Grizzly Details: Salmon Collapse Could Be Bad News for Bears

Scientific American
By Anne Casselman

Scientists are collecting hairs from live bears to prevent population declines as a result of declines in a principal food source: salmon.

For most of May, Chris Darimont, an environmental scientist at the University of California, Santa Cruz, poured liters of fermented cattle blood mixed with pureed rotten fish guts on 3,000 square kilometers of British Columbia’s coastal wilderness.

Darimont’s potent cocktail, dubbed “stink sauce” by his field crew, is key to a new study that examines the relationships among salmon and coastal grizzlies by mining their hair for unprecedented data on their diet and health.

For a 180-kilogram male grizzly, the stink sauce is akin to the call of the Pied Piper—it’s irresistible. And as the bear comes in for a whiff, the barbed wire circumventing the lure snags its hair. “We’re luring them in for a sniff of something and in return we’re taking their hair,” Darimont explains. “It’s sort of like forensic ecology.”

In the short term the study’s goals are to rapidly assess the health of coastal carnivore populations with respect to the salmon on which they depend, says Darimont, who moonlights as a research scientist for the Sidney, British Columbia–based Raincoast Conservation Foundation (RCF), which is spearheading and largely funding the five-year study.

Humans aren’t the only ones affected by poor salmon runs. The abysmal sockeye run on British Columbia’s Fraser River this summer was such big news it precipitated a federal inquiry into the fish’s disappearance. Pink and chum salmon runs along province’s central coast haven’t been doing so hot in recent years either, which raises questions about the long-term welfare of
coastal grizzlies that feast on them.

Government helicopter surveys conducted this fall along the Kimsquit River area north of Bella Coola, British Columbia, tallied below-average numbers of grizzly adults and cubs. This suggests poor salmon returns of prior years might be taking their toll on the bears, starving them of their primary prehibernation food source. “The science of it says you’re going to have a density of bears and productivity of the population proportional to the salmon base,” says Barrie Gilbert, a retired wildlife biologist from Utah State University in Logan.

Coastal grizzlies are a different beast altogether from their smaller interior cousins. The more salmon a male eats, the larger his skull grows; the more fish a female eats, the earlier she’ll reach reproductive maturity—and the more cubs she’ll have each year.

The population density of grizzly bears in Alaska’s salmon-rich areas runs 10 to 20 times higher than those in the sans-salmon interior of the state. The more fish in an ecosystem, the more grizzly bears that can be supported. Remove the salmon from an ecosystem and grizzly numbers drop, which is what happened over the short term when Owikeno Lake’s salmon stock went AWOL in coastal British Columbia in the late ’90s. “There are very few biologists who will argue that salmon aren’t a key limiting factor to grizzly bear numbers on the coast,” says Garth Mowat, a senior wildlife biologist with the British Columbia Ministry of Environment.

Grizzlies also act as nutrient conveyor belts, dispersing the nitrogen- and phosphorus-rich salmon carcasses across the forest floor like fertilizer. “As we lose either bears or salmon or both along the coast of British Columbia, then we’re also affecting the health of the forests,” Gilbert
says.

Stink sauce and the blood shed

But back to the juicy stuff: stink sauce, which is stored in a custom-built “blood shed”.

After the nearby barbed wire snags coarse guard hairs from the bear’s outer coat, the fur’s DNA is sequenced to divulge the number of individuals, their species and gender. “With those data alone we can track bear numbers over time… and see how they fluctuate with salmon numbers over time,” Darimont says.

Next, he and his colleagues will run stable isotope analyses on the hair to estimate of just how much salmon the bear consumed during the previous year’s salmon run. Finally, information on the bears’ hormonal states is extracted from the samples: Cortisol gives a picture of stress levels,
thyroxin provides an index of protein depravation or starvation, and sex hormones provide insight as to whether a female had cubs in the past year—all this from a clump of hair.

Darimont and his co-workers collected 550 hair samples this year from their study area near the town of Bella Bella, which lies halfway up British Columbia’s coast. The genetic work will roll in later this month, and the hormone work should take another three months. Together, this should provide the RCF with enough data to publish a “rapid but tentative” assessment.

“We’re hoping that we’ll get some very fine-scale intimate insight into the relationship between grizzly bear populations and health and salmon consumption,” Darimont says. “It’s the first work of its kind.”

By learning more about the relationships among salmon abundance and bear population dynamics, Darimont hopes to be able to determine whether there is a threshold of the spawning salmon biomass that would prevent the coastal grizzly bear population from declining. His data set could also help map out the consequences on bear numbers from human salmon harvesting—20 percent more or less, for example.

And things are starting to look up for grizzlies and at least one of their major food sources in the area. John Reynolds, a conservation biologist at Simon Fraser University near Vancouver, reports that the pink salmon have come back with a vengeance this year, on the heels of record lows in 2007 and 2008: “The grizzlies, of course, can eat the pinks, so it’s not all doom and gloom right now.”

Caption: WHERE’S THE FAT? Grizzlies aren’t as much interested in the protein in salmon as the fat, which fuels their high-energy winter hibernation.

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