Svalbard polar bears thrive in part due to ringed seal pups in the spring pack ice

Few people know that Arctic ringed seals (Phoca hispida, aka Pusa hispida) give birth and breed in the offshore pack ice in the spring, as it is seldom mentioned by either seal or polar bear specialists.

While it is true that some ringed seals give birth in stable shorefast ice close to shore, many others give birth well offshore in thick pack ice – where polar bears also live and hunt in the spring but where few Arctic scientists ever venture – and the existence of pack ice breeding ringed seals is one of the reasons that polar bears are such a resilient species.

ringed-seal-in-snow-cave_b-kelly-wikipedia

Ringed seal pup in a snow cave, B. Kelly photo (Wikipedia).

As a consequence, despite fears expressed by Ian Stirling, low shorefast ice and associated snow around Svalbard this winter (and any time in the past) is not necessarily a hindrance to polar bear survival because there are ringed seal pups available out in the surrounding pack ice – where bearded seals also give birth.

Of course, ringed seals pups are also available to Svalbard polar bears in the shorefast ice in the Franz Josef Land archipelago to the east (see map below) but it is the pups born in the offshore pack ice that are of interest here. The existence of pack ice breeding ringed seals may be why Norwegian biologists do not currently monitor ringed seals in the Barents Sea, despite many years of poor ice conditions around Svalbard in spring – this simply is not a species of concern.

barents-sea-ice-2017-feb-6_nis

The fact that distinct ringed seal ecotypes (or habitat-specific morphotypes) exist in the Arctic – one that gives birth and breeds in shorefast ice and another that gives birth and breeds in offshore pack ice, perhaps driven by competition for limited shorefast ice habitat – is a phenomenon a colleague and I discussed in a peer-reviewed book chapter published several years ago. Have a look at the excerpt below and see what you think.

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Crockford and Frederick (2011:72-76) [Some paragraphs are broken up here for easier online reading, and some passages are emphasized]:

Ringed seals in the Bering Sea inhabit both pack ice (large sheets or chunks of sea ice that drift with the currents and wind) and shorefast ice (immobile ice attached to land, aka “fast ice”). While it is often stated that animals found in the pack ice are groups of nonbreeding subadults that have been excluded from preferred territories in shorefast ice by older, dominant animals (Hammill and Smith 1991; Holst et al. 1999; Krafft et al. 2006, 2007; Laidre et al. 2008; Smith and Hammill 1981; Smith et al. 1991; Stirling and Øritsland 1995), it is has been clear for some time that the pack ice is not restricted to immature animals.

Evidence of ringed seal breeding in the pack ice comes from the Bering Sea (Burns 1970), Sea of Okhotsk, and Chukchi Sea (Fedoseev 1975), Davis Strait of the central Canadian Arctic (Finley et al. 1983), and the Barents Sea (Wiig et al. 1999).

Fedoseev (1975) concluded from his preliminary study in the North Pacific that pack ice–breeding seals were smaller at maturity as well as at birth and that they matured faster than shorefast ice–breeding seals.

Finley and colleagues provided morphometric evidence from Baffin Bay in support of Fedoseev’s work, of which we present a subsample here (see Table 4.1): Baffin Bay pack ice seals were not only smaller but matured about a year earlier than shorefast-breeding seals. In conclusion, Finley et al. (1983:171) suggested that the  age structure of the pack ice population they analyzed “approximates that of a normal breeding population.

crockford-and-frederick-2011-fig-4_1

Despite the evidence that two distinct morphotypes indeed exist— apparently the consequence of the population dividing itself among two distinct ecological habitats that are consistently available across the Arctic— ringed seals are characterized as giving birth invariably within shorefast ice habitats exclusively (e.g., Frost et al. 2004; Hammill and Smith 1991; Holst et al. 1999, 2001; Kingsley et al. 1985; Laidre et al. 2008; Lydersen and Gjertz 1986; Reeves et al. 2002; Smith and Hammill 1981; Smith et al. 1991; Stirling and Øritsland 1995).

Where pack ice–breeding is acknowledged to occur, it is largely dismissed as an insignificant phenomenon (e.g., Burns 1970; Kraftt et al. 2007; Nowak 2003). However, given that off shore pack ice throughout the Arctic remains virtually unsurveyed for all life forms, including polar bears and ringed seals (e.g., Aars et al. 2006), such a conclusion must be considered premature.

Some biologists have concluded that a substantial population of ringed seals must inhabit the off shore Arctic pack ice, based in part on estimates of the number of seals required to support known populations of polar bears,their primary predator (e.g., Derocher et al. 2002; Stirling 2002; Stirling and Øritsland 1995; Wiig et al. 1999).

Finley et al. (1983:171), based on a survey of Baffin Bay in the central Canadian Arctic, concluded that “the pack ice supports a much larger population of seals than does the bordering area of fast ice” and that these animals inhabit ice that lies well off – shore.

This off shore habitat is probably beyond the reach of biologists’ survey aircraft, which seldom manage to penetrate more than about 100 km north of the concentrated pack ice edge (e.g., Aars et al. 2008; Frost et al. 2004). As for polar bears, this limited survey effort has resulted in uncertainty regarding the global population size for ringed seal, which is currently estimated at about 7 million (Davis et al. 2008; Wiig et al. 1999; Reeves et al. 2002). [SJC now stated as “>3 million“]

While the primary productivity of pack ice over deep Arctic waters is presumed to be uniformly low and, thus, unsuitable habitat for seals and other marine mammals (e.g., Fischbach et al. 2007), this assumption is based on very few actual measurements collected over huge distances and time periods (Aars et al. 2008).

… In other words, it is apparent that young polar cod do not require ice that is positioned over shallow, continental shelf waters, and, therefore, neither do ringed seals nor the polar bears that prey upon them. If off shore pack ice over deep water is productive enough for polar cod, it should be suitable for ringed seals and polar bears also.

This perspective is important because it vastly expands what biologists consider suitable habitat for a number of arctic marine mammals and explains the ability
of this ecosystem to sustain these species over millions of years despite dramatic shifts in ice extent and persistence (e.g., Harington 2008).

The most recent evidence in support of these conclusions comes from a study of genetic diversity in the ringed seal, which showed virtually no regional subdivision across the Arctic (Davis et al. 2008).

The authors, in attempting to explain these results, suggest that preferred landfast ice breeding locations for mature ringed seals may be more limited than previously thought and propose that competition for landfast ice breeding territories may drive many mature seals off shore to breed in the pack ice.

This suggests to us an explanation for how and why ringed seal ecotypes develop and persist.

If strongly territorial, large bodied individuals of both sexes out-compete weakly territorial, smaller-bodied seals for access to limited coastal breeding habitats, smaller- bodied and more rapidly-maturing animals will end up breeding in the pack ice (see discussions of body size vs. relative growth rates in Crockford 2006).

Certainly, within landfast ice habitats, ringed seals of both sexes set up territories during the breeding season (Smith and Hammill 1981; Kraftt et al. 2007), and both reportedly show territorial aggression during this time (Smith and Hammill 1981; Stirling 1977, cited in Lydersen and Gjertz 1986).

In other species, a strong correlation has been found between body size and degree of territorial aggression, especially in males that stake out breeding territories; in some taxa, these traits appear to be positively linked to variations in plasma testosterone levels (e.g., Sinervo et al. 2000).

Thus, strongly territorial behavior and an associated larger body size that allows highly territorial animals to successfully outcompete smaller-bodied individuals for preferred breeding sites may drive continual recruitment of small-bodied seals into the off shore pack ice environment, leading to the development of two ecotypes that are morphological distinct distinct but genetically indistinguishable.

Note that such physically and behaviorally distinct ecotypes exist in many other species, see the entire paper for that discussion, cited below.

Crockford, S.J. and Frederick, G. 2011. Neoglacial sea ice and life history flexibility in ringed and fur seals. pg. 65-91 In T. Braje and R. Torrey, eds. Human and Marine Ecosystems: Archaeology and Historical Ecology of Northeastern Pacific Seals, Sea Lions, and Sea Otters. U. California Press, LA. pdf here.

Unfortunately, the tendency to ignore or dismiss the existence of pack-ice breeding ringed seal persists in the more recent literature, including the documents used to supporting listing ringed seals as ‘threatened’ with extinction by the US Fish & Wildlife Service under the Endangered Species Act (Kelly et al. 2010; Lowry 2016; USFSW 2012).

However, the evidence for the existence of ringed seal breeding in Arctic pack ice is not only very strong but helps to explain the long-term survival of polar bears and ringed seals despite massive changes in sea ice conditions over the last few hundred thousand years (Crockford 2017).

Recent papers

Crockford, S.J. 2017. Testing the hypothesis that routine sea ice coverage of 3-5 mkm2 results in a greater than 30% decline in population size of polar bears (Ursus maritimus). PeerJ Preprints 19 January 2017. Doi: 10.7287/peerj.preprints.2737v1 Open access. https://peerj.com/preprints/2737/

Kelly, B. P., Bengtson, J. L., Boveng, P. L., Cameron, M. F., Dahle, S. P., Jansen, J. K., Logerwell, E. A., Overland, J. E., Sabine, C. L., Waring, G. T. and Wilder, J. M. 2010. Status review of the ringed seal (Phoca hispida). NOAA Technical Memorandum NMFS-AFSC-212. www.afsc.noaa.gov/Publications/AFSC-TM/NOAA-TM-AFSC-212.pdf

Lowry, L. 2016. Pusa hispida. The IUCN Red List of Threatened Species 2016: e.T41672A45231341. http://www.iucnredlist.org/details/41672/0

US Fish & Wildlife Service (USFWS). 2012. Threatened status for the Arctic, Okhotsk and Baltic subspecies of the ringed seal. Federal Register 77: 76706–76738.

Selected references from Crockford & Frederick 2011 (see paper for others): contact me for copies if you can’t locate them

Aars, J., N. J. Lunn, and A. E. Derocher (editors) 2006 Polar Bears: Proceedings of the 14th Working Meeting of the IUCN/SSC Polar Bear Specialist Group, 20– 24 June 2005, Seattle, Washington, USA. Occasional Paper of the IUCN Species Survival Commission 32. IUCN, Gland (Switzerland) and Cambridge (UK)

Aars, J., T. A. Marques, S. T. Buckland, M. Andersen, S. Belikov, A. Boltunov, and Ø. Wiig 2009 Estimating the Barents Sea Polar Bear Subpopulation Size. Marine Mammal Science 25: 35–52.

Born, E. W., J. Teilmann, M. Acquarone, and F. F. Riget 2004 Habitat Use of Ringed Seals (Phoca hispida) in the North Water Area (North Baffin Bay). Arctic 57(2):129– 142.

Burns, J. 1970 Remarks on the Distribution and Natural History of Pagophilic Pinnipeds in the Bering and Chukchi Seas. Journal of Mammalogy 51(3):445– 454.

Crockford, S. J. 2006 Rhythms of Life: Thyroid Hormone and the Origin of Species. Victoria, Trafford.

Crockford, S.J. 2008 Be Careful What You Ask For: Archaeozoological Evidence of Mid-Holocene Climate Change in the Bering Sea and Implications for the Origins of Arctic Thule. In Islands of Inquiry: Colonisation, seafaring and the archaeology of maritime landscapes, edited by G. Clark, F. Leach and S. O’Connor, pp. 113– 131. Terra Australis 29 ANU E Press, Canberra.

Crockford, S., and G. Frederick 2007 Sea Ice Expansion in the Bering Sea during the Neoglacial: Evidence from Archaeozoology. The Holocene 17:699– 706.

Crockford, S.J. and Frederick, G. 2011. Neoglacial sea ice and life history flexibility in ringed and fur seals. pg. 65-91 In T. Braje and R. Torrey, eds. Human and Marine Ecosystems: Archaeology and Historical Ecology of Northeastern Pacific Seals, Sea Lions, and Sea Otters. U. California Press, LA.

Davis, C. S., I. Stirling, C. Strobeck, and D. W. Coltman 2008 Population Structure of Ice-Breeding Seals. Molecular Ecology 17:3078– 3094.

Derocher, A. E., Ø. Wiig, and M. Andersen 2002 Diet Composition of Polar Bears in Svalbard and the Western Barents Sea. Polar Biology 25: 448– 452.

Fedoseev, G. A. 1975 Ecotypes of the Ringed Seal (Pusa hispida Schreber, 1777) and Their Reproductive Capabilities. In Biology of the Seal, edited by K. Ronald and A.W. Mansfi eld, pp. 156– 160. Rapports et Proces-verbaux des Reunions, Conseil International Pour L’Exploration de la Mer 169.

Ferguson, S. H. 2006 The Influences of Environment, Mating Habitat, and Predation on Evolution of Pinniped Lactation Strategies. Journal of Mammalian Evolution 13:63– 82

Ferguson, S. H., M. K. Taylor, and F. Messier 2000 Influence of sea ice dynamics on habitat selection by polar bears. Ecology 81:761– 772.

Finley, K. J, G. W. Miller, R. A. Davis, and W. R. Koski 1983 A Distinctive Large Breeding Population of Ringed Seal (Phoca hispida) Inhabiting the Baffin Bay Pack Ice. Arctic 36:162– 173.

Fischbach, A. S., S. C. Amstrup, and D. C. Douglas 2007 Landward and Eastward Shift of  Alaskan Polar Bear Denning Associated with Recent Sea Ice Changes. Polar Biology 30:1395– 1405.

Frost, D. J., L. F. Lowry, G. Pendleton, and H. R. Nute 2004 Factors Aff ecting the Observed Densities of Ringed Seals, Phoca hispida, in the Alaskan Beaufort Sea, 1996– 99. Arctic 57:115– 128.

Hammill, M. O., and T. G. Smith 1991 The Role of Predation in the Ecology of the Ringed Seal in Barrow Strait, Northwest Territories, Canada. Marine Mammal Science
7:123– 135.

Kraftt, B. A., K. M. Kovacs, and C. Lydersen 2007 Distribution of Sex and Age Groups of
Ringed Seals Pusa hispida in the Fast- Ice Breeding Habitat of Kongsfjorden, Svalbard. Marine Ecology Progress Series 335:199– 206.

Labansen, A. L., C. Haug, and K. M. Kovacs 2007 Spring Diet of Ringed Seals (Phoca  hispida) from Northwestern Spitsbergen, Norway. ICES Journal of Marine Science 64:1246– 1256.

Laidre, K. L., I. Stirling, L. F. Lowry, Ø. Wiig, M. P. Heide-Jørgensen, and S. H. Ferguson
2008 Quantifying the Sensitivity of Arctic Marine Mammals to Climate- Induced Habitat Change. Ecological Applications 18(2, Suppl.):S97– S125.

Lee, J., and Taylor, M.K. 1994. Aspects of the Polar Bear Harvest in the Northwest Territories, Canada. Bears: Their Biology and Management 9, Part 1: A Selection of Papers from the Ninth International Conference on Bear Research and Management, Missoula, Montana, February 23-28, 1992, 237-243.

Lønne, O. J., and B. Gulliksen 1989 Size, Age and Diet of Polar Cod, Boreogadus saida (Lepechin 1773), in Ice Covered Waters. Polar Biology 9:187– 191.

Lydersen, C., and I. Gjertz 1986 Studies of the Ringed Seal (Phoca hispida Schreber 1775) in its Breeding Habitat in Kongsfjorden, Svalbard. Polar Research 4:57– 63.

Nowak, R. M. 2003 Walker’s Marine Mammals of the World. John’s Hopkins University Press, Baltimore.

Reeves, R. R, B. S. Stewart, P. J. Clapham, and J. A. Powel 2002 National Audubon Society’s Guide to Marine Mammals of the World. Alfred A. Knopf, New York.

Sinervo, B., Miles, D. B., Frankino, W. A., Klukowski, M., and D. F. DeNardo
2000 Testosterone, Endurance, and Darwinian Fitness: Natural and Sexual Selection on the Physiological Basis of Alternative Male Behaviors in Side- Blotched Lizards. Hormones and Behavior 38:222– 233.

Skúlason, S., and T. B. Smith 1995 Resource Polymorphisms in Vertebrates. Trends in Ecology and Evolution 10:366– 370.

Smith, T. B., and S. Skúlason 1996 Evolutionary Signifi cance of Resource Polymorphisms in Fishes, Amphibians, and Birds. Annual Review of Ecology and Systematics 27:111– 133.

Smith, T. G., and M. O. Hammill 1981 Ecol ogy of the Ringed Seal, Phoca hispida, in Its Fast Ice Breeding Habitat. Canadian Journal of Zoology 59:966– 981.

Stirling, I., and N. A. Øritsland 1995 Relationships between Estimates of Ringed Seal (Phoca hispida) and Polar Bear (Ursus mari-timus) Populations in the Canadian Arctic. Canadian Journal of Fisheries and Aquatic Sciences
52:2594– 2612.

Wiig, O., A. E. Derocher, and S. E. Belikov 1999 Ringed Seal (Phoca hispida) Breeding in the Drifting Pack Ice of the Barents Sea. Marine Mammal Science 15:595– 598.

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