Recent surveys of the sea otter (Enhydra lutris) population on Kuril Islands

Introduction

According to the Red List of threatened species of the International Union for Conservation of Nature (IUCN), the sea otter (Enhydra lutris) is an endangered species (Doroff et al. 2021). The species greatly suffered from overharvesting for its precious pelts and was on the edge of extinction approximately 100 years ago. After the implementation of protection measures, sea otter populations increased in number, but they remain endangered. Several populations have declined again (Doroff et al. 2003; Estes et al. 2005; Mamaev 2018; Tinker et al. 2021). The distribution area of sea otters is now smaller than the initial range, which once represented a continuous strip in the coastal waters of the North Pacific from Japan through the Kurils, southern Kamchatka, Commander and Aleutian Islands, coasts of North America, to California. Now, the range is fragmented into a series of islets (Doroff et al. 2021). Assessing the distribution and abundance of sea otters remains a significant challenge for conservation biology, and such studies are especially urgent for the Kuril Islands where sea otters have suffered particularly hard. At the end of the 19th century, various restrictions on sea otter harvesting were introduced, and in 1911, their extermination was completely banned. However, poaching on the Kuril Islands persisted for several decades, particularly in the 1910–1920s (Shin 2014). Although protection measures were later implemented, ‘scientific’ harvesting continued (Nikolaev 1969). For example, in 1940, Japanese author K. O. Miyatake reported on 332 sea otter stomachs (cited from Shitikov (1971)). This intensive harvesting led to a catastrophic population decline, with only several hundred sea otters remaining in the Kurils by the 1940s (Nikolaev 1969; Voronov 1974). The situation began to improve after the Kuril Islands became part of Russia in 1945, which led to the deportation of the Japanese population, the abandonment of most settlements, and a significant reduction in economic activity. Since then, sea otter numbers have increased to several thousand: 6804 were reported in 1980 (Kostenko et al. 2004), 7800 in 2000 (Kostenko et al. 2004), approximately 10,000 in 2010 (Kornev 2010), and 6010 (with a 95% confidence interval of 4492–11,314) in 2012 (Ovsyanikova et al. 2020). However, declines have been reported on some islands (Kornev 2010), and since 2012, there has been no comprehensive assessment of the entire archipelago’s sea otter population. The other populations, i.e. the American ones and those of Commander Islands, have been regularly monitored over the past decades (Nikulin et al. 2008; Mamaev 2018; Shelton et al. 2018), but Kuril populations assessed only sporadically. ‘A bibliography on the sea otter’ (Camp 2023) contains over 700 sources, but only seven focus on the Kurils. The bibliography is not complete in respect of the Russian sources (cited below), but they are still not numerous. The Kuril Islands are nearly uninhabited and distant from scientific centres, making access challenging. Any opportunity to gather new information on sea otters is invaluable. Such an opportunity arose during the Russian Geographic Society’s expedition from 2019 to 2023, which included studying nature conservation objects such as the sea otter. Initial results in 2019 confirmed the urgency of these studies, as sea otters were found to be few in number, with evidence of poaching observed (Popov and Scopin 2021). Subsequent surveys have provided new data on the sea otter populations over a 400 km section of the Kuril archipelago, offering insights into their overall state across the region.

Methods

The surveyed section of Kurils is located in its southern and central parts. It included the islands of Iturup, Urup, Broughton, Chirpoy and Simushir. Iturup, the southernmost island, is ribbon-shaped with uneven edges, stretching from the south-west to the north-east. It is 200 km long, 6.5–30 km wide, and covers an area of 3200 km². Iturup has a population of 6500, mostly concentrated in the middle of the island, with its southern and northern parts uninhabited. Urup Island, located northeast of Iturup, is also ribbon-shaped but shorter at 116 km long and up to 20 km wide, covering an area of 1400 km². Urup is more hilly and less suitable for development. There were several military posts over the past decades, but now they are abandoned and the island is almost uninhabited. There are two lighthouses on Urup, where a few people work. Mining for gold took place at the southern end of the island recently, and a shift camp was built there, but now the gold deposit is worked out, the mining stopped. Further northeast of Urup, two small islands Black Brothers are located. Chirpoy is one of them. It is composed of two volcanoes surrounded by solidified lava flows. Its area is approximately 20 km². It has no population. Northwards from Chirpoy, away from the main line of the Kuril ridge, lies the small Broughton Island. Its area is 7.32 km². North of these is Simushir, another elongated island 59 km long and 4–15 km wide. Broughton and Simushir Islands are also uninhabited. All the islands have a jagged coastline, and small islets are often found off the coast, which are potential refuges for sea otters (Komedchikov 2009).

During the expedition, researchers reached various coastal points by boat, setting up camps for several days. Sea otters were recorded by shore-based observers along the coastline from these camps. A small section of the shore of Iturup (Lion’s Mouth Bay, located at the south-western extreme of the surveyed section) was observed from a boat, because it is almost impassable. The boat moved at the distance of about 200 m from the coastline. The shores of Chirpoy (which are also almost impassable) had been also observed from a boat in the same manner, but a part of the shore (2 km) was observed from land.

The water surface was observed using binoculars, and sea otter occurrences were recorded with Global Positioning System and photographed using a Nikon 900 camera (85× magnification). The frequency of sea otter occurrences per kilometre of coastline was calculated, and abundance estimated, focusing on optimal habitats. The optimal habitats were identified based on cartography and information on the ecological requirements of the sea otters. They prefer shallow waters, the proportion of habitats less than 30 m deep was estimated as 84% of the total area (Bodkin et al. 2004) (although they are able to dive deeper); therefore, a depth of 50 m was considered the boundary of the habitats. If the depths gradually increase, and reach 50 m at a distance of at least 1 km from the coast, such coastal waters were considered optimal ones. On the contrary, if a sharp dump up to several 100 m occurs near the shore, and depths up to 50 m occupy a strip of less than 1 km, such shores were considered sub-optimal. The abundance was calculated only for optimal habitats. The length of the coastline was measured from satellite images with an accuracy of 1 km.

The total length of observation routes was 120 km on Iturup and 40 km on Urup in 2019, 33 km on Urup in 2021, 33 km on Iturup, 16 km on Urup in 2022, 17 km on Urup, 3 km on Broughton, and 20 km on Simushir in 2023. The routes performed in different years did not overlap. The coastline of Chirpoy was completely observed in 2021, with two 1-km sections observed in 2022 (Fig. 1).

Fig. 1.

The islands under study. Colours mark the survey of the sections of the coastline: green, surveyed in 2019; yellow, 2021; blue, 2022; pink, 2023; red, unsurveyed sections.

A literature search on the sea otters of Kuril Islands was compiled from the Russian science citation index and Google scholar databases. Moreover, the reports of relevant local organisations (dealing with the marine mammal counts) were analysed – Pacific branch of the Russian Federal Research Institute of Fisheries and Oceanography (‘TINRO’), Kurilsky nature reserve, ‘KurilGeo’ (the enterprise excavating gold deposit on Urup). The oral communications of the locals of Iturup were also collected.

Results

Nearly all sea otters were recorded on Urup Island. They were observed only twice on Iturup. This happened at two sites: two individuals were recorded at the extreme south-western observation point in 2019 (in entrance to the Lion’s Mouth Bay, where small islets are located), and one individual was observed at the extreme northeastern point in 2022 (Medvezhya Bay). The survey of Broughton Island yielded a similar result: only two sea otters were observed. Due to such a small number of records, calculations of the total number of sea otters for these islands were not performed. On Chirpoy and Simushir Islands, sea otters were not found at all.

On Urup Island, sea otters were observed at all surveyed sites, usually singly or in small groups of two to three individuals (Fig. 2), with one group of 11 individuals. They were mostly seen lying on their backs or moving slowly, occasionally diving for food. Some individuals moved along the coastline, but their speed did not exceed the speed of the observer, therefore the repeated counting of the same sea otters during the passage of one route did not occur. Sea otters have never been observed on land or on islets near the shore. The density ranged from 0 to 21 individuals per kilometre. The total length of the coastline with optimal habitats is 274 km, that is, almost the entire coastline of the island, the length of which is ~287 km. Calculation of abundance yielded numbers of several hundred individuals (Table 1). Evidence of poaching (skinned carcasses on shores) recorded in 2019 was not observed in subsequent years.

Fig. 2.

Sea otters, coastal waters of Urup Island. Photo © Igor Popov.

Discussion

The presented estimates of the number of sea otters on Urup Island were partly confirmed in 2021 by an alternative observation. ‘Kurilgeo’ reported counting the abundance around the island and indicated the number 691 (Kurilgeo 2021), which corresponds to the range determined in this work (although the methods of calculations and observations were not reported). An underestimate in subsequent years is possible because surveyed sections were rather short. However, the obtained data characterise the scale of the sea otters’ abundance: probably several hundred inhabit the coastal waters around the island. A more precise estimate is problematic based on available data. Moreover, it is hardly possible at all because it is technically challenging to observe 287 km of shore simultaneously. In addition to technical difficulties, storms and fog often hinder the surveys.

On Iturup Island, on the contrary, there was a contradiction with alternative counts. Occurrence of sea otters over the entire coastline was reported, including the central part of the island, where settlements are located (Ovsyanikova et al. 2020). However, neither our expedition nor locals observed sea otters in the central part of the island. According to locals, sea otters occur only in the northern part of the island on the Pacific side and in the southern part on both sides. Probably, a small number of them visit the central part of the island occasionally. Although sea otters have small home ranges (10–20 km) and usually do not disperse more than 40 km (Kenyon 1969), some individuals occur far from typical habitats (for example, at Chukotka Peninsula (Kochnev and Litovka 2010)). They float ‘aimlessly’ or are moved by storms, and such cases facilitate the spread and integration of populations. The central section of Iturup is hardly suitable for sea otters now because of human presence. Moreover, numerous fish traps on the western side of the island also harm marine mammals. Other information on the sea otters of Iturup is rather scarce. At the end of the 19th century, sea otters were abundant in the south-west of the island; several hundred were harvested annually until they were gone (Snow 1897). In the 1980s, ~600 sea otters were registered on Iturup (Kostenko et al. 2004). In 2000, 84 sea otters were counted on its western coast (Kornev et al. 2001); in 2012, 27 were counted on the northern coast (Kornev 2016). These data are incomplete but show that sea otters are not abundant on Iturup. Evidently, they are even less numerous than on Urup.

As for Chirpoy Island, its coastal waters are non-optimal for sea otters because the strip of shallow waters is narrow. However, at least a small number of them could inhabit it. Especially the bay on the eastern coast seems suitable, as there are dense aggregations of kelp (g. Alaria), which is a favourite habitat for sea otters. In the past, sea otters were reported from this island. In 2000, 11 sea otters were counted on Chirpoy and the neighbouring island Brat Chirpoev (Kornev et al. 2001). In 1970, sea otters were also recorded (Shitikov 1971). Moreover, there is a small islet called ‘Sea Otter’ between Chirpoy and Brat Chirpoev; this name may indicate a notable number of these animals in the strait between the islands in the past. In the 19th century, sea otters were reported for this area in the context of harvest description (Snow 1897). The next island, Broughton, seems more suitable for sea otters as it is surrounded by a wide strip of Alaria. Our data indicate that a small number of them occur on the island. Other data about this are not available. The island of Simushir remained unexplored during the expedition. We only found a small break ~10 km long in the distribution of the sea otters.

The obtained data indicate the fragmentation of the range and low numbers of sea otters. The relatively high number on Urup Island is less than expected. In the 1960s−1970s, their number was ~2000 (Kuzin et al. 1984). In 1967, the maximum of 2300 was reported (Nikolaev 1968). Afterwards, their number fluctuated, but it was usually bigger than now (Kornev and Korneva 2006). Currently, the number has declined to the level of the 1953–1956, when there were several hundred sea otters (Voronov et al. 1956; Nikolaev 1958; Voronov 1964).

Sea otters are not numerous on the other Kuril Islands. Relatively high numbers have been reported for the northernmost islands of Shumshu and Paramushir, but a decline has also taken place there. In 2020, the census indicated 1173 sea otters were present(Kornev 2020). According to 2012 data, there were 4003 of them (2370–7754 CI 95%) (Ovsyanikova et al. 2020). Previously, 15,447 sea otters were reported (Kornev and Korneva 2006). To the south of these islands, there is a gap: in 2012, only 753 individuals (328–3015 CI 95%) were counted over a section of the archipelago several hundred km long. It was supposed that the central section forms a barrier to sea otter spread because it is only marginally suitable for them; there are great depths at the shores and a lack of bays or other shelters (Ovsyanikova et al. 2020). However, this viewpoint about the barrier is an exaggeration because sea otters have occurred there, at least in the past (Snow 1897), and they can migrate several tens of kilometres. Probably, this zone is not optimal for sea otters, but it is not absolutely unsuitable.

On the southern section of the archipelago, sea otters are found mainly on the small islands of the Lesser Kuril ridge. According to recent observations (2022), there are 471 individuals (Linnik 2023). A tendency towards growth has been reported. In 2016, the number was estimated at only 150 (Nevedomskaya 2017). Sea otters not only increased in numbers but extended their range southwards a little. Since 2016, they have settled in small numbers on Hokkaido. Japanese mass media often report about them. Recently, 12 individuals were counted there (Anonymous 2023). In contrast to the main part of the range, a nature reserve had been created on the southern Kuril Islands, therefore, protection, monitoring, and, apparently, an increase in numbers are associated with this.

To sum up, according to recent observations, the total number of sea otters on Kuril Islands is ~3000. The initial or normal number is not known, but it was evidently much higher. Sea otters have been harvested for a long time. The whole archipelago was populated by the Ainu or their predecessors over several thousand years. Hunting and fishing were the main occupations of locals. Sea otters were of special interest not only because of pelts but also as food. The Ainu considered them a delicacy (Snow 1897). Since the end of the 18th century, Russian colonisers started harvesting sea otters, followed by numerous other visitors. The harvest by the Ainu was also significant at that time (Tezuka 2009). The harvest by Russians was controlled by the Russian-American Company. Its most active period on the Kurils was from 1828 to 1867. At that time, its base on Urup Island (Kurilorossia) progressed. In 1986, during the archaeological study of this colony, a large heap of kitchen midden (12 m × 7 m × 0.5 m) consisting of the bones of marine mammals, mainly sea otters, was found. There were ~15,000 bones (Shubin 1987). A similar object was found at a distance of 50 km from this site, where the temporary base of hunters existed (Shubin 2007). These remains originated only from sea otters skinned on land and eaten. The total number of harvested otters was much higher, as hunters skinned them at sea during their raids over the archipelago. This means that several hundred or even thousand sea otters were harvested annually during this period.

In the 1860s, the Russian-American Company collapsed with the loss of Alaska. Soon after, in 1875, the Kurils were given to Japan in exchange for the Japanese part of Sakhalin, and the Russian commercial harvest of sea otters in the Kurils stopped. However, other ‘users’ continued hunting. Large-scale hunting collapsed only at the end of the 19th century when ‘users’ realised the danger of the resource’s disappearance and started introducing protective measures. The harvest by the Ainu also stopped because the Japanese settled them in a single village and prohibited them from hunting altogether (Snow 1897). The final stage of sea otter hunting by foreign visitors was described in detail by Captain G. J. Snow, who was involved in it for a long time since 1872 (Snow 1910). According to his writings, from 1872 to 1895, 52 schooners were arranged for sea otter harvest. Of these, 13 were lost at sea, 17 were wrecked onshore, 5 were taken by Russians, and 14 stopped this business. Finally, only three remained. The maximum number of schooners simultaneously at sea was 12. They usually spent several months on the islands harvesting ~200 sea otters, rarely up to 300, and in the last years, ~100. The harvest from Japanese settlements also took place. This means that during 23 years, like during the previous period, ~1000 sea otters had been harvested annually. Japanese author E. Isino presented similar data: since 1873–1881 9760 were hunted, annual harvest did not exceed 1500 (cit. from Belkin (1966)). The hunting took place in a barbaric way, without any concern for the preservation of the resource. It turns out that across more than 100 years the annual extermination of several hundred or thousand sea otters was possible. This means that the total number was at least several tens of thousands. The question arises, why they are so scarce now and why they are declining after some recovery.

Sea otter hunting is almost universally prohibited, with the exception of a small area in south-east Alaska where Indigenous people are allowed to harvest a limited number (Raymond et al. 2019). However, numerous other factors negatively impact sea otters. In populated areas, disturbances from humans and fishing gear affect sea otters (Doroff et al. 2021). On Iturup Island and the northern islands, this is a concern, but most of the archipelago remains unaffected. Human presence and economic activities can lead to increased water pollution, turbidity, and eutrophication, all of which degrade sea otter habitats. In the studied area, these processes are either insignificant or entirely absent. Although gold excavation on Urup raised concerns, its impact on seawater was minimal, and sea otters were observed nearby. The presence of seagrasses (Zosteracea) along the coastline throughout the studied area (Iurmanov et al. 2022) indicates good water quality, as these plants are very sensitive to environmental disturbances. In the American section of the sea otter range, there has been an abnormal increase in predator activity. Killer whales (Orcinus orca) in the North Pacific and Aleutians (Doroff et al. 2003) and sharks (Carcharodon carcharias) in California (Tinker et al. 2016) have started attacking sea otters more frequently. This change in behaviour is attributed to the depletion of fish and other human-consumed marine life (Ellis 2004). For killer whales, the extermination of large whales may have caused this shift, leading them to target smaller mammals (Springer et al. 2003). However, there is no clear evidence for or against this explanation (Andersen Garcia et al. 2016). In the studied area, increased predator activity concerning sea otters has not been reported. Local experts believe that the killer whale population off the Russian coast is primarily fish-eating (Shulezhko et al. 2018). Although a sea otter carcass was once found in a killer whale’s stomach (Fomin et al. 2023), there have been no observed changes in killer whale behaviour (Ismail et al. 2023). White sharks appear to be absent altogether. Carcasses of bitten sea otters, which would indicate intensive predation, are not found on the shore. During our survey, killer whales were occasionally observed near kelp beds. In 2022, three individuals were seen moving slowly along the western coast of Urup, while sea otters remained amidst nearby algae aggregations (Fig. 3). The killer whales might have been patrolling for prey, but there were no indications of increased activity targeting sea otters. Parasites also contribute to the increased mortality of sea otters, alongside predators (Thomas and Cole 1996; Conrad et al. 2005; Johnson et al. 2009). Some of these parasites (Sarcocystis neurona, Toxoplasma gondii) are transmitted from land mammals (Miller et al. 2021), such as opossums (Didelphis virginiana), domestic cats, and other felids. Parasites can be carried into the sea by storm water drainage and infect sea otters. Dead individuals would then be found on the shore, but such instances are not known for the Kuril Islands. Potential land hosts for these parasites are nearly absent, with only a small number of domestic cats on some islands. The decline in the Commander Islands’ sea otter population was attributed to a lack of food resources (sea urchins and other benthic invertebrates) due to a sharp increase in sea otter numbers, following the classic ‘predator–prey’ model (Mamaev 2018). The same opinion was expressed about the Kurils (Kornev 2010; Linnik 2023). Local experts often emphasise this factor. However, it is doubtful as the sea otter numbers have remained very low for over a century, never approaching the maximum. The data on the other sea otter population also do not confirm this, because decline may take place even without nutritional limitation (Tinker et al. 2021).

Fig. 3.

Killer whale at the coastal zone of Urup. Photo © Igor Popov.

The situation on Kurils seems paradoxical: the islands are almost uninhabited, economic activity is weak, yet sea otter numbers are very small. The simplest probable explanation is poaching. Although we found only two skinned carcasses without heads on the shore, this evidence is significant given the low number of sea otters and the small probability of detecting such objects. Since sea otters are defenceless against modern hunting equipment, even a small group of poachers can cause significant harm. Collecting information on poaching is challenging, but some evidence exists. Advertisements for the sale of sea otter fur occasionally appear on the Russian site avito.ru, though they are not numerous. In 2022, two such advertisements were posted but are no longer available (https://www.avito.ru/sankt-peterburg/odezhda_obuv_aksessuary/shuba_iz_kalana_morskaya_vydra_2322541701; https://www.avito.ru/novosibirsk/odezhda_obuv_aksessuary/shkurka_morskaya_vydra-kalan_2585082194?slocation=653240). According to the Russian fur traders’ portal (https://shubki.info), sea otter pelts are sometimes offered, although fur traders do not recommend these purchases because they are illegal and unnecessary due to the availability of numerous alternatives. In 2005, reports indicated the sale of at least 600 sea otter pelts in Moscow and Kamchatka, most of which later appeared on Chinese markets (Doroff et al. 2021). These sea otters were likely illegally harvested in the Commander Islands reserve. At that time the collection of dead sea otters was allowed and organised, therefore, some of the pelts could be obtained in this way. However, this harvest was stopped officially in 2000, therefore the poaching is also probable. Similar activities might have occurred in the Kuril Islands. Poaching incidents were reported in subsequent years, with sea otter fur hats available on Kamchatka markets until at least 2010 (Kornev 2010). The demand from China seems to be a special incentive in the case of sea otter trade, because unlike other countries where fur is in use, in China the otter are of particular value. Especially in Tibet, products from otters are valued not only for beauty but also because of some kind of prejudice and beliefs; apart from being used as decoration, they are used as a ‘medicine’ (International Otter Survival Fund 2014; Yongdan 2018). Moreover, poaching because of ‘ignorance’ was reported in the past: locals or visitors killed sea otters without a significant benefit, not realising or ignoring the fact that it is endangered species; they made carpets for the floor from the sea otter pelts (Voronov 1974). We also heard about this during our expeditions. Numerous residents or visitors do not miss the opportunity to harvest any possible object, and the hunting is often considered a sport, therefore it is probable that sea otter suffered from these practices. We believe that poaching over several years has undermined the population on Urup and Iturup. However, the data for the entire archipelago is insufficient to identify the exact causes of the decline. In addition to poachers, the killer whales are still under suspicion because of evidence from the American side. The cause of the sea otter decline in Alaska was elevated mortality, but stranded carcasses were rare or absent (Tinker et al. 2021). This could happen because the sea otters were eaten at sea. It is incredible that American and Russian killer whales are isolated from each other and have different habits. The lack of data on the killer whale predation on Kurils may occur because of limited periods of surveys.

The sea otter populations on the Kuril Islands require further research, but the recent decline in their numbers and negative trends are evident. Urgent measures are needed to save them. Sea otters deserve attention not only for their own sake but also as a crucial part of the kelp community, one of the most productive ecosystems on Earth (Shelton et al. 2018). Kelp beds, formed by aggregations of brown algae, support many other organisms. Sea otters are essential in maintaining this balance by preying on sea urchins that feed on algae. If sea urchins overpopulate, they can disrupt the kelp ecosystem. Additionally, sea otter activity supports the health of ‘underwater meadows’ of seagrasses, as their physical disturbances enhance the diversity of seagrass microbiota and genetic structure (Foster et al. 2021). Seagrasses are threatened organisms declining worldwide (Short et al. 2011).

It is pertinent to mention the ‘shifting baseline syndrome’ (Pauly 1995; Papworth et al. 2009; Soga and Gaston 2018). Modern experts often cannot grasp the conditions from 100 to 200 years ago, leading to an underestimation of the potential abundance of fish and other animals in natural environments. Several authors cited small numbers when discussing the normal state of the sea otter populations on the Kurils: 5000 (Shitikov 1971), 6000–7000 (Belkin 1966), or ~10,000 (Klumov 1968). When the population reached approximately these numbers in 2000 (7830 recorded), the situation was deemed perfect (Kostenko et al. 2004). However, historical data on commercial harvests suggest a much larger scale of abundance. If the number of sea otters had been only 5000–10,000 in the 19th century, they would have been exterminated within 5–10 years.

Conclusion

The number of sea otters in the Kuril Islands is extremely small and continues to decline. The exact reasons for this decline are not well understood, but direct extermination has occurred in parts of the archipelago. Urgent measures are needed to preserve the population. The current number of sea otters is thousands of times less than their potential population size. When assessing the potential abundance of sea otters, it is necessary to refer to the ‘initial’ numbers that existed at least 200 years ago, rather than a presumed stable minimum level.