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Friday, April 24, 2009

All About Stellers

On Earth Day was The Whale Museum's annual marine naturalist gear-up, a day-long series of lectures that provides continuing education for working marine naturalists. One of the most informative talks this year was Candian biologist Peter Olesiuk talking about Steller sea lions.

(Note: While Steller sea lions are in fact stellar, their name comes from Georg Wilhelm Steller, a zoologist and explorer who is the namesake for several northwest critters including the Steller's Jay and Steller's Eider in addition to the sea lion. The misspelling of StellAr sea lion is a pet peeve of mine....)

There are two distinct populations of Steller sea lions: a western population that inhabits the Bering Sea and Aleutian Islands, and an eastern population that is found from SE Alaska/British Columbia south to California. While the western population is endangered and has experienced declines of as much as 70%, the eastern population is actually doing quite well, and has rebounded to historic levels after being hunted in the 1940s-1960s in an attempt to "control" fisheries for human purposes.

There are 14 breeding rookeries for the eastern population of Stellers, where males begin congregating in May to establish territories for when the females arrive in June. The animals will stay at the rookeries until August or later before dispersing for the winter. Males will lose up to 450 pounds while defending a breeding territory, a time in which they may stay put up to two months without leaving for food or water. In the San Juan Islands, we pretty much see only male sea lions and only in the winter; they're in this area when they've dispersed from rookeries to "beef up" over the non-breeding season. Females tend to do a lot of oceanic feeding during this time, but locally we have several "bull sites" where we see the males in the winter:

Once such bull site is Race Rocks, west of Victoria, BC. This foggy photo of males hauled out was taken last September.

Steller sea lions weren't seen often in the Salish Sea until the 1960s, when the population began to rebound from the hunting that had decemated it. Since the 1990s, their population has been booming, growing at a rate of more than 7% a year. The current population estimate is 18,250 Steller sea lions in British Columbia alone, compared to the low of 5,000 in the 1960s. Harbor seals have also been experiencing a population boom, and the high abundance of pinnipeds has likely been the reason for increased transient (marine mammal feeding) killer whale sightings in recent years.

A male Steller sea lion requires more than 60 pounds of food a day, or about ten times that of a harbor seal. Their top prey item is Pacific herring, but they also feed substantially on sandlance, salmon, dogfish, polluck, rockfish, and halibut.

Another bull site in BC is the Bell Chain Islets, pictured above in October 2007.

I first heard of Olesiuk due to some of his published work on killer whale population dynamics, although his current work focuses on sea lion population models. He shared the result of an interesting exercise he did determining, based on body mass, mortality, and productivity, how many transient killer whales ("Ts") the current population of Steller sea lions could sustainably support.

Remarkably, he found BC Stellers would only be able to support 26 Ts. If he included the whole eastern population of Stellers, this number jumped to 77 Ts, but still well below the current transient whale population of 200-250 whales. Of course, Ts feed on more than just Stellers - in fact, harbor seals and porpoises probably make up a larger proportion of their diet. To account for some of this, his final calculation included all Pacific northwest sea lions and harbor seals, which the model estimated would support about 300 Ts, closer to what we actually see. This little exercise is really interesting because it demostrates that a relatively small number of whales could have a huge impact on Steller sea lion populations, even if the population seems to be doing very well. As an undergrad, I got really into this sort of thing in my population biology class, where as an independent project I used Olesiuk's models to develop a population model for the Southern Resident killer whales.

Both Steller and California sea lions brave the harsh wave action at the Sea Lion Caves near Florence, Oregon, as shown here in January 2008. While most of the Steller breeding sites are in southeast Alaska and BC, there are a few in Oregon and northern California. Interestingly enough, there are no breeding sites in Washington.


Lancashire and Lakeland Outback Adventure Wildlife Safaris said...

Hi Monika - you didn't mention Mr Steller's best find his Sea Cow which sadly only lasted 25 years after he first described it...pity he didn't keep that one quiet..



Monika said...

Dave, how I would love to see a Pacific Northwest sea cow! You're right, he should have kept that one to himself. I'm still curious about his single report of the mysterious Steller's sea ape, as well....the only animal he described that has never been confirmed or seen again.