This time of year, Kristin Denryter’s phone buzzes constantly with alerts. But these aren’t regular notifications. Each one signals the status of a pregnant caribou from Southwestern Alaska’s Mulchatna herd.
The notifications come from devices implanted in the birthing canals of 49 collared females.
“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. Can it stand? Nurse? Follow its mother? These indicators help 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 in 2019.
A population crash
At the herd's peak 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 remains at roughly 15,000 animals — about half of the 30,000 that the state is hoping for.
Denryter says researchers have considered several theories, but one stands out.
“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.”
The population decline led to a total closure of subsistence hunting back in 2021, which remains in effect.
In 2022, the Alaska Board of Game approved a program to improve calf survival, which includes culling wolves and brown bears.
The program may be helping, according to the Alaska Department of Fish and Game. Based on a 2025 update from the department, the herd has seen a population increase of roughly 2,000 caribou in the last few years, and a greater calf-to-cow ratio.
Denryter’s research is not looking at caribou mortality specifically, 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.”
Was it disease?
Denryter’s study focuses instead on disease and nutrition as possible factors for low population numbers.
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 symptoms of the bacteria affecting the herd and was surprised not to.
“What we saw was, we had super high pregnancy rates, 96% in the first year of the study,” Denryter said, adding that calves born to mothers who tested positive for the bacteria did just as well as calves whose mothers didn’t.
She 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,” she said. “But we're just not seeing that level of issue on the landscape with Brucella right now.”
Are the caribou fat enough?
Along with tracking births, the researchers are studying the herd’s nutrition by measuring the body fat of female caribou. After sedating an animal, the researchers take ultrasound measurements of the fat on the 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 research is in its second year of a three year study. While early findings suggest disease and nutrition may not be the primary factors limiting the herd, Denryter and Crouse say more data is needed before drawing conclusions. 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.
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