Study on Iliamna Lake seal teeth adds to debate on conservation status

May 20, 2019

The elusive seals that reside in Bristol Bay’s Iliamna Lake have long been a mystery. While wildlife managers treat them the same as marine harbor seals, a new study supports what traditional knowledge has long held – that they are a distinct, freshwater population.

One of the Iliamna Lake seals seen just off a gravel beach on the east end of the lake, the seals' primary habitat.
Credit Jason Ching/University of Washington

In 2013, Sean Brennan was researching the life cycles of Chinook salmon in the Nushagak River for the University of Washington.

“I started to hear about issues surrounding whether or not the harbor seals in Iliamna Lake should be considered a distinct population segment from the general harbor seal taxon,” Brennan recalled.

Not much is known about the Iliamna seals’ ecology. Traditional knowledge and aerial surveys have provided some insight on the approximately 400 lake-dwelling animals. But Brennan said biologists haven’t been able to answer a simple question: Do the seals ever go to the ocean?

“I think one way to get at that without actually having radio collars that are able to last that long on individuals is to actually take advantage of the fact that the teeth of these seals is this lifelong record,” he said.

The seals keep the same canine teeth all their lives. Brennan said those teeth reveal clues about the animals’ life cycles.

A cross-section of the canine tooth of a harbor seal.
Credit Courtesy of Sean Brennan

“The canine teeth of seals grow like a series of stacked cones. So you actually have this lifelong chemical record of each seal,” he explained.

To figure out whether the seals go to the ocean, Brennan needed to find out what they ate. His team dissected the teeth of four seals – about one percent of the lake’s population – and compared their findings to water samples from the lake that measured its isotope ratios.

“So the first thing we knew was that the isotope ratios in the lake were very different from what we know is in the ocean,” he said.

The material laid down near the pulp cavity of the tooth represents the most recent time in that animal’s life. Moving up that series of stacked cones toward the tip of the tooth, researchers were able to get a chronological measurement of that individual’s life.

The study was funded by the Alaska Sea Grant, Arctic-Yukon-Kuskokwim Sustainable Salmon Initiative, North Pacific Research Board, Bristol Bay Regional Seafood Development Association and Bristol Bay Science Research Institute. Its results support what traditional knowledge from Elders and hunters has long espoused: Iliamna Lake seals reside in fresh water year-round.

“All individuals show that they’re born in the lake, they spend their entire life there, and early in life they tend not to eat adult sockeye salmon, even though they’re there,” Brennan said.

As they age, the seals become more reliant on salmon for food. But Brennan said that their findings indicate that it is highly unlikely the seals spend even a short amount of time in the ocean.

“A two-week trip to the ocean eating just marine foods there – you would see that picked up in the tooth easily. And all of the ratios that we measured in the tooth – we don’t see any evidence that they are making even these short trips to the ocean,” he said.

In 2016, the Alaska Department of Fish and Game and the National Marine Fisheries Service that oversee the Endangered Species listings declined a petition to manage the lake population separately from other harbor seals in the area.

According to Brennan, determining whether these seals are a distinct population is important given western Alaska’s rapidly warming climate and the proposed Pebble Mine, which would be built about 17 miles north of the lake. If the seals are distinct from marine populations in the region, such changes could affect them differently.

Peter Boveng leads the Polar Ecosystems Program at NOAA’s Marine Mammal Laboratory. He thinks more studies are needed before the conservation status can be reconsidered.

“The crux of that question is whether there’s any indication that they are ecologically significant to the broader taxon,” he said.

Boveng said this study supports previous research indicating that the Iliamna seals reproduce in the lake. But he takes issue with the notion that the seals’ diet makes them unique.

“Harbor seals are very generalist predators,” he said. “They eat all kinds of fish wherever they encounter them. The fact that they are eating some fish that may be found only in the lake isn’t really a unique trait or an evolved or adapted trait. We don’t believe that it’s something that’s different from what a marine harbor seal would do if you plopped it into the lake.”

Boveng also pointed to the small sample size in this and previous studies of the seals, saying they provided insufficient data to draw definitive conclusions about the seals’ place within its subspecies.

NOAA researchers are conducting additional genetic analysis on seal scat from a larger sample size. Both Brennan and Boveng said additional research is necessary to further etch out the seals’ place in its subspecies and their role in the lake’s ecosystem.

Contact the author at isabelle@kdlg.org or 907-842-2200.