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Status review of the ribbon seal (Histriophoca fasciata)

Executive Summary

This status review is intended to be a compilation of the best available information concerning the status of ribbon seals (Histriophoca fasciata), including the past, present, and future threats to this species. It was compiled by a National Oceanic and Atmospheric Administration (NOAA) Biological Review Team (BRT) in response to a petition filed by the Center for Biological Diversity to list the ribbon seal as threatened or endangered under the U.S. Endangered Species Act (16 U.S.C. 1531 et seq.) (ESA), primarily due to concern about threats to this species’ habitat from climate warming and loss of sea ice.
There are two key tasks associated with conducting an ESA status review:  The first is to delineate the taxonomic group under consideration; the second is to conduct an extinction risk assessment to determine whether the petitioned species is threatened or endangered. The ESA defines the term endangered species as “any species which is in danger of extinction throughout all or a significant portion of its range”. The term 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 horizon of the foreseeable future was determined to be the year 2050 because past and current emissions of greenhouse gases have already largely set the course for changes in the atmosphere and climate until that time, and because of enormous uncertainty about future social and political decisions on emissions that will dominate projection of conditions further into the future.

Species Background: The ribbon seal is a strikingly-marked member of the family Phocidae that primarily inhabits the Sea of Okhotsk, and Bering and Chukchi Seas. This species is strongly associated with the sea ice during its whelping, mating, and pelage molt periods, from mid-March through June. Most of the rest of the year is spent at sea; the species is rarely observed on land. The rates of survival and reproduction are not well known, but ribbon seals can live 20 to 30 years. They become sexually mature at 1-5 years of age, probably depending on environmental conditions, and adult females usually give birth every year to a single pup which is nursed for 3-4 weeks and then abandoned to fend for itself.

Species Delineation: Although there are two main breeding areas for ribbon seals, one in the Sea of Okhotsk and one in the Bering Sea, there is currently no evidence of discrete subpopulations on which to base a separation into distinct population segments. The population composing the entire species is the subject of this review. A molecular genetic analysis to identify geographic population structure is a high priority for research.

Extinction Risk Assessment: To assess the extinction risk, the BRT evaluated the risks based on specific demographic factors of the species, such as abundance, productivity, spatial structure, and diversity, as well as specific threats faced by the species, as outlined in Section 4(a)(1) of the ESA:

  • the present or threatened destruction, modification, or curtailment of its habitat or range,
  • over-utilization 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

Demographic factors: With a population likely comprising at least 200,000 individuals, ribbon seals are not currently at risk from the demographic issues of low abundance commonly associated with ESA listing decisions, such as demographic stochasticity, inbreeding, loss of genetic diversity, and depensatory effects. The current population trend is unknown, but a recent estimate of 49,000 ribbon seals in the eastern and central Bering Sea is consistent enough with historical estimates to suggest that no major or catastrophic change has occurred in recent decades. The species is thought to occupy its entire historically-observed range; there are no portions of the range in which ribbon seals have been reported to have disappeared or become extinct. A comprehensive survey of ribbon seal abundance, a new analysis of demographic data obtained from the former Soviet commercial harvest, and genetic studies of population structure are high priorities for research and monitoring.

Present or threatened destruction, modification, or curtailment of the species’ habitat or range: The main concerns about the conservation status of the ribbon seal stem from the likelihood that its 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 reliable assessment of the future conservation status of ribbon seals requires a focus on projections of the specific regional conditions, especially sea ice, and changes that could impact vital rates.
In contrast to the Arctic Ocean, where sea ice is present year-round, the ice in the Bering Sea and Sea of Okhotsk is seasonal in nature. Despite the recent dramatic reductions in Arctic Ocean ice extent during summer, the sea ice in the northern Bering Sea and Sea of Okhotsk is expected to continue forming annually in winter for the foreseeable future. The sea ice regimes in these seas will continue to be subject to large interannual variations in extent and seasonal duration, as they have throughout recorded history. While there may be more frequent years in which ice coverage is reduced, the late March to early May period in which ribbon seal reproduction occurs will continue to have substantial ice, particularly in the northern regions of the breeding range. In years of low ice it is likely that ribbon seals will adjust at least in part by shifting their breeding locations in response to the position of the ice edge as they have likely done in the past in response to interannual variability.
There could be impacts on ribbon seal survival and recruitment from more frequent years of reduced ice thickness and duration of seasonal ice coverage. Decreased availability of stable platforms for adults to complete their molt out of the water may lower survival, but it is not currently possible to quantify this impact or the extent to which ribbon seals may adapt by shifting locations for key life history events of breeding and molting. Weaned pups are likely dependent on sea ice for a 2-3 week period as they develop self-sufficiency in foraging. They enter the water regularly during this period, and therefore may not be particularly sensitive to modest reductions in coverage or quality, though they may be relatively limited in their capability to respond to rapidly deteriorating ice fields by relocating over large distances, a factor that could occur more frequently in the foreseeable future.
Ocean acidification, a result of increased carbon dioxide in the atmosphere, may impact ribbon 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. Because of ribbon seals’ apparent dietary flexibility and because the major effects of ocean acidification may not appear until the latter half of this century, this threat should be of less immediate concern than the direct effects of sea ice degradation.
Changes in ribbon seal prey, anticipated in response to ocean warming and loss of sea ice, have the potential for negative impacts, but the possibilities are complex. Some changes already documented in the Bering Sea and the North Atlantic Ocean are of a nature that could be ameliorative or beneficial to ribbon seals. For example, several fish species, including walleye pollock (Theragra chalcogramma), a common ribbon seal prey, have shown northward distribution shifts and increased recruitment in response to warming, at least initially. These ecosystem responses may have very long lags as they propagate through trophic webs. Apparent flexibility in ribbon seal foraging locations and habits may make these threats of lower concern than more direct impacts from changes in sea ice.
The threats associated with impacts of global warming on ribbon seal habitat, to the extent that they may pose risks to ribbon seals, were presumed to manifest throughout the current breeding and molting range (for sea-ice related threats) or throughout the entire range (for ocean warming and acidification) of the species, inasmuch as the finer-scale spatial distribution of these threats is not currently well understood. The question, therefore, of whether any of these poses a risk to the continued existence of ribbon seals in merely a significant portion of, rather than the entire, range was subsumed by this approach to the qualitative risk assessment.

Over-utilization for commercial, subsistence, recreational, scientific, or educational purposes: Recreational, scientific, and educational utilization of ribbon seals is currently at very low levels and is not projected to increase to significant threat levels in the foreseeable future. Commercial harvests by Russian sealers have at times been high enough to cause significant reductions in abundance and catch-per-unit-effort. The population apparently rebounded from a period of high harvest in the 1960s. Substantial but lower numbers were harvested for a few years in the early 1990s. Although Russian government quotas were recently in place that would allow large harvests (~18,000 annually), the actual takes are low because of poor economic viability. There is some effort in Russia to develop new uses and markets for seal products, but unless these are successful, the harvest is unlikely to increase in the near future. Subsistence harvest levels have been low historically, but could potentially increase in the future if ribbon seals are forced to use a reduced and more northerly ice field, which could put them in closer proximity to Alaska Native communities near the Bering Strait.

Diseases, parasites, and predation: A variety of pathogens (or antibodies), diseases, helminthes, cestodes, and nematodes, have been found in ribbon seals. The prevalence of these agents is not unusual among seals, but the population impact is unknown. There may be an increased risk of outbreaks of novel pathogens or parasites as climate-related shifts in species distributions lead to new modes of transmission. There is little or no direct evidence of significant predation on ribbon seals and they are not thought to be a primary prey of any predators. Polar bears and killer whales may be the most likely opportunistic predators in the current sea ice regime, but walruses could pose a potentially greater risk if reduced sea ice conditions force these pagophilic species into closer proximity in the future.

Inadequacy of existing regulatory mechanisms: There is little evidence that the inadequacy of existing regulatory mechanisms currently poses a threat to ribbon seals. However, there are no known regulatory mechanisms that effectively address reductions in sea ice habitat at this time. Also, it is unclear what regulatory mechanisms are in place to ensure that potential commercial harvests in Russia are conducted in a sustainable fashion.

Other natural or human factors affecting the species’ continued existence: Although some pollutants are elevated in ribbon seals, there is no conspicuous evidence of toxicity or other significant impacts to the species. Continued and expanded monitoring would be prudent, to document any trends in the contaminants of greatest concern.
Oil and gas exploration and development activities may include artificial-island construction, drilling operations, pipeline construction, seismic surveys, and vessel and aircraft operations. The main issues for evaluating the impacts of exploration and development activities on ribbon seals are the effects of noise, disturbance, and potential oil spills produced from these activities. Any negative effects on ribbon seals from noise and disturbance associated with development activities are likely to be minor and localized. Ribbon seals are also highly dispersed during the summer, open-water season so the rate of interactions with seismic surveys would likely be low, and in any case seals have not been shown to be significantly impacted by oil and gas seismic surveys. The threat posed to ribbon seals by oil spills will increase if offshore oil and gas development and shipping activities increase across their range as predicted. The potential impacts would be greatest during April-June when the seals are relatively aggregated, and substantially lower during the remainder of the year when they are dispersed in the open water throughout the North Pacific Ocean, Sea of Okhotsk, and Bering and Chukchi Seas.
Estimates from observed by-catch in commercial fisheries imply that less than 200 ribbon seals per year are taken, though mortalities are certainly under-reported in some fisheries. Because there is little or no fishery activity near aggregations of ribbon seals when they are associated with ice, and they are highly dispersed in the remainder of the year, by-catch is unlikely to be a significant threat to ribbon seal populations. For the same reason, competition from fisheries that reduce local abundance of ribbon seal prey is unlikely to be significant. Broad-scale reduction in a commercially-fished, primary prey species could have a significant impact, but the large groundfish fisheries in Alaska waters, at least, are well-managed to prevent depletion of the stocks.
The extraordinary reduction in Arctic sea ice that has occurred in recent years has renewed interest in trans-Arctic navigation routes connecting the Atlantic and Pacific Oceans via the Northwest Passage and the Northern Sea Route. The Chukchi Sea and Bering Strait would be the most likely areas for increased exposure of pelagic ribbon seals to ship traffic, because of the geographic constriction and the seasonal migration of part of the ribbon seal population around the beginning and end of the ice-free season. However, there is currently little or no information on direct impacts from shipping on seals in open water. Ribbon seals hauled out on sea ice may also be at risk from increased ship traffic, but likely only during spring and early summer, and then only by ice-reinforced ships. Assessing risk from increases in shipping and transportation is difficult because projections about future ship trends within the ribbon seal’s range are currently unavailable.
Several of the threats considered in this section on “Other natural or human factors affecting the species’ continued existence” were associated with specific regions or times of year when ribbon seal distribution is restricted, such as increased ship traffic in the Bering Strait region or oil and gas activities during the ribbon seal breeding and molting seasons. If such threats were to occur and cause a high rate of mortality or forgone reproduction, the species could be considered threatened or endangered in a significant portion of its range. However, none of the threats considered here is presently considered to be both sufficiently likely to occur and sufficiently high in impact, alone or cumulatively, to raise concern about them posing a risk of ribbon seal extinction or becoming endangered throughout a significant portion of its range.

Status of the ribbon seal population: Qualitative assessments of the balance between identified threats and a species’ capability to adjust must often be decided on the basis of expert opinion and on policy, especially policy regarding consideration given to uncertainty. Such policy is outside the scope of this status review. To assist in the process, however, the BRT addressed its summary conclusions on ribbon seal status using a scoring system in which each of the eight members assigned up to 10 likelihood points in support of the conclusion.
In consideration of all of the threats and potential threats identified above, the assessment of the risks posed by those threats, the possible cumulative impacts, and the uncertainty associated with all of these, the BRT drew the following conclusions:

  • Ribbon seals are not in current danger of extinction throughout all or a significant portion of their range. (78 out of 80 likelihood points)
  • The ribbon seal population is likely to decline gradually for the foreseeable future, primarily from slight but chronic impacts on reproduction and survival caused by reduced frequency of years with sea ice of suitable extent, quality, and duration of persistence. (51 out of 80 likelihood points)
  • Despite the expectation of a gradual decline, ribbon seals are not likely to become an endangered species within the foreseeable future throughout all or a significant portion of their range. (57 out of 80 likelihood points)

Finally, to reinforce the notion that reliable and effective assessments of species’ conservation status cannot be conducted without adequate estimates of abundance, the BRT concluded that despite the expectation of a declining ribbon seal population, it will likely not be possible to detect and document a significant overall decline unless monitoring is made a very high priority for both the Bering Sea and Sea of Okhotsk.

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