Japanese Sea Lion

One of the most recently extinct marine mammals

At almost the same time and for the same reasons, two marine mammals were wiped out in different places in the 20th century: the Caribbean monk seal and the Japanese sea lion. Both were hunted commercially: lamp oil was extracted from their blubber, leather was made from their hides, pipe cleaners from their whiskers, the canine teeth were used as jewelry or lucky charms, and the internal organs served medicinal purposes. At the end of the 19th and beginning of the 20th century, people also began catching sea lion pups and selling them to circuses or zoos.

In Remembering Extinction of Sea Lions on Dokdo Island (2021), the Russian journalist Aleksandra Bykova points out that illegal hunting of Japanese sea lions had already begun in the 17th century. When Japanese hunters learned of the huge sea lion populations said to live near the Dokdo island group, they set out for the islands. The fact that they had to enter Korean waters and intrude into the territory of their neighboring country did not matter to them.

The Japanese businessman and fisherman Nakai Yozaburo finally had the idea in 1904 of leasing Dokdo in order to be able to hunt sea lions on an even larger scale. Nakai Yozaburo contacted various ministries of the Japanese government and asked for Dokdo to be incorporated into the state. When the Japanese government, while preparing for war against Russia, became aware of the islands’ strategic importance, it claimed that Dokdo was terra nullius and began incorporating the island group into Shimane Prefecture in 1905. That same year, Nakai Yozaburo founded a fishing and hunting company there and Dokdo received a new official name: Takeshima.

With the appropriation of Dokdo and Japan’s occupation of the Korean Peninsula, the number of sea lions declined rapidly. Japanese fishermen hunted the marine mammals from 1903 to 1941. In 1904 alone, around 3,200 animals are said to have been killed, according to the South Korean news agency Yonhap News Agency in a 2019 article. Records on commercial fishing from the early 1900s show that around the turn of the century, roughly 3,000 Japanese sea lions were caught. By 1915 there were only 300 sea lions left, and in the 1930s only a few dozen.

From the 1940s onward, Japanese fishermen stopped hunting the sea lions, because by then they were practically already extinct. According to reports, the trawl nets of Japanese trawlers are said to have caught around 16,500 sea lions during Japan’s colonial rule—enough to wipe out a species.

Japanese aea lion – fact sheet

alternative names	Japanese sealion, Black sea lion, Dokdo sea lion
scientific names	Zalophus japonicus, Zalophus lobatus, Zalophus californianus japonicus
original range	Sea of Japan (Japan, Korea, Russia)
time of extinction	after 1951
causes of extinction	hunting, habitat loss, loss of prey animals
IUCN status	extinct

Population simulation illustrates the impact of hunting

To investigate the effects of human hunting on Japanese sea lions, researchers at Busan University in South Korea published a population simulation for the Japanese sea lion for the Dokdo island group in the Sea of Japan in 2022. The last reliable sea lion sightings from 1951 also come from Dokdo. The simulation is based on fragmentary historical and official reports on catch numbers in Korea and Japan and focuses on the period from 1900 to 1951. It shows that it took less than ten years for the original sea lion population to decline by 70 percent because of commercial hunting.

The population simulation was run with several hunting ban scenarios— with the result that if hunting of the sea lions had been stopped after six years, in 1910, it would have taken a full 120 years for the original population from 1900 to recover completely. After 14 years of hunting until 1920, it would take about 220 years for the population to recover. The scientists agree that the extinction of the Japanese sea lions was primarily caused by commercial hunting. The study clearly shows how great an influence overhunting has on natural population recovery.

Environmental changes could not be taken into account in the study, because almost nothing is known about the ecological role the Japanese sea lion once played in the marine ecosystem. Because it was long assumed that the similar-looking California sea lion and the Japanese sea lion were conspecifics, it was also assumed that they were identical in their habits and life cycle, and no one made the effort to study them. Japanese sea lions became a subject of research only after they were already extinct.

New findings on the ecological role

For a long time, almost nothing was known about what role the Japanese sea lion once played in the marine ecosystem. A study published in 2025 by Pusan National University has, for the first time, used stable isotope analyses of bone collagen to provide evidence of the species’ diet and way of life. The researchers examined bone remains discovered on Ulleungdo in 2021—the find dates from between 1548 and 1952 AD and is considered the most recent physical evidence of the species in Korea to date.

By analyzing carbon and nitrogen isotopes, the scientists were able to reconstruct which prey animals the Japanese sea lion preferred in different eras. The result: in the early and middle Chulmun period (ca. 6000–2000 BC), the animals stood at the top of the food chain and mainly hunted large predatory fish and squid. In later periods such as the Mumun period (ca. 1500–100 BC), there is evidence of a diet more strongly focused on coastal prey such as smaller fish and bottom-dwelling invertebrates.

The most recent samples from Ulleungdo finally show a clear shift toward pelagic prey animals, that is, species that live in the open sea. The study therefore demonstrates that Japanese sea lions were remarkably adaptable: over thousands of years, they were able to flexibly adapt their food spectrum to environmental changes and prey availability.

Comparisons with present-day marine mammals from the Sea of Japan—including the common minke whale (Balaenoptera acutorostrata) and the largha seal (Phoca largha)—also showed that the Japanese sea lion occupied a unique trophic niche. Its higher nitrogen values indicate that it stood above these two species in the food chain, meaning that it held an ecological role as an important coastal predator.

The study is an important contribution to the reconstruction of East Asia’s marine ecosystems. It shows that the Japanese sea lion, as an important component of coastal ecology, played a central role in the marine food web. Its eradication through overhunting is likely to have permanently disrupted the ecological balance of coastal regions, which may also have affected fish stocks.

Japanese Sea Lions: Further Reasons for Their Disappearance

It was not only hunting of the Japanese sea lions themselves that may have led to their disappearance. Since the marine mammals fed on fish—preferably anchovies and herrings—, commercial overfishing, which was not stopped until 1940, when there were hardly any fish left, led to a reduced food supply for the sea lions. Overfishing also ultimately proved fatal to the Japanese river otter (Lutra nippon). In Brehms verlorenes Tierleben (2007), Hanna Zeckau and Carsten Aermes also point out that when sea lions tried to eat fish caught in fishing nets, they sometimes sustained fatal injuries.

The loss of habitat through the destruction of coastal ecosystems may also have contributed to the decline in Japanese sea lion numbers. Some scientists have occasionally suggested that submarine warfare during the Second World War had a major negative impact on the sea lions’ habitat. In addition, Korean soldiers are said, according to rumors, to have used Japanese sea lions for target practice during the war.

Environmental changes may also have caused the rapid disappearance of the Japanese sea lions. In a 2009 article on sea lions of the genus Zalophus in the Encyclopedia of Marine Mammals , the American biologists Carolyn B. Heath and William F. Perrin point to the possibility that environmental changes or human overfishing may have occurred in connection with El Niño events (ENSO). The climate phenomenon may, for example, have led to a reduction in the main food source or to changes in the sea lions’ behavior, such as reproduction or migration.

The IUCN has listed the Japanese sea lion as extinct since 1994, since no animals have been found since the late 1950s despite extensive research on marine mammals in the sea lions’ former range. The Korean government placed the Japanese sea lion on its list of highly endangered species in 1998. In addition, the South Korean Ministry of Environment published leaflets for fishermen with pictures of the sea lion so that they would release these animals again should they accidentally become caught in nets.

It is always only about Dokdo or Takeshima

When it comes to the disappearance of the Japanese sea lions, the focus is usually only on Dokdo—an island group in the Sea of Japan to which both Japan and North and South Korea lay territorial claim. During the Japanese occupation, the archipelago was given the name Takeshima, and third countries call it Liancourt Rocks to avoid taking a position in the conflict over the territorial claims.

One might almost think that the Japanese sea lion lived exclusively around the Liancourt Rocks, but its range was much larger: it extended along the northwestern Pacific coast of Japan and Russia to Korea. In the Sea of Japan, which South Koreans incidentally call the East Sea, it occurred on the west and east coasts of Japan and on the east coast of Korea. In Russia, the sea lions are said to have been found along the Kuril island chain, on Sakhalin, the country’s largest island, and at the southern tip of the Kamchatka Peninsula. Japanese sea lions are also said once to have lived on the northern Ryūkyū Islands belonging to Japan.

In the literature, in order to document the species’ decline, the actually large range of Japanese sea lions is usually reduced to the Liancourt Rocks, or Dokdo or Takeshima. The reason for this may be that at the beginning of the 20th century, sea lions essentially existed in larger numbers only there. In Marine Mammals of the World (1998), the American zoologist Dale W. Rice also names Dokdo and the island of Ulleungdo, 90 kilometers away, as the most important Korean habitats of the marine mammals in the 1900s.

But what happened before 1900? Archaeological finds indicate that Koreans and Japanese living in coastal communities were already hunting sea lions in prehistoric times. The anthropologists Kyungcheol Choy and Michael P. Richards published a study on the subject in 2010. By analyzing bone collagen, the scientists were able to show that prehistoric coastal communities on the southeastern coast of Korea mainly fed on marine mammals, including sea lions.

Prehistoric hunting of sea lions to obtain food probably did not have major effects on population numbers, but commercial hunting with modern techniques certainly did. The California sea lion (Zalophus californianus), endemic to southern California and Mexico, which was also hunted for its hide and blubber, was therefore on the verge of extinction in the 19th century and was still extremely rare in the wild at the beginning of the 20th century.

That the Japanese sea lion had almost disappeared from Japan’s coastal waters in the early 1900s was probably also due to previous overhunting by Japanese fishermen targeting the meat and oil of the marine mammals. Kazue Nakamura wrote this in 1991 in An Essay on the Japanese Sea Lion.

The commercial hunting pressure must eventually have caused the geographic boundaries of the Japanese sea lions’ range to shrink, and the animals were ultimately found only on the previously untouched and uninhabited Liancourt Rocks (Dokdo) and on Ulleungdo . The coasts of both island areas are also known to have served the sea lions as places for reproduction and for rearing their young. Around 20,000 Japanese sea lions are said once to have lived on Dokdo and Ulleungdo.

Japanese killed all the sea lions?

South Korea and Japan disagree not only over who has territorial claims to Dokdo or Takeshima, but also over who is responsible for the extinction of the Japanese sea lion. Japan has a poor hand here thanks to its Killing Culture; the country still practices commercial dolphin and whale hunting and argues that hunting marine mammals is a Japanese tradition.

In 2020, an image shared on the social media platforms Facebook and Instagram caused a stir (JapanForward reported on it): a South Korean professor used, without permission, an image by the Japanese artist Yumiko Sugihara showing sea lions playing on the beach on Dokdo, turned it into a parody and changed its caption to: “Japan killed the Dokdo sea lions!” Japan had originally used the image to promote a summer exhibition for children in Shimane Prefecture.

Whether Japanese or South Koreans are responsible for the last Japanese sea lions cannot be said. What is certain is that Japan gave up the colony of Dokdo in 1945 and that the islands have again been administered by South Korea since 1953. To prove their innocence, the Japanese cite unconfirmed sightings, according to which a fisherman, for example, claimed to have seen sea lions in the Takeshima area in 1954.

Around 200 to 500 Japanese sea lions are also said to have been observed there in 1958, after South Korea had begun patrolling the area. The latter is said to come from a report by the Korea Hydrographic and Oceanographic Agency. It is said (at least according to Japanese sources) that hunting of the Japanese sea lions by South Koreans continued without conservation measures after the Japanese had left Dokdo. The last Japanese sea lion is said to have been observed in 1975.

In any case, the IUCN considers a 1951 sighting of 50 to 60 individuals on Dokdo to be the last confirmed evidence for the existence of the Japanese sea lion. All sightings reported in the 1970s and 1980s are considered unconfirmed. So we do not know who killed the last sea lions.

Recent reported sightings of Japanese sea lions are also ambiguous: in July 2003, individual sea lions were seen in Iwami in Tottori Prefecture in Japan and in March 2016 on the Japanese Koshikijima Islands. Based on photos, experts were able to confirm that they were eared seals, but it was not clear which species they belonged to. Perhaps they were simply California sea lions that had escaped from amusement parks or zoos.

Not a Subspecies of the California Sea Lion After All

Because of their visual similarity, scientists long assumed that Japanese sea lions were a subspecies of the California sea lion. A Japanese study from 2007 was able to prove the opposite: researchers at the Nagoya University Museum extracted ancient DNA from archaeological bone finds of Japanese sea lions and were able to show that the California and Japanese sea lions had already diverged evolutionarily in the early Pleistocene .

Additional certainty came in 2021 from the phylogenetic analysis of the complete mitochondrial genome of the Japanese sea lion based on skeletal remains excavated on the island of Ulleungdo: The Japanese sea lion is the sister species of the California sea lion. With an mtDNA match of more than 98 percent, the two species in the genus Zalophus are most closely related. After the California sea lion, the closest living relative of the Japanese sea lion is the Steller sea lion (Eumetopias jubatus), endemic to the northern Pacific, with a match of more than 93 percent.

As early as 2003, Sylvia Brunner analyzed the skull morphology of fur seals and sea lions for her study Fur seals and sea lions: Identification of species and taxonomic review. She noticed that the skull of the California sea lion differs significantly from that of the Japanese sea lion and the Galápagos sea lion (Zalophus wollebaeki). Until that time, scientists still assumed that both the Galápagos sea lion and the Japanese sea lion were subspecies of the California sea lion. Today, the division of the genus Zalophus in the family of eared seals (Otariidae) into these three species is generally accepted.

2025: fully decoded genome reveals evolution

A genome study published in August 2025 confirmed and refined the earlier findings—and decoded the entire genome of the Japanese sea lion for the first time. The study was based on DNA from 16 bone fragments from Dokdo and Ulleungdo. The analysis provides additional insights into the genetic diversity of the Japanese sea lion, its place in the eared seal family tree and previously unknown gene flow events.

The data show that the Japanese sea lion is the earliest-branching lineage within its genus and (in line with earlier estimates) split from the California sea lion around 2.2 million years ago. The study indicates that the genome of the Japanese sea lion contains genetic admixture: while the California and Galápagos sea lions show genetic influences from the Steller sea lion, identifiable genetic contributions in the Japanese sea lion come mainly from the northern fur seal (Callorhinus ursinus) and, to a lesser extent, from the Steller sea lion. These introgressions probably arose through matings and hybridization that occurred during earlier overlaps in ranges and seasonal migrations .

Despite a population decline that had already begun more than 100,000 years ago, the Japanese sea lion retained comparatively high genetic diversity until shortly before its disappearance—higher than that of some marine mammals that still exist today but are highly threatened. This argues against inbreeding or genetic impoverishment as causes of extinction. Based on the study, the scientists confirm historical sources according to which massive overhunting—especially between 1904 and 1925—caused the populations to collapse within a few decades. Females and young animals were deliberately killed, which greatly reduced the reproductive rate.

The study shows: even genetically healthy populations can, within a short time, be wiped out by intense human exploitation pressure. The genome of the Japanese sea lion now provides an important key to understanding evolution and biodiversity loss within the eared seals.

Sea lions are supposed to return to the sea of Japan

People apparently do not quite want to accept that Japanese sea lions are extinct: in 2007, the South Korean Ministry of Environment announced that South Korea, North Korea, Russia and China wanted to cooperate and conduct research in order to bring sea lions back to the Sea of Japan by 2010. This comes from an article in the Korea Times. The ministry admits that “the animals in Korea and Japan are almost extinct, but there is the possibility that a few animals still exist in Chinese and Russian waters.”

Accordingly, the collaboration was to begin with the search for still-existing Japanese sea lions in Chinese and Russian territories. In the event that no surviving animals were found, the South Korean government planned to relocate California sea lions from the USA in order to restore the coastal ecology.

Well then: Japanese sea lions were in fact found, but only skeletal parts during archaeological excavations on Dokdo and Ulleungdo in 2019 and 2020. The bone remains could be clearly assigned to Japanese sea lions by means of DNA analysis, as shown, for example, by this Korean study published in the Ocean Science Journal in 2019.

The 2007 idea of bringing California sea lions into the former habitat of the Japanese sea lions has apparently come to nothing.