Seal populations around the world are turning up with a troubling mix of threats: deadly bird flu spreading between mammals, industrial chemicals accumulating in their tissues, microplastics lodged in their digestive systems, and antibiotic-resistant bacteria that originate from human activity. At the same time, some populations are rebounding while others face collapse from vanishing sea ice. Here’s what researchers are finding and why it matters.
Bird Flu Is Killing Seals by the Thousands
The most dramatic recent finding is the spread of highly pathogenic avian influenza (H5N1) into seal and sea lion colonies across South America. In Peru alone, 5,224 sea lions were found dead or dying on beaches between January and April 2023, many showing neurological and respiratory symptoms. A single island off Peru’s coast, San Gallan, accounted for over 1,100 of those deaths. Thousands more sea lions died with similar symptoms in Chile during the same period.
What makes this especially alarming is the evidence of mammal-to-mammal transmission. Genomic analysis of an H5N1 outbreak in elephant seals in Argentina found that viruses from seals across Peru, Chile, Brazil, Uruguay, and Argentina form a distinct genetic cluster, sharing an identical set of mutations associated with adaptation to mammals. The ecological and genetic data together support the conclusion that the virus is spreading directly between mammals, not just jumping repeatedly from birds. Researchers also found evidence of occasional spillover back from mammals to birds, raising concerns about the virus continuing to evolve in marine mammal populations.
Industrial Chemicals in Seal Tissue
PFAS, the group of synthetic chemicals sometimes called “forever chemicals” because they don’t break down in the environment, are showing up at significant levels in Arctic seal populations. Ringed seals in East Greenland have been found with PFOS concentrations up to 1,330 nanograms per gram in their liver tissue. Seals in western Hudson Bay carried lower but still notable levels, up to 140 nanograms per gram. Other PFAS compounds are present too: one related chemical reached 39.2 nanograms per gram in East Greenland seals.
These numbers matter for two reasons. First, ringed seals are the primary prey of polar bears, so contaminants concentrate further up the food chain. Second, ringed seals are an important food source for many Inuit communities. In the Ittoqqortoormiit region of East Greenland, the estimated PFOS levels in ringed seal blood plasma averaged about half the highest concentrations found in the local human population, illustrating how tightly connected these food webs are.
Microplastics in Nearly Every Seal Examined
A study of 62 harbour seal and grey seal carcasses from German waters found microplastics in roughly 85% of the animals’ intestines. Researchers identified 540 suspected microplastic particles across the samples, split between fibers (42%) and fragments (58%). Harbour seals carried anywhere from zero to 81 particles per individual, averaging about 9.5. Grey seals ranged from zero to 43, averaging about 8.
The presence of microplastics in such a high percentage of wild seals confirms that plastic pollution has become a near-universal contaminant in marine food chains. Seals ingest these particles primarily through the fish they eat, meaning the problem reflects broader contamination of the ocean rather than direct contact with plastic debris.
Antibiotic-Resistant Bacteria From Human Sources
Nearly 40% of marine mammal samples from the North and Baltic Seas carried antibiotic-resistant E. coli, a finding that stood in stark contrast to fish from the same waters, which showed no resistant bacteria at all. Harbour seals and ringed seals predominantly carried a strain type (phylogenetic group B2) that is commonly associated with human infections, while harbour porpoises carried a different type more typical of environmental sources.
The resistant bacteria found in seals carried genes conferring resistance to multiple drug classes, including common antibiotics used to treat streptomycin-sensitive infections, sulfonamide-treated conditions, and tetracycline-responsive illnesses. The likely sources are sewage discharges, agricultural runoff, and healthcare wastewater entering coastal ecosystems. Seals, as long-lived predators that haul out on beaches and swim through coastal waters, accumulate these bacteria and serve as living indicators of how far antibiotic resistance has spread into wild environments.
Distemper Virus Remains a Background Threat
Phocine distemper virus has caused two catastrophic outbreaks in North Sea harbour seals, killing approximately 23,000 animals in 1988 and 30,000 in 2002. Both events caused fatal pneumonia across large portions of the population. In the years following each outbreak, antibody levels in the population gradually declined, indicating the virus stopped circulating and leaving younger seals with no immunity. That pattern of fading protection is what set the stage for the second outbreak 14 years after the first, and it raises ongoing concerns about future vulnerability.
Sea Ice Loss Is Devastating Seal Pups
Harp seals depend on stable pack ice to give birth and nurse their young. When ice conditions are heavy, pup mortality typically runs between 1% and 1.4%. When ice is thin or absent, the numbers reverse dramatically. In the worst year on record for the Gulf of St. Lawrence, when the winter climate index dropped to its lowest point and ice coverage hit a satellite-era minimum, harp seal pup mortality approached 100% in that region.
This isn’t a gradual decline. It’s a binary outcome: sufficient ice means most pups survive, and insufficient ice means most don’t. As Arctic and subarctic winters continue to shorten and warm, breeding habitat for ice-dependent seal species is disappearing on a timeline measured in decades rather than centuries.
Some Populations Are Recovering
Not all the news is grim. Grey seal pup production across the Greater North Sea and Celtic Seas is largely increasing or stable. In the UK, some colonies have grown rapidly. South-East England saw a 75% increase in pup production between 2014 and 2019. North-East England grew by 53%, and East Scotland by 28% over the same period. More established colonies in Scotland’s north coast and Orkney grew by just 1%, suggesting they may be approaching the carrying capacity of their habitat.
The Western Isles produced over 16,000 pups in 2019, and North Coast and Orkney led all UK colonies with more than 22,000. These recoveries reflect decades of legal protection and reduced hunting pressure, though the fast-growing colonies in England partly represent range expansion into areas where grey seals were previously scarce rather than pure population growth.
Shifting Food Webs
Long-term analysis of harbour seal bones spanning from 1928 to 2014 has revealed how coastal food webs have shifted over the past century. By measuring chemical signatures locked in bone tissue, researchers can reconstruct what seals were eating and how productive the surrounding ocean was at the time. In the Gulf of Alaska, the data show a change in the base of the food web since 1975, with primary production shifting in ways that don’t match historical patterns. These changes track with large-scale climate cycles and coastal upwelling patterns.
The picture varies by region. Food webs along the Washington coast and in the Salish Sea responded more to local freshwater discharge than to ocean-wide climate patterns, and showed no strong directional trend over the century. The takeaway is that seal diets and the ecosystems supporting them are not responding uniformly to environmental change. Some regions are experiencing fundamental shifts in productivity, while others have remained relatively stable, at least so far.