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NOAA Technical Memorandum NMFS-AFSC-255

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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 is an update to a previous review (Boveng et al. 2008) conducted 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 (ESA), primarily due to concern about threats to this species’ habitat from climate warming and loss of sea ice. This update was compiled by a National Oceanic and Atmospheric Administration (NOAA) biological review team (BRT) under a settlement agreement with the Center for Biological Diversity on August 30, 2011, in which NMFS agreed to apply a modified approach to analyzing extinction risk in the “foreseeable future”, and incorporate new information that has become available since the previous review.

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 2100 because a broad scientific consensus has developed around climate and sea-ice projections by the Intergovernmental Panel on Climate Change, and results of those projections are available through the end of the current century. Projections to the middle of the century are considerably more certain because past and current emissions of greenhouse gases have already largely set the course for changes in the atmosphere and climate until that time, so the BRT also considered potential outcomes from the present to the year 2050, as well.

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.

Extinction Risk Assessment:
To assess the extinction risk, the BRT first evaluated specific threats faced by the species, grouped in categories, or factors, as outlined in Section 4(a)(1) of the ESA:

  • 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 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-April 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 will likely 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. Preliminary analysis of recent satellite tracking results corroborate natural history observations that ribbon seals prefer to forage at the continental shelf break or in deeper water beyond. This preference may result in reduced condition, survival, or reproduction if the future ice edge recedes far from the shelf break during periods when ribbon seals are tied to the ice by a need to reproduce or molt, though their feeding is likely reduced and perhaps not as important during the molt.

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 a wide array of calcifying organisms, but also through more general impacts on physiology and energetics of a broader suite of fish and invertebrates. The nature and timing of such impacts are extremely uncertain. Because of ribbon seals’ apparent dietary flexibility, the species may have greater resilience to these impacts, unless there are major trophic shifts that persist for periods of a ribbon seal generation or more.

Changes in ribbon seal prey, anticipated in response to ocean warming and loss of sea ice, also have the potential for negative impacts, but again 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. However, the possibility of cumulative or multiplicative effects involving both loss of sea ice and changes in prey communities is important to consider.

Overutilization 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. A recent outbreak of disease in ice-associated pinnipeds including ribbon seals, in which no infectious agent has yet been identified, may be evidence of that risk. 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 sharks may be underestimated as ribbon seal predators, and 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 at elevated levels 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 seismic surveys for oil and gas. 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 shipping trends within the ribbon seal’s range are currently unavailable.

Threats Assessment:
A structured approach was used to elicit BRT members’ expert judgment about the significance of threats facing ribbon seals. The threats were grouped by ESA Section 4(a)(1) factors (excluding Inadequacy of Existing Regulatory Mechanisms), and each individual threat was scored for its significance in two components: extent and likelihood. Extent was defined as the portion of the population that would experience reduced survival or reproductive success if the threat condition or event were to occur. Judging the extent of a threat required considerations such as the geographic scope, the age classes of ribbon seals affected, and the duration of that threat. Likelihood was defined as the probability that the threat will occur within a specified time period in the foreseeable future.

The extent and likelihood of each threat were scored on 5-level scales, separately for the Bering Sea and the Sea of Okhotsk, and in consideration of two time intervals: from now to the year 2050, and from now to the year 2100. The former interval was included to provide a sense of how some of the threats are anticipated to change, and to provide a reference similar to the time frame used in the previous assessment of ribbon seal status. The latter interval corresponds to the foreseeable future for the primary threats related to climate disruption by greenhouse gas emissions.

The extent and likelihood scores for the period from now to 2100 were combined as a product to yield a threat score that could range from low to extreme (numerically from 1-25). These individual threat scores were then considered in a judgment about the overall score, using the same scale, for all the threats included in each ESA Section 4(a)(1) factor. In assigning an overall factor score, BRT members considered that a factor score should typically be at least as high as the individual threat scores within that factor, and in some cases higher because multiple threats may have cumulative effects that amplify the overall significance of the factor.

For ribbon seals in both the Bering Sea and Sea of Okhotsk, the BRT judged the threats of destruction or modification of habitat to be of greater significance than the threats from the other factors. In the Bering Sea, loss of suitable ice for molting and pup maturation, and diminished prey from effects of ocean acidification were judged to be the greatest threats to habitat, although confidence about the score for the latter threat was low. In the Sea of Okhotsk, losses of ice suitable for whelping, nursing, mating, and molting were judged to pose the greatest threats. Threats of overutilization were judged to be of low significance and threats from disease, parasites, and predation were judged to be of moderate significance in both regions. Threats from other natural or man-made factors were judged to be of moderate significance in both regions. Across all four factors, the threats were judged to be more significant in the Sea of Okhotsk than the Bering Sea, reflecting greater concern about ice loss and regulation of petroleum development in the Sea of Okhotsk. There was substantial variability in the threat scores and factor scores among the BRT members, with strong consensus only in the factor comprising threats of overutilization in the Bering Sea.

Demographic risks:
Threats to a species’ long-term persistence are manifested demographically as risks from low abundance, poor productivity, disrupted spatial structure and connectivity, and reduced genetic and ecological diversity. These demographic risks thus provide the most direct indices or proxies of extinction risk. The current and foreseeable future status of each of these risks was assessed in turn by responding to a set of questions adapted from guidelines used to assess salmonid populations but based on general conservation biology principles applicable to a wide variety of species.

The BRT members’ assessments of the significance of demographic risks to the persistence of ribbon seals were summarized in numerical scores. Scoring was modeled on similar approaches used in other ESA status reviews and was designed to elicit expert judgment about the likelihood that the known and potential threats will impact a species’ persistence. Specifically, each BRT member considered the risk that the population may be placed in danger of extinction by demographic problems with abundance, productivity, spatial structure, or diversity, and then assigned a score to each of these demographic categories as follows: 1 = very low or zero risk, 2 = low risk, 3 = medium risk, 4 = high risk, and 5 = very high risk. To assess future risks of extinction, the BRT members scored the risks that problems associated with those demographic categories will—between now and 2050, and between now and 2100—place the species in danger of extinction.

For the period from now to 2050, demographic risks were judged to be low to very low (average of BRT members’ scores ranging from 1.1 to 2.0). With a population likely comprising at least 200,000-300,000 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 observations by hunters and a recent estimate of 61,100 ribbon seals in the eastern and central Bering Sea are consistent enough with historical accounts 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.

For the longer time frame, from now to 2100, a recognition of the increasing significance of diminished sea ice and increased prevalence of ocean acidification was reflected in risk scores that ranged from low to medium (1.6 to 2.8). The greatest risks perceived by the BRT were in the category of spatial structure, stemming from loss of habitat patches and connectivity that is anticipated to become significant in the latter half of this century.

To express a single, summarized judgment about extinction risk, each BRT member was asked to allocate 10 likelihood points to various time intervals, indicating his or her judgment about the time until ribbon seals would reach a population level of 5,000 individuals, representing a hypothetical extinction threshold or minimum viable population. The time intervals were ‘now to 2025’, ‘2026 to 2050’, ‘2051 to 2075’, ‘2076 to 2100’, and ‘>2100’. Averaged over the 11 BRT members, a likelihood of 0% was ascribed to the combined intervals from now to 2050, 4% was ascribed to the interval 2051 to 2075, 13% was ascribed to 2076 to 2100, and 83% was ascribed to the period beyond 2100. In other words, the BRT's collective judgment was that there is a 4% + 13% = 17% chance that the ribbon seal population will decline to 5,000 individuals before the end of the current century. The range among BRT members in this judgment was from 0% to 50%, reflecting the variation that results from sparse and uncertain information underlying this assessment.

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