Focus on northern fur seals

Flaherty and Leu

Did you know a mature male northern fur seal can weigh over 600 pounds? That’s a lot larger than Flaherty and Leu, the two fur seals who joined the Seattle Aquarium last March. They’re still young and growing, though: Flaherty is three years old and currently weighs in at approximately 90 pounds; Leu is four years old and tips the scales at around 120 pounds. When fully grown, they’ll weigh between 350 and 600 pounds, depending on the season. Mature female fur seals, on the other hand, are much smaller. An adult male fur seal can be up to six times larger than an adult female! Sexual dimorphism is the term for size difference between males and females—and the sexual dimorphism between male and female fur seals is the largest of any marine mammal species.

nothern fur seals - male vs. female size

Northern fur seals are also distinguished by their big eyes, which help them see at night; the large flippers that propel them through open water; and, maybe most notably, their thick, luxuriant fur. That fur made them especially attractive to hunters for many years. As a result, northern fur seals are listed as “depleted” under the Marine Mammal Protection Act. Their total population, estimated at 1.1 million, is far less than it used to be—and their numbers are declining.

What’s the reason behind the population decline? That’s a question that’s been on many researchers’ minds, and the answers have been slow in coming. Field research on northern fur seals is no easy task since the animals breed in the summer on remote islands in the North Pacific and, until recently, no one truly understood where they went the rest of the year. As it happens, more than 50 percent of the wild northern fur seal population breeds on the Pribilof Islands in Alaska—so it’s there that researchers from the National Oceanic & Atmospheric Administration’s (NOAA’s) National Marine Fisheries Service (NMFS) focused their efforts. Researchers attached satellite-linked time depth recorders (SLTDRs) to the animals; the SLTDRs then transmitted data on location, time, and depth of dives in real time so that animals could be followed while at sea. The collected data confirmed that northern fur seals are amazing distance swimmers and travel thousands of miles in search of food. Much more remains to be learned about northern fur seals, especially with regard to their population decline.

Fur seals in zoos and aquariums are a rarity as well: just seven are on exhibit in the United States, including our own Flaherty and Leu. The Seattle Aquarium has a long history with northern fur seals. We’re one of just three facilities in the United States—and the only one on the West Coast—to exhibit them. In 1983, our facility became the first in the world to have a northern fur seal conceived and born in human care. A total of five fur seals born here lived to adulthood. Isaac, the final of these, was born in 2000 and moved to the New England Aquarium in 2009 as part of a collaborative breeding loan. While there, he sired our own Flaherty—which made us especially happy to welcome him to the Aquarium last year. It’s a fur seal full circle!


Interested in learning more about northern fur seals? Read our fur seal fact sheet, then come to the Aquarium to meet Flaherty and Leu—and check out the interactive kiosk next to our fur seal exhibit!

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