Western Alaska’s Mulchatna caribou herd was once one of the largest herds in the state before the population began to decline roughly three decades ago. While the herd’s numbers have stabilized in recent years, they haven't rebounded. An Alaska Department of Fish and Game study aims to find out why by focusing on two key factors: disease and nutrition.
This time of year, wildlife physiologist Dr. Kristin Denryter’s phone is constantly buzzing with alerts. But these are no regular notifications. Each one signals the status of a pregnant Mulchatna caribou. The notifications come from devices implanted in the birthing canal of 49 collared females, which monitor calving success in real-time.
“I get text messages every day and emails every day for every collar to tell me ‘no birth detected,’” Denryter said. “And then eventually, hopefully, I'll start getting alerts that say 'birth detected.'”
When a birth is detected, Denryter flies to the GPS location to observe the calf — checking if it can stand, nurse, and follow its mother. These indicators help researchers assess survival rates.
The research is part of a three-year study which seeks to understand why the Mulchatna caribou herd — which once numbered around 200,000 across a huge region of western Alaska — has failed to recover after plummeting to just over 12,000 animals in 2019.
At its peak population in 1997, 4,770 caribou were harvested for subsistence to support 48 villages across the Bristol Bay, Kuskokwim River, and Lake Clark regions.
Today, the herd is hovering at roughly 15,000 animals, well below the state's minimum management objective of 30,000. Denryter says there are a few theories for the crash.
“Most folks hypothesized that it was overgrazing the range of nutritional carrying capacity. So basically, you had too many animals on the landscape,” Denryter said. “They ate themselves out of house and home, and then there weren't enough food resources, and that's what drove the initial population decline.”
The population decline led to a total closure of subsistence hunting back in 2021, which will remain in effect until 2028.
In 2022, the Alaska Board of Game approved an Intensive Management program—including targeted removals of wolves and brown bears—to improve calf survival. And according to a 2025 update from the department, the program may be helping. Cow to calf rations and population numbers are up slightly.
Denryter’s research is not specifically looking at Caribou mortality, but the team has noticed trends through their close monitoring of the herd. During the first year of the study, five collared caribou died, three of which were killed by predators.
“It's such a small sample size that we really can't say a whole lot about it,” Denryter said. “Except that it does look like, of the mortalities we've documented, that there have been quite a few attributed to predators. So predators probably are an important part of the ecology of the Mulchatna caribou herd.”
Researchers are now focusing on disease and nutrition as possible factors for low population numbers, and they are looking at impacts to survival rate and successful calf rearing.
The herd was exposed to Brucellosis — a bacterial infection that can cause infertility, pregnancy loss, and poor calf health in other species — almost a decade ago. Denryter says she expected to see these symptoms in the herd and was surprised to find no evidence of the bacteria affecting the herd.
“What we saw was, we had super high pregnancy rates, 96% in the first year of the study,” Denryter said. “We also saw that most of those calves that were born to mothers that were seropositive for Brucella did just as well as mothers as calves born to mothers that were seronegative for Brucella.”
Denryter says it’s unclear whether caribou as a species are unaffected by the bacteria or if the herd was exposed without developing an active infection.
“It's still possible, if not probable, that when an animal is actively infected, it may cause them problems,” Denryter said. “But we're just not seeing that level of issue on the landscape with Brucella right now.”
Along with tracking births, researchers are studying the herd’s nutrition by measuring the body fat of female caribou. After sedating the animals, the researchers take ultrasound measurements of the fat on a caribou's rump.
John Crouse, a wildlife researcher on the team, developed the method roughly a decade ago.
“We're trying to determine, are they heavy enough? Are they fat enough?” said Crouse. “And what levels [are needed] to achieve pregnancy? What levels [are needed] to successfully carry a fetus through to birth?”
Crouse says early findings suggest that winter nutrition is not limiting the herd. Caribou enter the colder months in lean condition but gain weight through the winter – suggesting that they’re eating enough lichen, which is their primary winter food source.
Before this study, researchers only measured Mulchatna caribou body fat in the fall. Denryter says the new findings contribute to understanding the herd’s seasonal cycle.
“What we're learning is that being lean in October means you know that over the summer time, during lactation and bug harassment season, these lactating animals are thinner,” said Crouse. “But the consequence is that they're not continuing to lose [weight] over winter. They're actually gaining over winter.”
This data is some of the first of its kind. Denryter says very few herds have had these techniques applied to them to determine how fat they are seasonally, and they were surprised by the results.
“It's not something we expected at all,” said Denryter. “And so one of the things that we hope to see going forward is that these techniques are applied to other herds so we can learn whether Mulchatna is atypical or if this is just how caribou make a living.”
The study is now in its second year. This spring, researchers will continue monitoring pregnant females, then return in the fall for additional fat measurements.
Once the study is complete, Denryter and Crouse will present their findings to the Alaska Department of Fish and Game and offer management recommendations for the herd’s future.
