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Status review of the ringed seal (Phoca hispida)

 

Executive Summary

A Biological Review Team (BRT) convened by the National Marine Fisheries Service assessed the best available information concerning the status of ringed seals (Phoca hispida) and past, present, and future threats to the species in response to a petition filed by the Center for Biological Diversity. The petition seeks to list the ringed seal as threatened or endangered under the U.S. Endangered Species Act (ESA), primarily due to concern about threats to the species’ habitat from climate warming and diminishing ice and snow cover.

The BRT’s review included delineating population structure within the species and assessing the risk of extinction at present and in the foreseeable future. The review is intended to inform the Secretary of Commerce’s decision whether to list the species as endangered or threatened in all or part of its range. The ESA defines an endangered species as “any species which is in danger of extinction throughout all or a significant portion of its range.” Threatened species is defined as “any species which is likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range.” The foreseeable future was not considered to be fixed but, rather, threat specific. Ice and snow habitats are affected by climate which is forecasted to continue changing directionally at least until the end of the century in response to greenhouse gas (GHG) forcing. Variability in the magnitude—but not the directionality—of climate change increases over time and is reflected in inter-model variability and in levels of certainty assigned to the BRT’s assessments of threats and demographic risks.

Species Background: Ringed seals are one of the smallest true seals (Phocidae), a group of marine carnivores descended from terrestrial mammals. The distribution of phocids is biased toward polar seas reflecting evolutionary adaptations to ice-covered waters. Ringed seals are the most strongly ice-associated seal, coming out of the water exclusively on sea ice except in marginal seas and freshwater lakes where ice disappears seasonally. Their global distribution has expanded and contracted with changing sea-ice cover, and today they inhabit all the seasonally ice-covered seas of the Northern Hemisphere as well as Lake Saimaa (Finland) and Lake Ladoga (Russia). After reaching sexual maturity, typically at 4-8 years—and when environmental conditions are favorable—female ringed seals produce a single pup each year. The pups are born in subnivean lairs (snow caves) excavated above breathing holes in the ice, where they are nursed for 5-9 weeks. Males typically become sexually mature at 5-7 years. Survival rates are not well known, but ringed seals can live in excess of 40 years. Ringed seals eat a wide variety of prey but tend to prefer small, schooling species that form dense aggregations, such as cods, smelts, and crustaceans. Population estimates are highly uncertain in most areas and range from the low millions in the Arctic to less than 300 in Lake Saimaa.

Species Delineation: Five subspecies of ringed seals inhabit the Arctic Ocean, Sea of Okhotsk, Baltic Sea, Lake Ladoga, and Lake Saimaa and are discrete breeding populations. The Arctic Ocean subspecies may be comprised of multiple discrete subpopulations, but population structure is not fully resolved.

Extinction Risk Assessment: For each subspecies, the BRT evaluated 17-18 threats grouped by the ESA Section 4(a)(1) factors:

  • the present or threatened destruction, modification, or curtailment of its habitat or range,
  • overutilization for commercial, recreational, scientific, or educational purposes,
  • disease or predation,
  • the inadequacy of existing regulatory mechanisms, or
  • other natural or manmade factors affecting its continued existence

The BRT also assessed the risks to population persistence posed by those threats in demographic terms (abundance, productivity, spatial structure, and diversity). The Arctic Ocean, Sea of Okhotsk, Baltic Sea, and Lake Ladoga subspecies all number several thousand or more seals and are not believed to be currently at risk from the effects of demographic stochasticity, inbreeding, loss of genetic diversity, or depensation. The Saimaa seal, however, numbers fewer than 300 individuals and shows substantially lower genetic diversity than do the other subspecies. Saimaa ringed seals have been listed as endangered under the ESA since 1993.

Present or threatened destruction, modification, or curtailment of the species’ habitat or range: Diminishing ice and snow cover are the greatest challenges to persistence of all of the ringed seal subspecies. Climate models consistently project overall diminishing ice and snow cover at least through the current century with regional variation in the timing and severity of those losses. Increasing atmospheric concentrations of GHGs, including carbon dioxide (CO2), will drive climate warming and increase acidification of the ringed seal’s ocean and lake habitats. Acidification threatens changes in prey communities on which ringed seals depend.
Ice loss will be greatest in the summer and fall months when the ringed seal’s use of ice as a resting platform is at a minimum. In those months, however, ice remains important to prey populations such as Arctic cod, and ringed seal populations will be affected by diminished prey populations. Increased competition with northward-expanding, subarctic species may also affect prey densities. The greatest impacts to ringed seals of diminished ice cover will be mediated through diminished snow accumulation. While winter precipitation is forecasted to increase in a warming Arctic, the duration of ice cover will be substantially reduced, and the net affect will be lower snow accumulation on the ice. Model forecasts indicate that throughout the range of ringed seals, there will be substantial reductions in on-ice snow cover. Snow depth limits the formation of subnivean lairs, and birth lairs require depths of at least 50-65 cm. Such depths typically are found only where 20-30 ­­cm or more of snow has accumulated on flat ice and drifted along pressure ridges or ice hummocks. Within the century, snow cover is forecasted to be inadequate for the formation and occupation of birth lairs over most of the species’ range. Without the protection of the lairs, ringed seals—especially newborn—are vulnerable to freezing and predation. As populations decline, the significance of currently low-level threats—including ocean acidification, increased human activity, and changes in populations of prey, predators, competitors, and parasites—may increase.

Overutilization for commercial, subsistence, recreational, scientific, or educational purposes: Subsistence and commercial harvests of Arctic ringed seals have been large in the past, but there is no evidence that they have contributed to large-scale population declines. Commercial harvests in the Sea of Okhotsk and predator-control harvests in the Baltic Sea, Lake Ladoga, and Lake Saimaa caused population declines in the past but have since been restricted. Current harvest levels appear to be low and sustainable. Recreational, scientific, and educational uses are minimal and not projected to increase significantly in the foreseeable future for any of the subspecies.

Diseases, parasites, and predation: Ringed seals have co-evolved with numerous parasites and diseases, and those relationships are presumed to be stable. Evidence of distemper virus, for example, has been reported in Arctic ringed seals, but there is no evidence of impacts to ringed seal population size or productivity. Abiotic and biotic changes to ringed seals’ habitat potentially could lead to exposure to new pathogens or new levels of virulence, but the BRT considered the potential threats to ringed seals as low.
Ringed seals are commonly preyed upon by polar bears and Arctic foxes, and less commonly by other terrestrial carnivores, sharks, and killer whales. Predation on newborn pups by gulls and ravens is typically prevented by the pups’ concealment in subnivean lairs. When the pups are prematurely exposed, however, predation by birds—as well as terrestrial carnivores—can be substantial.

Inadequacy of existing regulatory mechanisms: Harvests and incidental takes by fisheries and commercial activities are reasonably well regulated throughout the range of ringed seals. Currently, however, there are no effective mechanisms to regulate the global GHG emissions that are driving—via climate warming—destruction of ringed seal habitat. The BRT implicitly considered impacts of inadequate regulation of GHG emissions by way of the emissions scenarios used in forecast models; the scenarios were all “non-mitigated”, meaning that they assumed no globally-significant framework for regulating or reducing emissions would be implemented.

Other natural or human factors affecting the species’ continued existence: Drowning of seals in fishing nets and disturbance by human activities remain conservation concerns in Lake Saimaa and Lake Ladoga. Reduced productivity in the Baltic Sea subspecies in recent decades resulted from pollutants impairing fertility. Petroleum development, commercial fisheries, increased ship traffic, and pollutants pose moderate risks to the Arctic, Okhotsk, and Baltic subspecies. Their significance would increase, however, for any populations diminished by the effects of climate change or other threats.

Status of the ringed seal subspecies: The BRT reviewed published data and consulted with other experts to evaluate the specific threats and demographic risks to population persistence for each subspecies of ringed seals. Threats and demographic risks were scored quantitatively, and the level of certainty in scores was also recorded. The BRT concluded:

Arctic ringed seals: Persistence of the Arctic subspecies likely will be challenged as decreases in ice and, especially, snow cover lead to increased juvenile mortality from premature weaning, hypothermia, and predation. The depth and duration of snow cover are forecasted to decline substantially throughout the range of Arctic ringed seals. Risks to abundance, productivity, spatial structure, and diversity currently are low. In the foreseeable future, however, it is expected that abundance and productivity will decline and spatial structure will be disrupted by rapid loss of habitat. Initially, impacts may be somewhat ameliorated if the subspecies’ range retracts northward with sea-ice habitats. By 2100, however, average snow depths will fail to meet the 20-30 cm minimum needed for successful formation and maintenance of birth lairs in a substantial portion of the subspecies’ range. Thus, within the foreseeable future, it is likely that the number of Arctic ringed seals will decline substantially, and they will no longer persist in substantial portions of their range.

Okhotsk ringed seals: Before the end of the current century, ice is forecasted to be limited to the northernmost regions of the Sea of Okhotsk during the whelping and nursing periods. On-ice measurements of snow cover are not available for the Sea of Okhotsk, but model results indicate inadequate (< 20 cm average depth) snow for birth lairs throughout the subspecies’ range in the most recent decade. Okhotsk ringed seals apparently depend on sheltering in the lee of ice hummocks as snow cover is inadequate for lair construction in much of the habitat. In the foreseeable future, the diversity of the subspecies will be at moderate risk while its abundance, productivity, and spatial structure will be highly at risk. Okhotsk ringed seals likely will decline to levels that threaten their persistence as a consequence of a decrease in sea-ice habitat suitable for whelping, nursing, and molting. The range of Okhotsk ringed seals is bounded by land to the north, and the opportunity to retract its range with the ice is limited accordingly.

Baltic ringed seals: Substantial reductions in sea-ice extent by mid-century, coupled with deteriorating snow conditions, will substantially alter the habitat of Baltic ringed seals and lead to decreased survival of pups. In the foreseeable future, risks to demographic attributes are expected to be moderate (diversity) to high (abundance, productivity, and spatial structure). The range of Baltic ringed seals is bounded by land to the north, and the opportunity to retract their range with the ice is limited accordingly. Degradation of ice and snow habitats is likely to cause substantial population declines and threaten the Baltic ringed seal’s persistence within the foreseeable future.

Ladoga ringed seals: Persistence of Ladoga ringed seals will be challenged by decreased ice habitat suitable for whelping and nursing as well as increased pup mortality from hypothermia and predation due to insufficient depth or duration of snow cover. At present, there is a moderate risk to the subspecies’ persistence since, as a landlocked population, Ladoga ringed seals cannot disperse to new habitats. Within the foreseeable future, persistence will be challenged by moderate risks to diversity and high risks to abundance, productivity, and spatial structure. Degradation of ice and snow habitats is likely to cause substantial population declines and threaten the Ladoga ringed seal’s persistence within the foreseeable future.

Saimaa ringed seals: Saimaa seals currently are listed as endangered under the ESA. The population remains low (< 300 seals) and pup mortality remains high. Forecasts of decreasing snow depth and seasonal duration likely will increase pup mortality through hypothermia and predation. Declining ice cover represents a further loss of habitat. The present risk to population persistence is moderate to high in terms of productivity and diversity and high to very high in terms of abundance and spatial structure. In the foreseeable future, the risks are expected to be high to very high for all of the demographic attributes. Degradation of ice and snow habitats is likely to cause substantial population declines and further threaten the Saimaa ringed seal’s persistence within the foreseeable future.



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