Ice Seals of Alaska

DOI: 10.25923/4488-8843

L. Quakenbush¹, A. Bryan¹, J. Crawford¹, J. Olnes¹, and R. Stimmelmayr^2,3

¹Arctic Marine Mammal Program, Alaska Department of Fish and Game, Fairbanks, AK, USA
²North Slope Borough, Department of Wildlife Management, Utqiaġvik, AK, USA
³Institute of Arctic Biology, University of Alaska, Fairbanks, AK, USA

Headlines

• Ice seal populations in the Pacific Arctic (ringed, bearded, spotted, and ribbon seals) remain healthy.
• Monitoring the subsistence harvest since the 1960s shows no sustained change in body condition, a stable or younger age of maturity, high pregnancy rates, and pup survival past weaning.
• As predicted with warmer water, fewer ringed seals are eating Arctic cod, and more are eating saffron cod; however, no concurrent changes in health are evident.

Introduction

Climate change in the Arctic is expected to affect sea ice-associated seal populations, or “ice seals,” by reducing sea ice extent and the length of time sea ice is available for resting, pupping, pup rearing, and molting (see essay Sea Ice). Four species of ice seals occupy the Bering, Chukchi, and Beaufort Seas, west and north of Alaska, and are harvested by Alaska Natives for subsistence: ringed (Pusa hispida), bearded (Erignathus barbatus), spotted (Phoca largha), and ribbon seals (Histriophoca fasciata) (Nelson et al. 2019; Fig. 1). Ringed and bearded seals were listed as ‘Threatened’ under the Endangered Species Act (ESA) in 2012 due to concerns with predicted sea ice decline over the next century (U.S. Federal Register 2012a,b). Current populations of all four species are large (>100,000s) and the subsistence harvest for all four species is sustainable (Nelson et al. 2019).

Sampling of the annual ice seal harvest, coordinated by the Alaska Department of Fish and Game, has provided important information regarding seal population health and status since the 1960s. Coastal Indigenous communities that harvest seals provide measurements and tissues used to assess diet, body condition, age of maturity, pregnancy rate, and proportion of pups in the harvest (Fig. 1). The longevity of this biomonitoring program allows for comparisons over time that provide insight into individual and population level responses to recent environmental changes.

Diet

Changes in seal prey availability may occur as sea ice decreases and ocean temperatures increase (see essay Sea Surface Temperature), which may cause shifts in seal diet. For example, stomach contents from ringed seals, collected since 2010, suggest changes in their winter (December-May) diet are occurring. Arctic cod, saffron cod, rainbow smelt (Osmerus mordax), and Pacific herring (Clupea pallasii) are the four most common fish identified in ringed seal stomachs in winter. Fewer ringed seals ate Arctic cod during the 2010s than 2000s, whereas more ate saffron cod, rainbow smelt, and Pacific herring (Fig. 2). Walleye pollock were not identified in seal stomachs in the 2000s but have been identified at low frequency in the 2010s (Fig. 2).

These shifts in diet fit with known and predicted changes in fish distribution. Arctic cod (Boreogadus saida) have shifted their range northwards and are predicted to become less available on the continental shelf (waters <200 m deep), and therefore less available to seals (Marsh and Mueter 2020). Saffron cod (Eleginus gracilis) and walleye pollock (Gadus chalcogrammus) are predicted to become more available due to their preference for warmer waters (Stevenson and Lauth 2019; Deary et al. 2021). This change in diet is predicted to have negative effects on the energetics, body condition, and productivity of seals because saffron cod contain less energy by mass than Arctic cod (Florko et al. 2021).

Body condition

Blubber thickness was used as an index of body condition (Crawford et al. 2015) to determine if seal health has responded to changes in environmental conditions over time. It is important to account for known seasonal fluctuations and other sources of variability in blubber thickness when considering trends (Quakenbush 2020), before we can determine if there were decades when seals were on average thinner (unhealthy) or fatter (healthy). In general, the average blubber thickness of all four seal species has not undergone sustained negative changes since the 1960s (Fig. 3). Blubber thickness values dipped slightly for ringed, bearded, and spotted seals in the 2010s, a decade that included two unusual mortality events (UMEs). One primarily occurred in 2011 and 2012 when 657 seals were sickened by an unknown disease (NOAA Fisheries 2024a) and another in 2018 and 2019 when 275 seals were found dead. The later UME was attributed to malnutrition caused by extremely warm ocean temperatures in the Bering Sea during the consecutive winters of 2017/18 and 2018/19 (NOAA Fisheries 2024b). However, average blubber thickness rebounded in the early 2020s and was above average for spotted seals.

Productivity

When resources are limited, it takes longer to acquire the energy needed to grow, and females mature at an older age (Laws 1956). When resources are not limited, seals grow quickly and mature at a younger age. The age of maturity has been consistent (spotted and ribbon) or occurred at a younger age in recent decades (ringed and bearded) for all species over the sampled period (Fig. 4a).

Once mature, and when conditions are good, females can give birth to one pup per year (Riedman 1990). The percentage of adult females that are pregnant each decade is used to indicate good or poor reproductive decades. Pregnancy rate has remained high (>75%) for all four species throughout the study period (Fig. 4b).

Proportion of pups in the harvest

Although female body condition and pregnancy rates are currently high, and maturation is occurring at a younger age, if pups do not survive, the population will eventually decline. Females nurse their pups on the sea ice; therefore, it is important for sea ice to remain stable until pups are weaned. As sea ice becomes less stable in the spring and recedes earlier (see essay Sea Ice), the nursing period could become shorter, making it harder for pups to survive. Because the harvest in Alaska occurs after pups are weaned, and seals are harvested opportunistically without deference to age, we can use the proportion of pups relative to other ages in the harvest to know that pups are being born and surviving past weaning. The proportion of pups in the harvest has remained high (>40%) and increased over time for ringed, bearded, and spotted seals since the 1970s (Fig. 5).

In summary, information analyzed from the ice seal subsistence harvest in Alaska indicates that all four species are as healthy and productive in the early 2020s as they have been in past decades. Stomach contents have shown some changes in diet between the 2000s and 2010s, such as fewer ringed seals consuming Arctic cod and more consuming saffron cod and other fish. Changes in diet are likely indicative of broader ecosystem changes affecting prey species composition and distributions. Despite less sea ice and potential shifts in prey availability and quality, integrated health indices such as body condition, productivity, and pup survival are stable or above average, leading us to conclude that the changes in the Pacific Arctic marine environment through 2023 have not resulted in long-term negative impacts to ice seals. Our conclusion agrees with observations from the seal hunters that provide samples; they report seals are generally healthy.

Methods and data

Samples and morphometric data were collected from seals harvested for Alaska Native subsistence purposes during 1998-99, 2000s, 2010s, and 2020-23 at communities under a biological monitoring program conducted by Alaska Department of Fish and Game (ADF&G). See Crawford et al. (2015) and Quakenbush (2020) for detailed methods. Data from this period were compared to data previously collected by ADF&G from the subsistence harvest and research cruises during 1962-69, 1970s, 1980-82, 1984-85, and 1990-91. In the analysis of body condition, we also included blubber thickness data from 1,282 ringed seals harvested near Point Hope as part of the Cape Thompson Project Chariot study (1960-61; Johnson et al. 1966).

Data for each seal included collection date, species, sex, standard length, and blubber thickness. Tissues include the stomach, female reproductive tract, and a tooth or claw for aging. Samples from all locations and seasons were pooled by decade (except for diet, which were from December-May only) to compare changes over time. Seal age was determined by counting annual growth rings in a tooth or claw. Stomach contents were identified using taxonomic keys to the lowest taxonomic level. Changes in prey categories over time were analyzed using logistic regression models. Blubber thickness was measured over the sternum between the front flippers and changes by decade were analyzed using linear mixed effects models that accounted for known sources of variability, including seasonal variation in blubber thickness, seal length, and sex. The remaining unexplained variability, also known as the model residuals, can then be plotted by decade to determine if there were periods when seals were on average thinner or fatter. Female reproductive tracts were evaluated for reproductive status and condition to evaluate productivity using changes in the age of maturity and pregnancy rate by decade. Age of maturity was estimated as the age at which 50% of females had ovulated at least once and analyzed using probit regression models (DeMaster 1978). Pregnancy rate was defined as the proportion of mature females with a corpora lutea in the year of harvest, except that seals with a corpora lutea but no fetus by 1 November were considered not pregnant. Pregnancy rate data were analyzed using logistic regression models. We assessed changes in the proportion of pups harvested using logistic regression models.

Acknowledgments

These data are available due to sample contributions by subsistence hunters from their ice seal harvests, and the support of their communities and Tribal Councils. Support is provided by the North Slope Borough Department of Wildlife Management (NSB) and the Ice Seal Committee. J. Burns, K. Frost, and the late L. Lowry established this biomonitoring program and collected and maintained data prior to 2000. Many biologists, technicians, and college interns have helped with sample collection and processing. The late W. Walker identified many fish to species using otoliths. Funding was provided by the National Oceanic and Atmospheric Administration, National Marine Fisheries Service (NMFS, Projects NA05NMF4391187, NA08NMF4390544, NA11NMF4390200, and NA22NMF4390239). Samples were collected under NMFS research permits 358-1787, 15324, 20466, and 26254 issued to ADF&G and permits 814-1899, 17350, 21386, and 26667 issued to NSB.

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November 12, 2024