The endangered Hawaiian monk seal faces numerous threats — from entanglement in fishing lines to loss of habitat, shark predators, human disturbances, and diseases.
Now the seals face a viral threat — H5N1, or Highly Pathogenic Avian Influenza — as it continues to circulate around the world, infecting tens of millions of birds and at least 48 different mammal species, including seals and sea lions.
The Marine Mammal Center of California is taking a proactive step by running a trial on the effectiveness of H5N1 vaccines on two monk seal pups at its specialized hospital in Kona.
An outbreak among the seals in Hawaii, said Sophie Whoriskey, TMMC’s associate director of Hawaii Conservation Medicine, would be absolutely “catastrophic.”
“It’s a really vulnerable population,” she said. “They’re still endangered. There’s been so much effort by so many multiple partners in the Hawaiian Islands to bring that population back.”
Only about 1,600 seals are left in the wild, mostly in the remote isles of Papahanaumokuakea Marine National Monument, but increasingly in the main Hawaiian isles.
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Scientists have seen the bird flu strike numerous seal populations in other parts of the world, including in Argentina, where it wiped out a population of southern elephant seals in 2023
The H5N1 outbreak at Peninsula Valdes killed more than 17,000 elephant seals, including about 97% of their pups, according to an article in “Nature Communications.” Concurrently, the virus was found in dead terns.
Whoriskey said elephant seals are similar to monk seals in anatomy, and were “naive” to avian influenza, meaning they had never been exposed to the virus and therefore, had no natural immunity to it.
“So that raises our concern level for the Hawaiian monk seals,” she said, “because similarly, they don’t have any natural exposure to it yet.”
Seal pup pioneers
Two male monk seal pups rescued over the summer and under the care of Ke Kai Ola have been chosen for the vaccine trial.
Monk seal pup RU28, identified by his flipper tag, was born to Hawaiian monk seal RH38 on Kauai, but was still underweight upon weaning at 5 weeks old.
Monk seal experts brought RU28 to Ke Kai Ola after determining the pup was underweight and unlikely to survive without intervention.
Another pup, RU99, was rescued from the eastern shoreline of Molokai.
Volunteers found him alone, with no mother in sight, at an estimated age of just 2 weeks old. Since he had not appeared to have nursed for the usual 5- to- 7-week period, the pup was rescued and brought to Ke Kai Ola to be fattened up.
Both pups have been fed fish shakes and slurries and brought back to a healthy weight, according to Whoriskey, and ideal for participation in the trial. TMMC is using donated vaccines developed by Zoetis for cattle.
Monk seal RU28 received his first shot of the vaccine Oct. 6, followed by a booster on Oct. 27, and has been doing well, she said.
“He did great,” she said. “We didn’t see any signs of a reaction to the vaccine. He just kept right on eating and playing and doing all of his normal monk seal behaviors. So we’re really encouraged by that.”
Monk seal RU99 got his first dose, with the second one scheduled the week after Thanksgiving.
Scientists will analyze blood samples from both seals to examine whether they developed antibodies resistant to the H5N1 virus, as was the case with the elephant seals.
TMMC conducted an earlier vaccine trial with six elephant seals at its headquarters in Sausalito, Calif., in July and August.
The results were encouraging, she said, as they appeared to respond well to the vaccine, and mounted an appropriate antibody response.
One elephant seal did develop hives, she said, but was in the control group receiving a placebo, and TMMC determined they were triggered by another environmental allergen.
Additionally, Whoriskey said the U.S. Fish and Wildlife Service pioneered a bird flu vaccine trial with critically endangered California condors, with 80% having had a positive antibody response.
“Obviously it’s different, birds to mammal, so we couldn’t use the same exact vaccine or the same results of their study to inform us,” she said, “but we’re following in their footsteps.”
Both monk seals will be released back to the wild, as is the routine for rescued monk seal pups once they become healthy.
H5N1 in the isles
Hawaii has so far been minimally impacted by avian influenza, with only one major outbreak detected in November 2024 at a duck sanctuary in Wahiawa in Central Oahu.
State officials tested 10 birds at Susie’s Duck Sanctuary after a backyard flock of birds suddenly fell ill, and confirmed the dreaded H5N1 virus in pet ducks, geese, and one wild zebra dove.
More than 70 waterfowl, including two of the sanctuary’s ducks adopted by a family at Pearl Harbor, either died or were euthanized, and the site was quarantined.
That same month, results from a healthy, wild duck tested by the USFWS at the James Campbell National Wildlife Refuge on Oahu’s North Shore also came back positive.
But there were no signs of further spread among animals or people, including at a Mililani pet fair where the ducks had been last November, nor elsewhere until recently.
In October, the Hawaii Department of Health warned of a presumptive case in a sick duck found at the University of Hawaii at Manoa campus on Oahu. In November, a second migratory duck found along the Wailea shoreline on Maui also was confirmed to have H5N1.
State officials say the risk of infection to poultry and humans in Hawaii remains low.
But these recent signs are concerning to Whoriskey and her team, especially during Hawaii’s migratory bird season.
“It’s been a really effective virus,” Whoriskey said. “It’s managed to travel around the whole globe with migratory birds, and I think we were safe for a long time because we’re so isolated in Hawaii. Now it’s made its way here, too.”
She said, “It definitely makes us that much more nervous, and more empowered to keep going with this vaccine trial to try to see if we can prevent a catastrophic outbreak.”
Most endangered seals
Hawaiian monk seals are endemic, meaning found nowhere else in the world but Hawaii, according to the National Oceanic and Atmospheric Administration.
They are also considered one of the most endangered seal species in the world, and continue to be protected by state and federal laws.
Only about 1,600 seals remain in the wild, mostly in the remote isles of Papahanaumokuakea Marine National Monument, while increasingly more are being born in the main Hawaiian isles.
A decade ago, there were only about 1,100 remaining in the wild.
NOAA has worked for years to bring the species back from the brink of extinction, with scientists and volunteers making painstaking efforts to rescue seals in need of help.
The Marine Mammal Center opened Ke Kai Ola, a dedicated monk seal hospital in Kona at a cost of $3.2 million in 2014, and has since partnered with NOAA to rescue and rehabilitate the seals.
Most are malnourished pups from Papahanaumo- kuakea, but others, including RU28’s mother, needed intervention due to a tooth infection, hooks lodged in stomachs and mouths, or treatment for toxoplasmosis, a parasitic disease.
During peak birthing season, monk seal mothers and pups need protection along popular shorelines, where humans or unleashed dogs may disturb or harm them.
NOAA Fisheries celebrated a milestone a few years ago, as the monk seal population in 2021 surpassed 1,500 for the first time in more than 20 years.
Vaccinating Hawaiian monk seals is not new for NOAA, which launched a program in 2015 to protect them against morbillivirus — a known threat causing marine mammal deaths worldwide.
Since 2016, NOAA has been vaccinating monk seals, including those at Papahanaumokuakea against morbillivirus. Scientists believe it has achieved herd immunity at some isles.
For wild seals, NOAA has developed a technique using a pole syringe to vaccinate a seal from a safe distance.
The next steps after the avian influenza vaccine trial, however, would be in collaboration with NOAA, which oversees management of the seals.
NOAA said in a statement that it relies on the best available science to protect, conserve, and recover more than 160 endangered and threatened species.
“We will take the Center’s results into consideration as we work to ensure the survival of our nation’s at-risk marine species,” said NOAA spokesperson Rachel Hager in an emailed statement.
Whoriskey is hopeful.
“We know so little,” she said. “There’s so much that we have to extrapolate from medicine and other animals because obviously, this vaccine wasn’t designed for Hawaiian monk seals. Generally, they’re really, really safe.”
There are many questions yet to be answered, with the biggest being whether the vaccines are effective or not.
If the seals’ blood samples show some sign of antibody response, she said, that offers some reassurance it could protect the vulnerable monk seals against the virus — and that, she said, “is better than nothing.”
HAWAIIAN MONK SEALS
The mammals are endangered under the Endangered Species Act.
>> Its scientific name is neomonachus schauinslandi.
>> An estimated 1,600 remain in the wild, with nearly 1,200 at Papahanaumokuakea, and 400 in the main Hawaiian isles.
>> Monk seal population declined for approximately six decades but today is increasing due to NOAA’s recovery efforts. Still, it is only about a third of its historic size.
>> Protected against harassment and harm by state and federal laws. NOAA recommends viewing seals from a distance of at least 50 feet on shore and in the water; 150 feet for mom-and-pup pairs.
>> Threats include shark predation, entanglement in fishing gear, habitat loss, contaminants, diseases, and human-seal interactions, including intentional killing by humans.
>> See a seal in need of help? Call NOAA’s marine mammal hotline at 888-256-9840.
Source: NOAA Fisheries