just how many harbor seals there are in the bay. But until recently, they could only guess because they didn't know whether they were actually counting harbor seals or spotted seals.
SMALL: "Spotted seals have not received a lot of attention. In terms of their geographic range, we know that they occupy Bristol Bay and probably down the Aleutian chain. But to really know how many are there at certain times of the year, we really haven't done the work. It's complicated because you can't distinguish between the two seals. It's hard enough when you have them in hand, but a lot of the work we do is through aerial surveys. When you're up in the air, there's no way you can tell the difference."
About the only way you can know for sure is to examine the seal's DNA. For the last two summers, Small and several colleagues captured as many seals as they could. And from each took a tiny skin sample that was sent to the National Marine Fisheries Service for positive identification.
SMALL: "It's one tool to look at the different lineages between the spotted seal and a harbor seal. Last year we caught a total of 39 animals and five of them were spotted seals. This year we caught a few more total number of seals but it looks like only two or three are spotted seals."
Sorting out Bristol Bay's seals is important to learn just how each species is faring in the bay. With four years of aerial population counts and about 150 skin samples, Small thinks the harbor seal population is stable at around 18,000 animals. He says spotted seals probably number around 1,600 animals, but since they come and go frequently from the bay, that number could be higher.
Small also says Bristol Bay is a good place to study the impacts of climate change because the two seal species may respond very differently to a warmer climate. Harbor seals, for example, may extend their range as receding sea ice pushes spotted seals farther north.
SMALL: "I think these two species would be beneficial because they're at an area where their ranges overlap. If changes start to occur within the Bristol Bay area that folks are starting to see in the Arctic, then the opportunity is there to see how these species will change with the changing ecosystem."
OUTRO: This is Arctic Science Journeys Radio, a production of the Alaska Sea Grant Program and the University of Alaska Fairbanks. I'm Doug Schneider.
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Thanks to the following individual for help preparing this script:
Arctic Science Journeys is a radio service highlighting science, culture, and the environment of the circumpolar north. Produced by the Alaska Sea Grant College Program and the University of Alaska Fairbanks. The shortcut to our ASJ news home page is www.asjnews.org.
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Seal travel map
Spotted seals are famous for their wandering ways. See just how far one spotted seal traveled. (Courtesy ADF&G.)
Related Web sites
Spotted Seal (ADF&G)
Harbor Seal (ADF&G)
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