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NOAA Technical Memorandum NMFS-AFSC-211

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Status review of the bearded seal (Erignathus barbatus)

 

Executive Summary

On March 28, 2008, the National Marine Fisheries Service (NMFS) initiated a status review of the bearded seal under the U.S. Endangered Species Act (ESA) (73 FR 16617). Then on May 28, 2008, we received a petition from the Center for Biological Diversity to list the bearded seal as a threatened or endangered species under the ESA, primarily due to concern about threats to this species’ habitat from climate warming and loss of sea ice. To assist with determining whether listing of the bearded seal under the ESA is warranted, a Biological Review Team (BRT) was convened by the NMFS to assess the best available information concerning the status of ringed seals (Erignathus barbatus) and past, present, and future threats to the species.

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 NMFS’s decision about whether to list the species as endangered or threatened. The ESA defines an endangered species as “any species which is in danger of extinction throughout all or a significant portion of its range”. A 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 habitats are affected by climate which is forecast to continue changing directionally at least until the end of the century in response to greenhouse gas forcing. Variability in the magnitude—but not the directionality—of projected 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: The bearded seal is a member of the true seals (Phocidae), a group of mammals highly adapted for a primarily aquatic existence. The species diverged about 11 million years ago from its nearest common ancestor with other extant seal species, prior to the emergence of the white fetal or natal coat that is typical of most northern phocid species and thought to be a specific adaptation to reproducing on sea ice. Nevertheless, they inhabit the seasonally ice-covered seas of the Northern Hemisphere where they whelp and rear their pups, and molt their coats on the ice in the spring and early summer. After reaching sexual maturity— typically at 5-6 years—most female bearded seals produce a single pup each year. Most pups are born in April, but March and May births are common in some areas. The pups are able to enter the water within hours of birth. They begin to forage while still under maternal care during a nursing period of about 24 days. Males typically attain sexual maturity at 6-7 years. A lifespan of about 20-25 years is normal, with a maximum of about 30 years. Like all phocids, bearded seals shed and regenerate their hair and epidermis annually but the process is more prolonged and diffuse than in other species. The timing and duration of molting seems to vary substantially among regions and individuals, but molting generally occurs in April to June or July. Bearded seals feed primarily on benthic organisms, including epifaunal and infaunal invertebrates, and demersal fishes and so are closely linked to areas where the seafloor is shallow. However, they are also able to switch their diet to include schooling pelagic fishes, when advantageous.

Species Delineation: Two subspecies of bearded seals are widely recognized: E. b. barbatus often described as inhabiting the Atlantic sector, and E. b. nauticus inhabiting the Pacific sector. The geographic distributions of these subspecies are not separated by conspicuous gaps, and there are regions of intergrading generally described as somewhere along the northern Russian and central Canadian coasts. The BRT defined longitude 112° W in the Canadian Arctic Archipelago as the North American delineation between the two subspecies and 145° E as the Eurasian delineation between the two subspecies. Based on evidence for discreteness and ecological uniqueness of bearded seals in the Sea of Okhotsk, the E. b. nauticus subspecies was further divided into an Okhotsk DPS and a Beringia DPS, so named because the continental shelf waters of the Bering, Chukchi, Beaufort, and East Siberian Seas that are the bearded seals range in this region overlie much of the land bridge that was exposed during the last glaciation and that has been referred to as Beringia.

Extinction Risk Assessment: The BRT evaluated 19 specific threats to bearded seals 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.

Present or threatened destruction, modification, or curtailment of the species’ habitat or range: The BRT assessed the observed and projected changes with significant potential to impact the bearded seal’s range and habitat, including both the physical and biological components of habitat, such as changes in sea ice, ocean temperature, ocean pH (acidity), and associated changes in bearded seal prey species.

The main concern about the conservation status of bearded seals stems from the likelihood that their sea-ice habitat has been modified by the warming climate and, more so, that the scientific consensus projections are for continued and perhaps accelerated warming in the foreseeable future. A second major concern, related by the common driver of carbon dioxide (CO2) emissions, is the modification of habitat by ocean acidification, which may alter prey populations and other important aspects of the marine ecosystem. A reliable assessment of the future conservation status of each bearded seal species segment requires a focus on projections of specific regional conditions, especially sea ice. Global climate models selected for their consistency with the observed records of regional sea ice were used to project mean, minimum and maximum ice extent in each of the months of March, April, May, June, and July, for each of the decadal periods centered on the years 2030, 2050, and 2090. In a few regions, where the global climate models were poor at hindcasting the observed sea ice, less direct means of predicting ice cover—such as comparison of air temperature predictions with past climatology—were necessary.

For bearded seals, the presence of sea ice is considered a requirement for whelping and nursing young. Similarly, the molt is believed to be promoted by elevated skin temperatures that, in polar regions, can only be achieved when seals haul out of the water. Thus, if suitable ice cover is absent from shallow feeding areas during times of peak whelping and nursing (April/May), or molting (May/June and sometimes through August), bearded seals would be forced to seek either sea-ice habitat over deeper waters (perhaps with poor access to food) or coastal regions in the vicinity of haul-out sites on shore (perhaps with increased risks of disturbance, predation, and competition). Both scenarios would require bearded seals to adapt to novel (i.e., suboptimal) conditions, and to exploit habitats to which they may not be well adapted, likely compromising their reproduction and survival rates. Research suggests that, during the time of whelping and nursing, bearded seals prefer areas where the percent concentration of sea ice is >25%.  Lacking a more direct measure of the relationship between bearded seal vital rates and ice coverage, the BRT assumed that this preference relationship reflects the species requirements for sea-ice coverage and defined areas within the current core distribution of bearded seals where the ice projections were below 25% concentration, as inadequate for whelping and nursing. The BRT also assumed that ice coverage <15% would be insufficient for molting. In the Chukchi, Beaufort, East Siberian, Kara, Laptev, and Greenland Seas, as well as Baffin Bay and the Canadian Arctic Archipelago, little or no decline in ice extent is expected for the months of March – May during the remainder of the century; in most of these areas there will be a moderate decline of June ice cover and a substantial decline of July ice cover. More southerly waters, such as the Sea of Okhotsk, Bering Sea, Hudson Bay, and Barents Sea, are expected to have substantial reductions in average and minimum March - May ice extents by the middle or end of this century; June and July ice cover in these areas has historically been sparse and variable in June – July and will be even more so in the future. Although the distribution of bearded seal breeding and molting areas are not well documented, these more southerly parts of their range are likely to very important.
Ocean acidification, a result of increased carbon dioxide in the atmosphere, may impact bearded seal survival and recruitment through disruption of trophic regimes that are dependent on calcifying organisms. The nature and timing of such impacts are extremely uncertain. Changes in bearded seal prey, anticipated in response to ocean warming and loss of sea ice, have the potential for negative impacts, but the possibilities are complex. Ecosystem responses may have very long lags as they propagate through trophic webs. Because of bearded seals’ apparent dietary flexibility, this threat may be of less immediate concern than the threats from sea-ice degradation.

Overutilization for commercial, subsistence, recreational, scientific, or educational purposes:  Recreational, scientific, and educational utilization of bearded seals is currently at low levels and is not projected to increase to significant threat levels in the foreseeable future for any of the DPSs. Bearded seals’ solitary nature has made them less suitable for commercial exploitation than many other seal species. Still, they may have been depleted by commercial harvests in some areas of the Sea of Okhotsk and the Bering, Barents, and White Seas during the mid-20th century. There is currently no significant commercial harvest of bearded seals and significant harvests seem unlikely in the foreseeable future. Bearded seals have been a very important species for subsistence of indigenous people in the Arctic for thousands of years. The current subsistence harvest is substantial in some areas but there is little or no evidence that subsistence harvests have or are likely to pose serious risk to the species.

Diseases, parasites, and predation: A variety of diseases and parasites have been documented to occur in bearded seals. The seals have likely co-evolved with many of these and the observed prevalence is typical and similar to other species of seals. Abiotic and biotic changes to bearded seal habitat potentially could lead to exposure to new pathogens or new levels of virulence, but the BRT considered the potential threats to bearded seals as low.
Polar bears are the primary predators of bearded seals, but other predators include brown bears, killer whales, sharks, and walruses. Predation under the future scenario of reduced sea ice is difficult to assess; polar bear predation may decrease, but predation by killer whales, sharks and walrus may increase.

Inadequacy of existing regulatory mechanisms: Harvests and incidental takes by fisheries and commercial activities are reasonably well regulated throughout the range of bearded seals. Currently, however, there are no effective mechanisms to regulate the global greenhouse gas emissions that are driving—via climate warming—destruction of ringed seal habitat. The BRT implicitly considered impacts of inadequate regulation of greenhouse gas 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: Contaminants, oil and gas industry activities, fisheries, and shipping all have potential to impact bearded seal populations. Compared to the far-reaching changes expected in sea ice and ocean conditions, the future changes and impacts from these other factors were judged by the BRT as likely to be more localized and less significant.

Status of the bearded seal species segments: The BRT reviewed published data and consulted with other experts to evaluate the specific threats to population persistence for the E. b. barbatus subspecies and the Beringia and Okhotsk DPSs of bearded seals. Threats were scored quantitatively and the level of certainty in scores was recorded. Risks posed by the inadequacy of existing regulatory mechanisms were not included as part of this scoring. The BRT also assessed the risks to population persistence posed by those threats in demographic terms (abundance, productivity, spatial structure, and diversity).

E. b. barbatus: The present population size is very uncertain, but was estimated to be about 188,000 individuals. Sea ice of sufficient concentration is expected to persist in the Canadian Arctic Archipelago and Baffin Bay through the end of the century.  Throughout the rest of E. b. barbatus’s range, however, the reduction in sea ice is expected to be more dramatic, particularly in the Barents Sea which will become ice free in June within the next few decades. The impacts of the loss of sea ice in the Barents Sea, and the reduction of sea ice in the Kara Sea, could theoretically be offset by the creation of more suitable sea ice habitat in the Laptev Sea which currently has high concentrations of ice and low numbers of bearded seals, though we are not aware of past examples in which similar mitigative shifts in habitat have occurred during rapid climatic changes.

The BRT judged the decoupling, or spatial separation of sea ice from benthic feeding habitat to be a moderate to high threat to the subspecies E. b. barbatus; this, and a decrease in sea-ice habitat suitable for molting were judged to be nearly the same strength and the most significant threats to the subspecies. Both threats stem from the anticipated reduction in sea ice area and seasonal persistence, and these combined with other sea-ice, climate, and ocean acidification threats were reflected in a moderate to high score overall for threats associated with habitat modification or loss. Threats within the other ESA Section 4(a)(1) factors were judged to be low or moderate. 

The present risks that this subspecies is in danger of extinction due to demographic problems of abundance, productivity, spatial structure, and diversity were all judged to be low or very low in severity, reflecting the evidence that the population is large and apparently occupying its historic range. Within the foreseeable future, the risks that demographic problems might place the population in danger of extinction were consistently judged to be higher than at present (medium in severity), reflecting the anticipated decline in sea ice habitat over the remainder of the 21st century and consequent impacts on spatial structure (loss of habitat patches) and productivity (changes in vital rates of reproduction and survival).

Beringia DPS:  The present population size is very uncertain, but was estimated to be about 155,000 individuals. In the East Siberian, Chukchi and Beaufort Seas the average ice extents during the whelping, nursing, mating and part of the molting period are all predicted to be very close to historical averages out to 2095 with typically 25% or greater ice concentration over the entire shelf zones. End of century projections for the Bering Sea in April-May suggest that there will be sufficient ice only in small zones of the Gulf of Anadyr and in the area between St. Lawrence Island and Bering Strait.  In June, suitable ice is predicted to disappear as early as mid-century.  To adapt to this regime, bearded seals would likely have to shift their nursing, rearing and molting areas to the ice covered seas north of the Bering Strait.

The most significant threats to the persistence of bearded seals in the Beringia DPS were judged to be the same ones as for the E. b. barbatus subspecies, plus the threat of a decrease in prey density or availability due to changes in ocean temperature and ice cover. These and other threats related to loss of sea ice habitat and climate-related changes in ocean conditions were scored in the moderate to high range, leading to an overall score of high for threats of habitat modification or loss. Threats within the other ESA Section 4(a)(1) factors were judged to be low or moderate. The threat-specific and overall habitat factor scores were generally higher for the Beringia DPS than for E. b. barbatus, likely a measure of greater concern about declines in ice extent that would separate sea ice suitable for pup maturation and molting from benthic feeding areas. The risk of this is likely greater in the range of the Beringia DPS, where the spring and summer ice edge may retreat to deep water of the Arctic Ocean basin, than in much of the range of E. b. barbatus, where summer ice refugia are anticipated to occur in the Canadian Arctic Archipelago and northern Greenland.

The present risks that the Beringia DPS is in danger of extinction due to demographic problems of abundance, productivity, spatial structure, and diversity were all judged to be low or very low in severity, again on the basis of a large population occupying its historic range.  Within the foreseeable future, the risks that demographic problems might place the population in danger of extinction were consistently judged to be higher than at present (medium to high in severity), and higher than the corresponding risks to E. b. barbatus.

Okhotsk DPS: The present population size is very uncertain, but was estimated to be about 95,000 individuals. By the end of this century, the Sea of Okhotsk is predicted to frequently be ice free, or nearly so, during April – June, the crucial months for those important life history events. The ice-covered area is much smaller in the Sea of Okhotsk than the Bering Sea and, unlike in the Bering Sea, there is no marine connection to the Arctic Ocean. Over the long-term, bearded seals in the Sea of Okhotsk do not have the prospect of following a shift in the average position of the ice front northward as seals in the Bering Sea would. The greatest threats to the persistence of bearded seals in the Okhotsk DPS were judged to be decreases in sea ice habitat suitable for whelping, nursing, pup maturation, and molting. These threats, which were scored as high, are more severe in the range of the Okhotsk DPS than in the ranges of the Beringia DPS and E. b. barbatus. The overall score for habitat-related threats also was greatest for the Okhotsk DPS (high, tending toward very high). Although the scores for the other ESA Section 4(a)(1) factors were in the low to moderate range, they tended to be higher than those for E. b. barbatus and the Beringia DPS.

The present risks that the Okhotsk DPS is in danger of extinction due to demographic problems of abundance, productivity, spatial structure, and diversity were all judged to be low in severity but the numerical scores were somewhat higher than for the other species segments. This may be a result not only of stronger threats identified above, but also of the somewhat lower confidence (certainty scores) ascribed to the threats scores for the Okhotsk DPS. Within the foreseeable future, the risks that demographic problems might place the Okhotsk DPS in danger of extinction were consistently judged to be higher than at present (high for abundance, productivity, and spatial structure; medium for severity), and the highest of the three species segments.

 


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