Spotted seals (Phoca largha) are medium-sized, ice-associated seals found across the North Pacific, from the Bering Sea and Sea of Okhotsk south to the coasts of China, Korea, and Japan. Named for the dark, irregular spots that dapple their silvery-gray coats, they are closely related to harbor seals and were once considered the same species. Their life cycle is tightly bound to seasonal sea ice, where they give birth, nurse pups, and mate, but they spend much of the year on long coastal migrations that can cover thousands of kilometers. That combination of ice dependence and wide-ranging movement makes them both fascinating and increasingly vulnerable.
Range and Migration
Spotted seals breed on the pack ice of the Bering, Chukchi, and Okhotsk Seas, as well as in the Bohai Sea off northeastern China, during late winter and early spring. Once breeding wraps up and the ice melts, the seals disperse along coastlines to summer feeding grounds. A satellite-tagging study of young spotted seals from Peter the Great Bay in the Sea of Japan found that they migrated north and east along the mainland coast, traveling between roughly 2,300 and 3,100 km to reach their summer habitats in the northern Tatar Strait and Aniva Bay.1PubMed Central. Migrations of young spotted seals (Phoca largha) from Peter the Great Bay, Sea of Japan/East Sea, and the pattern of their use of seasonal habitats Once settled at a summer site, each seal stayed within a compact, isolated area from August through October before reversing course in November. The return journey followed the same route but at a faster pace, and by December the seals were back at their natal islands.
This faithfulness to specific summer sites and natal islands suggests a degree of site fidelity that researchers are still working to understand. A separate tagging effort on Baengnyeongdo Island off the Korean Peninsula tracked a free-ranging spotted seal for 107 days. That individual stayed within a roughly seven-kilometer radius of its preferred haul-out rocks for most of the monitoring period, moving back and forth between just two resting spots.2Fish and Aquatic Sciences. First satellite tracking of a free-ranging spotted seal (Phoca largha) from the Baengnyeongdo Island For an animal capable of crossing thousands of kilometers seasonally, that kind of local attachment during the non-breeding months is striking.
Diving, Feeding, and Diet
Spotted seals are benthic generalists during their foraging season: they eat a wide variety of fish, shrimp, and cephalopods, and they find most of their food close to the ocean floor. Oceanographic tracking in the Pacific Arctic has shown that they mostly dive to near-bottom depths, frequently reaching the sea floor itself.3Continental Shelf Research. Oceanographic influences on spotted seal foraging in the Pacific Arctic In the shallow shelf waters they prefer, those dives are typically modest, often under 100 meters, though the exact depth depends on local bathymetry.
What the seals eat matters not only for their own health but also for understanding the broader Arctic food web. Fatty acid and stable isotope analyses from the Bering Sea have revealed that spotted seals derive a large share of their nutrition from food chains rooted in sea-ice algae. During cold periods with extensive ice coverage, roughly half to two-thirds of the fatty acids in their blubber traced back to ice-algal production rather than open-water phytoplankton.4PubMed. Carbon sources and trophic relationships of ice seals during recent environmental shifts in the Bering Sea The same study found clear evidence of resource partitioning among Arctic seal species: spotted seals and ribbon seals occupied a similar dietary niche, while bearded seals and ringed seals fed on distinctly different prey. That overlap with ribbon seals raises questions about how the two species will compete if warming waters push both toward the same shrinking habitat.
How Spotted Seals Sense Their World
Like other phocid seals, spotted seals hunt in waters that are frequently murky, dimly lit, or both. Vision plays a role, but their whiskers are arguably the more impressive sensory tool. Seal whiskers have a distinctive undulating shape, with repeating bumps along the shaft, that suppresses the kind of self-generated vibrations that would otherwise create background noise as the seal swims. This design dramatically improves the signal-to-noise ratio of the sensory system, and studies estimate that seals can use their whiskers to track a fish swimming as far as 180 meters away, a detection range comparable to dolphin echolocation.5PubMed Central. Creating underwater vision through wavy whiskers: a review of the flow-sensing mechanisms and biomimetic potential of seal whiskers
Their eyes also have specialized adaptations for life in cold water. Marine mammals face a basic problem: blood flowing to the eyes from the warm body core can be cooled dramatically by the surrounding water, and the retina’s photoreceptors work best within a narrow temperature range. Spotted seals and other marine mammals address this through a specialized vascular structure called the ophthalmic rete, a dense network of blood vessels behind the eye that acts as a heat exchanger. This structure helps maintain a stable operating temperature for the retina and also dampens blood-flow fluctuations that could affect vision.6PubMed. Comparative anatomy of the ophthalmic rete and its relationship to ocular blood flow in three species of marine mammal
Underwater Vocalizations
Spotted seals are vocal animals, especially during the breeding season, but their calls are not particularly elaborate compared to some other marine mammals. Underwater recordings from Liaodong Bay in China identified four major call types that accounted for about 78% of all vocalizations: knocks, growls, drums, and sweeps.7PubMed. Classification of underwater vocalizations of wild spotted seals (Phoca largha) in Liaodong Bay, China These calls are brief, typically lasting between 12 and 270 milliseconds, and are pitched low, with peak frequencies below 600 Hz. Researchers described the overall repertoire as extensive but limited in complexity. That distinction matters: a wide variety of call types gives the seals communicative flexibility, but the calls themselves lack the layered, frequency-modulated structure you hear in, say, humpback whale song or walrus clicks.
Breeding behavior involves courtship displays that take place almost entirely underwater. Observations of captive spotted seals documented underwater mating during April and early May, and the courtship sequence was described as more complex and variable than what is typically seen in other seal species, with most pre- and post-mating behaviors and vocalizations drawn from the animals’ year-round behavioral repertoire.
The Energy Cost of Molting
Every year, spotted seals go through a complete molt, shedding and regrowing their entire coat of fur. This is one of the most energetically expensive events in their annual cycle. Metabolic measurements of captive spotted and ringed seals showed that resting metabolic rate increased sharply during the molt, peaking around the time the new coat was about halfway grown in. For some individuals, metabolic costs more than doubled compared to non-molting periods.8Conservation Physiology. Molting strategies of Arctic seals drive annual patterns in metabolism Air temperature also influenced metabolic rate in spotted seals, with higher ambient temperatures pushing costs up further.
During the molt, spotted seals spend more time hauled out on land or ice, which makes sense: growing new skin and fur is easier when the animal is not losing heat to cold water. This haul-out behavior makes them more visible and more countable, which is useful for researchers conducting population surveys. But it also makes them more vulnerable to disturbance. Boat traffic, aircraft overflights, and human foot traffic near haul-out sites can cause molting seals to flush into the water, interrupting a process that already demands enormous energy reserves.
The Sibling Species Problem
Spotted seals and harbor seals look similar enough that field identification can be tricky, especially in areas where their ranges overlap in the North Pacific. For decades, scientists debated whether they were actually one species. Genetic work has settled the question: the two are distinct species with no evidence of hybridization at nuclear DNA markers, even in zones where they co-occur.9PubMed Central. Surreptitious sympatry: Exploring the ecological and genetic separation of two sibling species
That same genetic study did turn up something interesting. Out of 20 tagged seals, six carried mitochondrial DNA haplotypes typical of spotted seals rather than harbor seals, despite being identified in the field as harbor seals. Comparison with large reference datasets of both species ruled out misidentification and incomplete lineage sorting, pointing instead to the two species being reciprocally monophyletic in their mitochondrial DNA, meaning their maternal lineages are completely sorted into two non-overlapping groups. Bayesian analysis of nuclear DNA confirmed two discrete genetic clusters with no signs of admixture. In other words, these two species live alongside each other in some areas but are not interbreeding, at least not to any detectable degree. The ecological separation, likely driven by spotted seals’ dependence on pack ice for breeding while harbor seals use terrestrial sites, appears to be holding firm.
Sea Ice, Climate, and Body Condition
Because spotted seals rely on sea ice for pupping, nursing, and mating, declining Arctic and sub-Arctic ice coverage is a central concern for the species’ future. Pups born on the ice need stable platforms for several weeks while they nurse and build up blubber reserves. If the ice breaks up or melts earlier than usual, pups can be separated from their mothers prematurely.
A long-term study of body condition in Bering Sea seals found that, overall, body condition in ice-associated seals declined over the study period. Pups of all species showed a particularly steep annual decline. Spotted seals were something of an exception: across all ages and sexes, they showed a slight annual increase in body condition that partially offset the broader downward trend.10Regional Studies in Marine Science. Genomic comparison of male and female spotted seals in the Liaodong Gulf, China But spotted seal pups still showed declining body condition over time, tracking the pattern seen in ribbon and harbor seal pups. The finding suggests that adult spotted seals may be adapting their foraging behavior or benefiting from shifts in prey availability, while their pups, dependent on ice for those crucial first weeks, remain vulnerable.
The broader picture is sobering. As ice retreats and the timing of freeze-up and breakup shifts, the entire sympagic (ice-based) food web that supplies roughly half or more of a spotted seal’s caloric intake could be disrupted. If the ice-algal production that underpins their food chain shrinks, the seals will have to rely more heavily on open-water food webs, which may not be as productive in the same areas.
Pollutants in Their Blubber and Tissues
Spotted seals occupy a mid-to-upper trophic level in marine food webs, which means they accumulate pollutants from the prey they eat. This bioaccumulation has been documented in several studies across the species’ range. In the Bohai and northern Yellow Seas, trace elements including mercury, arsenic, cadmium, zinc, and others were detected in all tissue samples from stranded spotted seals. Zinc was the dominant contaminant across all tissue types, and concentrations of several elements, especially cadmium, increased with body length, a sign of ongoing accumulation over the animal’s lifetime.11PubMed. Tissue distribution and trophic magnification of trace elements in typical marine mammals in Bohai and north Yellow Seas
Heavy metal accumulation patterns differ between tissues. A study comparing wild and captive spotted seal pups in the Bohai Sea found that metals like aluminum, vanadium, and mercury accumulated preferentially in the hair of wild pups, while zinc and selenium were highest in feces. The overall pattern was hair accumulating more metals than feces, and feces more than blood.12PubMed Central. Differences in Heavy Metal Accumulation in Wild and Captive Spotted Seal (Phoca largha) Pups in the Bohai Sea That distribution matters for researchers: it means hair sampling, which is non-invasive, can be a useful proxy for monitoring contaminant exposure in wild populations.
Organic pollutants are also a concern. Blubber samples from spotted seals in the western Sea of Japan contained organochlorine pesticides, specifically DDT and HCH, in every individual tested. DDT concentrations were especially high and variable, ranging over more than an order of magnitude between individuals, with average levels far exceeding HCH concentrations.13Marine Pollution Bulletin. Organochlorine pesticides (HCH and DDT) in blubber of spotted seals (Phoca largha) from the western Sea of Japan DDT was banned in most countries decades ago, but it persists in the environment and continues to cycle through marine food webs, concentrating in the blubber of top predators.
The Liaodong Gulf Population
Not all spotted seals live in the vast expanses of the Bering or Okhotsk Seas. One of the most isolated and endangered subpopulations breeds in Liaodong Bay, in China’s Bohai Sea. This is the southernmost breeding population, and it exists under very different pressures from its northern relatives. The Bohai Sea is shallow, semi-enclosed, and surrounded by some of China’s most industrialized coastline. Pollution, shipping, coastal development, and fisheries bycatch all create risks for the seals there.
Genomic analysis of spotted seals in the Liaodong Gulf has revealed a recent population decline, detected through demographic modeling based on genome-wide markers.14Regional Studies in Marine Science. Genomic comparison of male and female spotted seals in the Liaodong Gulf, China The combination of a small, isolated population, ongoing anthropogenic stresses, and limited genetic exchange with other spotted seal groups makes this subpopulation particularly fragile. China designated the spotted seal as a national first-class protected species, and the Liaodong Bay breeding area is a focal point for conservation efforts, including monitoring, anti-poaching enforcement, and occasional rescue and rehabilitation of stranded pups.
Historical Commercial Harvest
Spotted seals were hunted commercially in the Sea of Okhotsk from the early 1970s through the mid-1990s as part of a broader harvest of ice-associated seals. During that period, harvest records documented roughly 90,000 spotted seals taken, making them the third most commonly harvested species behind ringed and ribbon seals.15PubMed Central. The commercial harvest of ice-associated seals in the Sea of Okhotsk, 1972-1994 What stands out is the age structure of the catch: molted pups made up over 52% of all spotted seals harvested, a much higher proportion of young animals than was seen in the harvests of ringed or bearded seals. That heavy targeting of juveniles would have had outsized effects on recruitment and population growth, since those are the animals that have not yet had a chance to reproduce.
The Okhotsk commercial hunt ended in the mid-1990s following the dissolution of the Soviet Union and growing international pressure over marine mammal conservation. Today, subsistence harvests of spotted seals continue among Indigenous communities in Alaska and the Russian Far East at much lower levels. Alaska Native hunters take spotted seals for food and materials, and assessment work on ice seal harvests in Alaska has concluded that current subsistence take is sustainable. Outside of those traditional harvests, direct killing of spotted seals is now rare, though bycatch in commercial fisheries and entanglement in marine debris remain ongoing sources of mortality.
Biomimetic Potential of Seal Whiskers
The engineering world has taken notice of the spotted seal’s whiskers, or more precisely, the whiskers shared across the phocid seal family. The undulating geometry of a seal whisker, with its regularly spaced bumps and valleys, solves a problem that plagues underwater sensors: vortex-induced vibrations. When water flows past a smooth cylinder, it creates alternating vortices that shake the structure, generating noise. The wavy cross-section of seal whiskers disrupts that vortex pattern and dramatically reduces self-generated vibration.16PubMed Central. Creating underwater vision through wavy whiskers: a review of the flow-sensing mechanisms and biomimetic potential of seal whiskers Engineers have been developing whisker-inspired flow sensors for underwater robotics, aiming to give autonomous underwater vehicles the ability to detect and track objects by reading the hydrodynamic wakes they leave behind, much the way a seal tracks a fleeing fish through dark water. The estimated 180-meter detection range that seal whiskers achieve gives some sense of why this is worth pursuing: that kind of passive sensing could complement or even replace active sonar in certain applications, with no energy cost for emitting a signal and no acoustic disturbance to marine life.