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Ringed Seal
(Pusa hispida)

IUCN STATUS (2010) - LEAST CONCERN

Arctic Ringed Seal  - LEAST CONCERN; 

Baltic Sea Ringed Seal - VULNERABLE;

Lake Ladoga Ringed Seal - VULNERABLE;

Lake Saimaa Ringed Sea - CRITICALLY ENDANGEREDl;

Sea of Okhotsk Ringed Seal - DATA DEFICIENT.

Distribution and Numbers

So named because of the ring-shaped marks on its coat, there are generally recognised to be five separate subspecies of the ice-inhabiting ringed seal. The most numerous subspecies, the Arctic ringed seal, P.h. hispida is found in all of the Arctic Ocean seas and the Bering Sea, ranging as far south as Newfoundland and northern Norway. It is general considered 'unwise to attempt an estimate' of Arctic ringed seal abundance, owing to its widespread and remote distribution (Frost & Lowry, 1981; Reeves et al., 1998). The retreat of the ice at the end of the last Ice Age caused the Baltic ringed seal to become separated from the Arctic ringed seal about 11,000 years ago, and also caused the Saimaa and Ladoga ringed seals to be trapped in their respective freshwater lakes about 8,000–9,000 years ago (Muller-Wille, 1969; Hyvarinen & Nieminen, 1990).


There are currently estimated to be fewer than 10,000 in the Northen and central Baltic, most of which are in the Bothnian Bay (Härkönen et al.,1998). About 2,000 Ladoga seals (P.h. ladogensis)are presently found in Lake Ladoga in western Russia (Verevkin, 2003), while only about 220-270 Saimaa seals (P.h. saimensis) remain in Lake Saimaa in eastern Finland (Sipilä, 2003).

A survey in the late 1960s resulted in a population estimate for the Okhotsk ringed seal P.h. ochotensis of about 800,000 - 1,000,000 seals in the Okhotsk Sea and northern Japan (Bychkov, 1971), but present abundance must be considered to be unknown (Kovacs et al. 2008).

Phoca hispida - Image 1

P.h. botnica
Photo: Anna Roos,
Swedish Museum of Natural History

Status


Arctic and Okhotsk Ringed Seals
The effects of natural events and human-caused activities on these subspecies are difficult to determine due to the lack of accurate population data. There is concern however that oil and gas exploration and extraction in many parts of their respective ranges, particularly in the Chukchi and Beaufort Seas, may cause disturbance as well as possible pollution of the seals, their habitat and food supply. It has also recently been reported that the waters off Sakhalin Island in the Okhotsk Sea are about to be opened up to massive oil and gas development, and that tanker traffic and the extreme weather conditions in the area may lead to a major spill there. Potential disturbance and pollution from vessel traffic on the proposed Northern Sea Route is also a cause for concern.

Ringed seals are one of the most important subsistence prey for native hunters throughout their range, but it is difficult to determine the extent of this hunting. Some published figures suggest that the annual kill in Alaska was about 3,000 seals in the 1990s (Kelly, 1998), about 50,000 per year were killed in Canada in the 1970s and 1980s (Reeves et al., 1998). 50,000 - 70,000 were killed each year from 1979-1983 in Greenland (Reijnders, et al. 1993), and 7,300 were commercially hunted in Russia in 1988. It was reported that Inuit natives from the Canadian territory of Nunavut, in a move towards commercial rather than subsistence hunting, planned to undertake a hunt of up to 2,000 ringed seals a month, mostly for export to Japan, China and Taiwan. Plans were also announced to market sealskins as well as to process ringed seal meat into food products, organs and bones into health products, and to render fat into oil capsules for sale as a dietary supplement. A pilot hunt was planned for the summer of 1999, with the intention of carcases being shipped to Newfoundland for processing until a plant could be built in the Arctic. In a related move it was reported that the Inuit Tapirisat of Canada and the Nunavut government are working together to overturn the section of the U.S. Marine Mammal Protection Act which bans the import of sealskins from Canada.
Arctic climate change may be affecting the seals' ice habitat, populations having been shown in the past to be very sensitive to changes in the ice conditions. Rising pollutant levels in ringed seals, even in the more northerly locations, have also recently been reported, indicative of a general rise in pollution of the marine environment on a global scale. Russia's Emergency Situations Ministry reported in December 1998 that leaking radioactive waste in the country's Arctic seas is resulting in excessive radiation levels, reaching 100 times normal levels in some places. The radioactive waste is held in Arctic dumps used by the Soviet Union in the 1960s.
Baltic Ringed Seal
It is estimated that there were a minimum of 180,000 Baltic ringed seals at the start of the 20th century (
Härkönen et al, 2008). Intensive hunting and pollution however have reduced their numbers considerably to about 25,000 before the 1940s, due to an extermination campaign by the Swedish and Finnish authorities. A further decline to about 5,000 in the mid-1980s has been attributed to reproductive failure due to organochlorine contamination of the environment (Harding & Härkönen, 1999; Härkönen et al, 2008). Hunting was gradually limited in several countries, and then banned altogether throughout the Baltic Sea in 1988 by Recommendation 9/1 of the Helsinki Commission (HELCOM). Recently however there has been mounting pressure, especially from Finland and Sweden, for the Recommendation to be changed in order to re-allow hunting of the various seal species in the Baltic Sea, and there are concerns that this may lead to Baltic ringed seals being hunted once more. Research published in 1997 calculated that the level of hunting that the Baltic ringed seal population could sustain was close to zero. Baltic ringed seal numbers are only now starting to recover gradually, and particular concerns are still being expressed regarding the viability of the Gulf of Finland population (Stenman et al., 2005). A recent tagging study of adult seals from three main areas in the Baltic showed great area fidelity and no overlap in distribution, suggesting three separate stocks. This lack of movement between stocks could have severe consequences for seals from the small and unstable populations in the Gulf of Finland and Estonian waters, where breeding has repeatedly failed in recent years due to lack of breeding ice (Härkönen et al., 2008).
Toxic chemicals and metals in the heavily-polluted Baltic Sea have been blamed for pathological changes, disease and sterility of females in Baltic ringed seals (Helle et al., 1976). Although measures have been taken and this problem has been reduced to some extent, environmental contaminants remain a great threat to the health of the ringed seal population. Pollution-caused uterine occlusions, which severely reduce reproductive rates and which are probably caused by organochlorines such as PCBs, peaked at the end of the 1970s but are continued to persist in the Bothnian Bay population (Bergman & Olsson, 1986). A number of Baltic ringed seals from the small population in the Gulf of Finland died mysteriously in 1991-92. It was thought that the most cause may have been some form of neurotoxic compound (Stenman et al., 2005), although this was not verified. Russia's Emergency Situations Ministry reported in December 1998 that chemical weapons dumps in the Baltic Sea were causing heavy metal and arsenic contamination.
Several tens of ringed seals, mostly young seals, die each year from entanglement in fishing nets and gear, while seal predation also causes some conflict between fishermen and seals. An additional threat to the population comes from the disturbance of ringed seal breeding sites by ship traffic through the ice. There is currently concern that the construction and development of oil and chemical terminals in the Gulf of Finland, along with increased vessel traffic in the area, may cause disturbance to the small ringed seal population there as well as increase the potential risk of oil spills and winter habitat degradation (Trukhanova et al., 2010). The Baltic ringed seal is listed as Vulnerable on the IUCN Red List (Kovacs e al., 2008) and as an Appendix III species under the Bern Convention. The subspecies is also protected through legislation by various countries within its range.

Phoca hispida - Image 2

P.h. ladogensis
Photo: Jouni Koskela,
Finnish Forest and Park Service

Ladoga Seal
It is estimated that there were about 20,000 Ladoga seals at the start of the 20th century (Chapskiy, 1932), but hunting up to the 1940s severely depleted the population. Hunting was banned in 1980 (Sipilä & Hyvarinen, 1998), but illegal hunting and some killing of seals by fishermen still occurs. Since the 1950s the Ladoga seal has suffered from entanglement in fishing nets, with around 15% of the population (200-400 seals) per year dying in this way since the early 1990s (Verevkin et al., 2009). Recent studies have concluded that the pollution of Lake Ladoga, caused mainly by increased industrialisation around the Lake, is a serious threat, with high levels of toxic chemicals in the water and mercury being found in the seals' body tissues.

A serious threat to the population comes from disturbance caused by increasing recreational activities during the summer on the rocky islets in the northern parts of the lake, which are favourite haul-out sites for the seals. Increased vessel traffic is also a concern as it may result in disturbance, and has also resulted in oil spills and consequent pollution. The Ladoga seal is listed as Vulnerable on the IUCN Red List (Kovacs et al., 2008) and as an Appendix II species under the Bern Convention. The species is also included in the Russian Red Data Book (Red Data Book of the Russian Federation, 2000).
Saimaa Seal
It is estimated that there were about 700 Saimaa seals at the beginning of the 20th century. Hunting subsequently killed many of the seals, and the subspecies was also eradicated from some parts of the lake by fishermen. In danger of extinction, the Saimaa seal was protected by Finnish law in 1955 (Tonder and Jurvelius, 2004). However its numbers continued to decline rapidly, numbering as few as 180 by the early 1980s, before stabilising to some extent at current levels. The subspecies' high pup mortality was an important factor in the decline and it is thought probable that mercury pollution in the lake was also partly responsible. Hunting of the Saimaa seal is forbidden.
Lake Saimaa is characterised by several exposed water areas, connected to each other by narrow sounds. There are about 10 breeding areas and it is possible, because of the lake's sub-divided nature, that the population is divided into two or more sub-populations. Some seal sanctuaries have been set aside since 1982. Two national parks currently cover areas within the seals' range, while a national shoreline conservation programme established extra protection for 6-8 of the breeding areas in 2006. Metsahallitus (the Finnish Forest and Park Service), the agency that is responsible for the monitoring and protection of the subspecies, is aiming to raise the number of Saimaa seals to a minimum of 400 by the year 2025. The population is currently slowly growing at about 2% per year due to protection measures (Metsähallitus, 2007). An annual WWF-sponsored survey of pups takes place by small hovercraft in the spring (after the lairs have collapsed and the survey will not cause too much disturbance to mothers and pups). The 2000km survey counted 51 pups in 2008 and 44 in 2009 (Metsähallitus, 2010)
Phoca hispida - Image 3

P.h. saimensis
Photo: Jouni Koskela,
Finnish Forest and Park Service
The most serious threat to the Saimaa seal is now from subsistence fishing, and there has been a significant mortality of seals, especially of juveniles, due to entanglement and drowning in fishing nets and traps. In an attempt to alleviate this problem, voluntary agreements now exist which forbid fishing around the main breeding sites from 15 April to 30 June, covering about 325 sq.km (Metsahallitus, 2007). This area covers approximately 50% of birth lair locations. The other main threats at present are house building along the shoreline, snowmobile traffic, and recreation around the lake which destroy suitable habitat for the subspecies and cause disturbance, particularly during the winter breeding season. Pollution of the lake by toxic chemicals and heavy metals has been a serious problem in the past, but lake pollution levels and concentrations of pollutants in Saimaa seal tissues have both decreased significantly in the last 15-20 years (Sipilä, 2003).
Until 1991 artificial regulation of the Lake Saimaa water level for a hydro-electric plant resulted in seal birth lairs collapsing, causing high pup mortality and lowered weight of weaned pups. Such artificial lowering of the lake's levels is not now permitted during the period of winter ice cover. Natural lowering of the lake's levels occurs from time to time, but the effects of this have been reduced in recent years by the artificial regulation of two lakes which empty into Lake Saimaa. The Saimaa seal is listed as Endangered on the IUCN Red List and as an Appendix II species under the Bern Convention.

Life history and annual cycle


Arctic and Okhotsk Ringed Seals
The behaviour of these subspecies varies quite widely between regions. The seals however generally remain in contact with drifting pack ice or shorefast ice for most of the year and rest, pup and moult there. In autumn and early winter, as openings in the ice start freezing over, the seals create breathing holes in the ice. They continue to maintain these holes, sometime through ice up to 2m thick, with their strong claws. During winter and early spring the Arctic ringed seals dig caves or lairs into the drift snow that has gathered above the breathing holes, these often multi-chambered lairs offering protection from the extreme cold and polar bear predation. Seals typically maintain two or more lairs simultaneously, the distance between these lairs ranging up to 4.5km (Smith & Stirling, 1975). The Okhotsk ringed seal may sometimes haul out on land and, unlike the Arctic subspecies, does not give birth in lairs (Pikharev, 1941, Fedoseev, 1964).

Pups are generally born in the lairs from mid-March to mid-April. They are born with a white coat which they shed within 4-6 weeks for a coat that is silver on the front, dark grey on the back, and with only a few spots. Maternal body weight at the time of birth is about 74kg (Lyderson et al., 1992). Nursing is thought to last for about 40 days (Hammil et al., 1991), and nursing mothers make foraging trips, spending altogether about 50-80% (individual variation) of their time in the water (Lydersen et al., 1992). Despite their foraging effort, however, mothers still lose about 0.65kg/day average during the nursing period (Lydersen, 1995).  A study of pups aged 25 to 39 days old at Svalbard (Lydersen and Hammill, 1993) found that these pups entered the water at least once a day, and altogether spent about 50% of their time in the water and 50% on the ice (inside their lairs). When they were in the water, 20% of their time was spent diving, and they dived to the sea bed, with average dive times of about 1 min, although maximum dive times were between 5 and 12 min. Two pups with VHF tags attached were both found to use 6–7 breathing holes (Lydersen et al. 1993). A study of pregnancy energetics showed that ringed seal pups are energetically more costly to their mothers compared to other phocid seals, because of their unusually high activity level. However, their swimming and diving ability is believed to be an evolutionary adaptation to avoidance of predators, mainly polar bears (Lydersen, 1995). Little is known about the Arctic ringed seal's mating system, but mating is thought to take place in mid-late May, presumably under the ice near the birth lair. Moulting generally takes place in mid-May to mid-July, the seals basking on the ice. Quite a lot of weight is lost by the moulting seals as they virtually stop feeding during this period. The adult coat varies widely but is generally dark grey to silver with numerous rings and spots, especially on the back.
Many seals are reported to migrate (e.g. north-south or inshore-offshore) on a seasonal basis in response to ice availability and there is evidence of long-distance migration and dispersion, particularly for juvenile seals. Arctic ringed seals, mostly pups and subadults, are the major prey of polar bears and arctic foxes. Other predators include wolverines, wolves, dogs, killer whales, walruses and large birds. Diet varies widely but Arctic and Okhotsk ringed seals are known to eat cod, smelt, herring, crustaceans, zooplankton, squid and sculpins (Sinisalo, 2007).
Baltic Ringed Seal
Baltic ringed seals usually associate in small groups during the moult and during ice formation. Groups of ringed seals are found hauled out on rocks in the Gulf of Riga and the Gulf of Finland from May to November, but tend to occur singly during the ice-free period in the Bothnian Bay. During the winter however the seals are found alone, spread out in the ice fields, and are often aggressive to other individuals. The pups are born in lairs on the pack ice in late February - early March.
Peak moulting takes place from mid-April to the beginning of May (Bergman, 2007). After the moult, the seals spend several months feeding very intensively at sea, to build up their energy stores once more. In June and July seals in a telemetry study spent 70–85% of their time diving, decreasing to about 50% of their time in late autumn, and doubled their weight by November to 100kg or more (Härkönen et al., 2008). Larger seals dive more deeply than smaller seals, and males tended to make longer dives than females. The seals tended to forage during the day and haul out at night, and this pattern is probably related to the pattern of movement favoured prey species - herring, for example being close to the sea bed suring the day, when they may be more easily caught by diving seals (Härkönen et al., 2008). Baltic ringed seals feed mainly on fish such as herring, smelt, whitefish, sculpin and perch. They also feed on crustaceans, particularly isopods, during the winter. Sea eagles are known predators of Baltic ringed seal pups.
Ladoga Seal
The Ladoga seal differs in appearance from the Baltic and Saimaa seals, being smaller, darker and having longer whiskers. During ice-free periods Ladoga seals gather together in herds on skerries and rocky island shorelines, the herds consisting of up to 300 seals. The seals use a rich repertoire of sound communication within these herds, both in air and underwater. Underwater sounds recorded in a summer herd were of six types: 'click, burst pulse, knock, chirp, yelp and low frequency growl'. The clicks could be used for echolocation of obstacles or distances from the bottom and the knocks are thought to be used in long-distance communication (Kunnasranta et al., 1996). The breeding behaviour of the Ladoga seal has not yet been documented, but it is thought that the pupping season lasts from March to the beginning of April. The birth lairs of the Ladoga seal are typically situated 40-60km from the shore, near hummocked or ridged ice, although in northern areas of the lake they are mostly found in snowdrifts along the shoreline. Lairs in the northern part of Lake Ladoga have sometimes been attacked by dogs, red foxes and wolves. The pups are normally born with white coats, although some dark pups with black hairs in their coats have recently been observed. It is thought that lactation lasts 1.5-2 months and that mating takes place during early lactation (Philatov, pers. comm.). In the summer the seals concentrate in small areas in the northern part of the lake. Most of the seals have a dark brown coat, some with light rings and others with light vein-like patterns. Some seals however have a light coat with a black belt around the body and indistinct rings and brown spots. Ladoga seals feed mainly on prey such as smelt, vendace, ruffe and burbot (Agafonova, 2007).
Saimaa Seal
Saimaa seals are solitary animals and are usually found hauled out alone or in pairs along shorelines. The mating behaviour of the subspecies has not yet been documented, but the females are known to give birth to their grey-coated pups from late February to early March. The pups are born in lairs constructed in snow-drifts on the ice, the lairs having been built close to the shore of small islands and islets in late December. Pups are weaned after 7-9 weeks and start to look for their own food from mid-April to late June (Sipilä). Adult seals appear to remain in the vicinity of the same area year-round, often using the same shoreline or even the same rock. There is a great variety in coloration among Saimaa seals, but individuals are usually fairly dark. The Saimaa seal also uses click vocalisations, but other vocalizations have been reported as rare (Kunnasranta et al., 1996).
Phoca hispida - Image 4

P.h. saimensis
Photo: Jouni Koskela,
Finnish Forest and Park Service
Underwater visibility in Lake Saimaa is very poor due to high concentrations of humus, in some areas visibility being as low as 2m, and the seals tend to feed on fish in total darkness. The longest dive so far measured for a Saimaa seal has been 23 minutes. Moulting takes place on land in late May - early June. Saimaa seals feed on small schooling fish such as vendace, roach, smelt and perch. The subspecies has no natural predators.

Vital statistics


The measurements of Arctic and Okhotsk Ringed Seals vary greatly between regions and also during the year. Most adults measure 1.1-1.5m in length and 50-70kg in weight, males tending to be longer than females. Pups are born measuring 60-65cm in length and weighing 4.5-5.5kg. Females reach sexual maturity at 4-8 years, males at 5-7 years. Individuals have been known to live up to 43 years of age (Lydersen, 1995; Fedoseev, 1964).

The Baltic Ringed Seal is the largest of the subspecies, adults weighing from 50-125 kg (Helle, 1980) and measuring 1.5-1.75m in length (Bergman, 2007). Females reach sexual maturity at 3-6 years, earlier than males. Baltic ringed seals have been known to live to 40 years of age. Adult Ladoga Seals weigh 60–70kg, while pups are born measuring 50-60cm in length and weighing 4-5kg (Popov, 1979). Females reach sexual maturity at 4-5 years, males at 6-7 years. Adult Saimaa Seals measure up to 1.5m in length and weigh 45-100kg. Males reach sexual maturity at 5-6 years, females probably one year earlier. About 50 Saimaa seal pups are born every year and little over half of them reach the age of sexual maturity.

Pup development profile

Ringed seal subspecies

Weight of newborn

Weight at weaning

Duration of  lactation

References

Pusa hispida hispida

4,6 – 5,4 kg

18,7- 22,1 kg

39-44 days

Lydersen, 1995, Hammill et al. 1991, Lydersen, Kovacs, 1999, Smith, Hammill, Taugbol, 1991

P.h. botnica

3,3-4,6 kg

15-22,5 kg

28-42 days

Zheglov, 1975, Tormosov, Esipenko, 1990, Bergman, 2007,

P.h. ladogensis

4-5 kg

15 kg

35-54 days

Popov, 1979, Sipila T., personal communication, Filatov, personal communication

P.h. ochotensis

3,5-4 kg

6-9 kg

30 days

Fedoseev, 1964

P.h.saimensis

3,3 - 7,3 kg

15-20 kg

35-54 days

Sipila, 2003, Hyvarinan, Nieminen, 1990, Sipila T., personal communication


Internet links

Young ringed seal pup on the ice with its mother, beside a breathing hole (note, it is more usual for ringed seal pups to be hidden in snow lairs than to be exposed on the ice, as in this clip)

A juvenile ringed seal hauls out on a small piece of floating ice beside two adults (location unknown)

Ringed seal in the water at breathing hole (location unknown)

Saimaa ringed seal hauled out on a small rock and another Saimaa ringed seal seal returns to the water from a rock

More information may be be found from the following websites:

All ringed seals:  The IUCN Red-list ringed seal page

Arctic ringed seals: The NOAA Fisheries Office of Protected Resources. This site gives references to stock assessments and other key documents

Saimaa seals:  The Metsähallitus website .....   Saimaa seals brochure to download


References

Agafonova E.V., Verevkin M.V., Sagitov R.A., Sipila T., Sokolovskaya M.V., Shahnazarova V.Yu. (2007). Kolchataya nerpa v Ladozhskom ozere i na ostrovah Valaamskogo arhipelaga. - Vammalan Kirjapaino Oy.  – 61p.

Bergman A., Olsson M. (1986) Pathology of Baltic grey seal and ringed seal females with special reference to adrenocortical hyperplasia: Is environmental pollution the cause of widely distributed disease syndrome? - Finnish Game Research, 44:47–62. 

Bychkov V. A. (1971). A review of the conditions of the pinniped fauna of the USSR. In "Scientific Principles for the Conservation of Nature" (L. K. Shaposhnikov, ed):59-74.

Chapskiy K.K. (1932) Ladozhskiy tyulen i vozmozhnost ego promyisla. – Otchet dlya Leningradskogo Issledovatelskogo instituta ryibolovstva: 147 – 157.

Fedoseev G.A. (1964). Ob embrionalnom, postembrionalnom roste i polovom sozrevanii ohotskoy kolchatoy nerpyi, - «Zool. Zhurnal», V.42. - p.8

Frost, K. J. and Lowry, L. F. 1981. Ringed, Baikal and Caspian seals Phoca hispida Schreber, 1775; Phoca sibirica Gmelin, 1788; and Phoca caspica Gmelin, 1788. In: S. H.

Ridgway and R. Harrison (eds), Handbook of marine mammals, Vol. 2: Seals, pp. 29-53. Academic Press.

Hammill M.O., Lydersen C., Ryg M., Smith T.G. (1991). Lactation in the ringed seal (Phoca hispida). – Can. J. Fish. Aquat. Sci. 48: 2471-2476.

Harding K.C., Härkönen T. (1999) Development in the Baltic grey seal (Halichoerus grypus) and ringed seal (Phoca hispida) populations during the 20th century. Ambio 28:619. -627.

Härkönen, T., Stenman, O., Jussi, M., Jussi, I., Sagitov, R. and Verevkin, M. 1998. Population size and distribution of the Baltic ringed seal (Phoca hispida botnica). In: M. P. Heide-Jorgensen and C. Lydersen (eds), Ringed seals in the North Atlantic, pp. 167-180. NAMMCO Scientific publication.

Harkonen T, Jüssi M, Jüssi I, Verevkin M, Dmitrieva L, et al. (2008) Seasonal Activity Budget of Adult Baltic Ringed Seals. PLoS ONE 3(4): e2006. doi:10.1371/journal.pone.0002006

Helle E., Olsson M., Jensen S. (1976) PCB Levels Correlated with Pathological Changes in Seal Uteri. Ambio 5: 261-263.

Helle E., Sulkava S., Sipila T., Danilov, P. (1998) Phoca hispida spp.//Kotiranta, H., Uotila, P. Sulkava, S. & Peltonen, S-L. (eds.) Red Data Book of East Fennoscandia. - Ministry of the Environment, Finland, Finnish Environmental Institute & Botanical Museum, Finnish Museum of Natural History, Helsinki: 196-199.

Hyvarinen H., and Nieminen M. (1990). Differentiation of the ringed seal in the Baltic Sea, Lake Ladoga and Lake Saimaa. Finnish Game Research 47: 21–27. 

Isenberg D.S., Gutierrez C.M., and Cummings B. (2008) Before the secretary of commerce petition to list three seal species under the endangered species act: ringed seal (Pusa hispida), bearded seal (Erignathus barbatus ), and spotted seal (Phoca largha ),” Diversity. 144pp.

Kelly, B. P. 1988. Ringed Seal. Pages 57-75 in J. W. Lentfer, editor. Selected Marine Mammals of Alaska: Species Accounts with Research and Management Recommendations. Marine Mammal Commission, Washington, D.C.

Kovacs K., Lowry L. & Härkönen T. (2008). Pusa hispida. In: IUCN 2009. IUCN Red List of Threatened Species. Version 2009.1

Kunnasranta, M., Hyvärinen, H. and Sorjonen, J. (1996).  Underwater vocalizations of Ladoga ringed seals (Phoca hispida ladogensis Norq.) in summertime.  Mar. Mamm. Sci. 12(4): 611–618.

Lavigne, and L. F. Lowry. (1993). Seals, Fur Seals, Sea Lions, and Walrus. IUCN/SSC

Lydersen, C. (1995).  Energetics of pregnancy, lactation and neonatal development in ringed seals (Phoca hispida).  In Whales, seals, fish and man (A.S. Blix, L. Wallac and Ø. Ulltang, eds).  Elsevier Science B.V.

Lydersen, C., Hammill, M.O., Ryg, M.S. (1992). Water flux and mass gain during lactation in free-living ringed seal (Phoca hispida) pups.  J. Zool., Lond., 228: 361-369.

Lydersen, C., Hammill, M.O., Ryg, M.S. (1993).  Differences in haul-out pattern in two nursing ringed seal (Phoca hispida) pups.  Fauna norv. Ser. A 14: 47–49.

Lydersen, C. & Hammill, M.O. (1993).  Diving in ringed seal (Phoca hispida) pups during the nursing period.  Can J. Zool., 71: 991–996.

Lydersen C. (1995) Energetic of pregnancy, lactation and neonatal development in ringed seals (Phoca hispida. In: Blix A.S., Walloe L., Ulltang O. (eds) Whales, seals, fish and man. Elsevier science BV, Amsterdam: 319-327.

Lydersen С., Kovacs K. (1999) Behaviour and energetics of ice-breeding, North Atlantic phocid seals during the lactation period. – Marine ecology progress series, 57: 265-281.

Metsahallitus, 2007.  www.metsa.fi

Muller-Wille L. L. (1969). Biometrical comparison of four populations of Phoca hispida Schreb. in the Baltic and White Seas and Lakes Ladoga and Saimaa. Commentations Biologicae Societas Scientiarum Fennica 31:1–12.

Pikharev G.A. (1941). Tyuleni yugo-zapadnoy chasti Ohotskogo morya, «Izv. TINRO», V.20.

Popov L.  (1979) Ladoga Seal. In Mammals in the Seas, Vol. II: pinniped species summaries and report on sirenians. FAO Fisheries Series No.5: 70-71.


Red data book of the Russian Federation (2000), v. 1 "Animals" // V.E. Sokolov, M.Ya. Smelova, V.A. Bychkov (eds). - AST, Astrel’.

Reeves, R. R. 1998. Distribution, abundance and biology of ringed seals (Phoca hispida): an overview. NAMMCO Scientific Publications 1: 9-45.

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Sinisalo T. (2007). Diet and Foraging of Ringed Seals in Relation to Helminth Parasite Assemblages. - Juvascula studies in biological and environmental science. – Juvascula University Printing House. – 41 pp.

Sipilä T. (2003) “Conservation biology of Saimaa ringed seal (Phoca hispida saimensis ) with reference to other European seal populations . Department of Ecology and Systematics Division of Population Biology University of Helsinki Finland,” - New York, vol. 2. – 40p.

Sipilä T., Hyvärinen, H.  (1998). Status and biology of Saimaa (Phoca hispida saimensis) and Ladoga (Phoca hispida ladogensis) ringed seals. In Ringed seals in the north Atlantic//Heide-Jørgensen M. P., Lydersen C. - The North Atlantic Marine Mammal Commission Scientific Publications, №1: 83-99.

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