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The Arctic is warming at an unprecedented rate, with unknown consequences for endemic fauna. However, Earth has experienced severe climatic oscillations in the past, and understanding how species responded to them might provide insight into their resilience to near-future climatic predictions. Little is known about the responses of Arctic marine mammals to past climatic shifts, but narwhals ( Monodon monoceros ) are considered one of the endemic Arctic species most vulnerable to environmental change. Here, we analyse 121 complete mitochondrial genomes from narwhals sampled across their range and use them in combination with species distribution models to elucidate the influence of past and ongoing climatic shifts on their population structure and demographic history. We find low levels of genetic diversity and limited geographic structuring of genetic clades. We show that narwhals experienced a long-term low effective population size, which increased after the Last Glacial Maximum, when the amount of suitable habitat expanded. Similar post-glacial habitat release has been a key driver of population size expansion of other polar marine predators. Our analyses indicate that habitat availability has been critical to the success of narwhals, raising concerns for their fate in an increasingly warming Arctic.

The elongated, spiraled tusk of male narwhals (Monodon monoceros) grows continuously throughout the life of the whale and is most likely a secondary sexual trait used in male–male hierarchical competition and possibly in female mate choice. Sex determination in narwhals is typically based on the presence (male) or absence (female) of an erupted tusk, but anomalies such as females with tusks, tuskless males or double-tusked whales occur, although infrequently. In this study, we collected reproductive data and recorded the presence or absence of a tusk in narwhals from the Inuit hunt in Greenland (1993 and 2010–19) with the purpose of estimating the frequency of tusk anomalies. We found that of the 173 whales, 2.9% displayed tusk anomalies. Tusked females constituted 1.5% of sampled females, tuskless males 2.8% of sampled males and double-tusked males 0.9% of sampled males. Biological information on a tusked female, a tuskless male and a double-tusked male was collected and is presented here. The tusked female was sexually mature, and 18 ovarian scars (indicating pregnancies) documented a long reproductive lifespan. The complete female tusk was estimated to be between 146 and 151 cm in length. The tuskless male was sexually maturing, as indicated by body dimensions, and the double-tusked male was sexually immature, with the two tusks measuring <90 cm in length. Although narwhals exhibit extremely low levels of genetic diversity, tusk anomalies persist in the populations, perhaps facilitated by the reproductive ability of whales with tusk anomalies.

The narwhal (Monodon monoceros) is a high‐Arctic species inhabiting areas that are experiencing increases in sea temperatures, which together with reduction in sea ice are expected to modify the niches of several Arctic marine apex predators. The Scoresby Sound fjord complex in East Greenland is the summer residence for an isolated population of narwhals. The movements of 12 whales instrumented with Fastloc‐GPS transmitters were studied during summer in Scoresby Sound and at their offshore winter ground in 2017–2019. An additional four narwhals provided detailed hydrographic profiles on both summer and winter grounds. Data on diving of the whales were obtained from 20 satellite‐linked time‐depth recorders and 16 Acousonde™ recorders that also provided information on the temperature and depth of buzzes. In summer, the foraging whales targeted depths between 300 and 850 m where the preferred areas visited by the whales had temperatures ranging between 0.6 and 1.5°C (mean = 1.1°C, SD = 0.22). The highest probability of buzzing activity during summer was at a temperature of 0.7°C and at depths > 300 m. The whales targeted similar depths at their offshore winter ground where the temperature was slightly higher (range: 0.7–1.7°C, mean = 1.3°C, SD = 0.29). Both the probability of buzzing events and the spatial distribution of the whales in both seasons demonstrated a preferential selection of cold water. This was particularly pronounced in winter where cold coastal water was selected and warm Atlantic water farther offshore was avoided. It is unknown if the small temperature niche of whales while feeding is because prey is concentrated at these temperature gradients and is easier to capture at low temperatures, or because there are limitations in the thermoregulation of the whales. In any case, the small niche requirements together with their strong site fidelity emphasize the sensitivity of narwhals to changes in the thermal characteristics of their habitats. Narwhals avoid areas with warm temperatures. Both their prey capturing and diving behavior show their clear preference for cold habitats. They preferentially select habitats where the temperature ranges between 0 and 1°C. Photo: Carsten Egevang

IntroductionAge estimation is an important tool for understanding the life history of animal populations, and several techniques have been developed, each with its own strengths and limitations.MethodsIn this study, we apply a novel age estimation method that utilizes trace element signals with seasonal components obtained through laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) on tusks of 16 narwhals. We model tusk growth as a stochastic process that, by hypothesis, tracks the number of elapsed annual cycles. For each tusk, we estimate this process in order to derive model-based age estimates.ResultsWe show that this method provides objective and reproducible age estimates without requiring visually distinguishable growth layers. Age estimates are compared with those from other methods, specifically manual counting (visual reading) of growth layers, radiocarbon dating, and using aspartic acid racemization. Our model provided age estimates ranging from 16 to 60 years of age and showed strong agreement with manual counts for 14 of 16 individuals, with two critical exceptions differing by 9 and 14 years, respectively.DiscussionWe end with a discussion of modeling challenges and deficiencies related to this particular tusk dataset. Although demonstrated on narwhal tusks, we discuss how this method can be generalized to other mineralized materials with a layered structure.

The Atlantic walrus ( Odobenus rosmarus rosmarus ) is an Arctic endemic species that is under increasing threat due to declines in their sea ice habitats. Walruses rely directly ( e.g ., resting, giving birth) and indirectly ( e.g ., tight coupling between sympagic and benthic productivity) on sea ice, and are particularly sensitive to disturbance by increasing anthropogenic activities ( e.g ., shipping) that are taking place concomitant with sea ice declines throughout their Arctic range. Management and conservation of Atlantic walrus require monitoring of their distribution and assessments of regional and range-wide abundance trends. Atlantic walrus population assessments are typically based on counts of walruses hauled out at terrestrial sites during aerial or boat-based surveys, and increasingly, using satellite imagery. We compiled a comprehensive, distribution-wide dataset of all known Atlantic walrus terrestrial haul-out sites from relevant national datasets to promote accessibility and consistency of use among multiple users, including Indigenous groups, scientific researchers, and resource managers.

A decreasing trend in narwhal (Monodon monoceros Linnaeus, 1758) abundance has been identified in a small population in Scoresby Sound, Southeast Greenland. We hypothesize that excessive hunting has affected life history and population dynamics of this population. Biological information and samples collected from the Inuit hunt, from satellite-tagged narwhals and from official hunters’ reports, were used to estimate age, growth, and reproduction. During 2007 through 2019, a decreasing proportion of young and increasing proportion of older whales were harvested. Male and female body length and male tusk length increased significantly, while body mass of both sexes showed a nonsignificant increase. The probability of catching a female decreased significantly, while a nonsignificant decline of catching a pregnant female was observed in both biological samples and hunters’ reports. Narwhal swimming speeds correlated with fluke widths indicated that larger whales attain greater speeds. The decline in juveniles and females is probably due to an opportunistic hunting practice targeting the easiest-to-catch whales, where bigger whales are faster and more difficult to catch. The cumulative effect of overharvest with a declining proportion of females, an overrepresentation of large males, and a lack of calves and juveniles has detrimental implications for this small narwhal population.

One of the last pristine marine soundscapes, the Arctic, is exposed to increasing anthropogenic activities due to climate-induced decrease in sea ice coverage. In this study, we combined movement and behavioral data from animal-borne tags in a controlled sound exposure study to describe the reactions of narwhals, Monodon monoceros , to airgun pulses and ship noise. Sixteen narwhals were live captured and instrumented with satellite tags and Acousonde acoustic-behavioral recorders, and 11 of them were exposed to airgun pulses and vessel sounds. The sound exposure levels (SELs) of pulses from a small airgun (3.4 L) used in 2017 and a larger one (17.0 L) used in 2018 were measured using drifting recorders. The experiment was divided into trials with airgun and ship-noise exposure, intertrials with only ship-noise, and pre- and postexposure periods. Both trials and intertrials lasted ∼4 h on average per individual. Depending on the location of the whales, the number of separate exposures ranged between one and eight trials or intertrials. Received pulse SELs dropped below 130 dB re 1 μPa 2 s by 2.5 km for the small airgun and 4–9 km for the larger airgun, and background noise levels were reached at distances of ∼3 and 8–10.5 km, respectively, for the small and big airguns. Avoidance reactions of the whales could be detected at distances >5 km in 2017 and >11 km in 2018 when in line of sight of the seismic vessel. Meanwhile, a ∼30% increase in horizontal travel speed could be detected up to 2 h before the seismic vessel was in line of sight. Applying line of sight as the criterion for exposure thus excludes some potential pre-response effects, and our estimates of effects must therefore be considered conservative. The whales reacted by changing their swimming speed and direction at distances between 5 and 24 km depending on topographical surroundings where the exposure occurred. The propensity of the whales to move towards the shore increased with increasing exposure (i.e., shorter distance to vessels) and was highest with the large airgun used in 2018, where the whales moved towards the shore at distances of 10–15 km. No long-term effects of the response study could be detected.

To maintain a sustainable harvest of marine mammals, knowledge on key life history parameters such as age is essential. In West Greenland, humpback whale numbers were low during the 1980s due to past commercial whaling. However, the moratorium, which was enforced in 1986, had a positive effect on their abundance and by 2010 it was possible for Greenland to reopen the subsistence hunt on humpback whales in West Greenland. In this study, eyes were collected from 12 humpback whales taken in the subsistence hunt from 2010-2015 and used for age and growth estimation. Aspartic acid racemization (AAR) and bomb radiocarbon dating techniques were performed on the eye lens nuclei to obtain independent age estimates. Through AAR, 11 individuals were estimated to be younger than 20 years old and a single individual estimated to be 46 years old. Radiocarbon dating supported these estimates as all individuals had post-bomb pMC values. The Gompertz growth curve showed that humpback whales reach their asymptotic lengths around age ten and hence at a relatively young age. In conclusion, half a century after commercial whaling ended, the humpback whales feeding off West Greenland appear to consist mainly of young individuals. An age distribution that is likely attributed to a historical overexploitation and since a rapid increase in numbers following the moratorium.

Eyes from 75 narwhals (Monodon monoceros) were collected in West Greenland in 1993 and 2004 for the purpose of age estimation. Age estimates were based on the racemization of l-aspartic acid to d-aspartic acid in the nucleus of the eye lens. The ratio of d- and l-enantiomers was measured using high-performance liquid chromatography. The aspartic acid racemization rate (kAsp) was estimated to be 0.001045/year ± 0.000069 SE by regression of d/l ratios on age estimated by length from 15 young narwhals (≤298 cm) and by earplug laminations from 13 fin whales (Balaenoptera physalus). The d/l ratio at age 0 ((d/l)0) was estimated to be 0.0288 by regression of d/l ratios against the estimated age of the 15 young narwhals. The intercept of the regression slope, providing twice the (d/l)0 value, was 0.05759 ± 0.00147 SE. The maximum estimated age was a 115-year-old (±10 SE) female. Asymptotic body length was estimated to be 396 cm for females and 457 cm for males, and asymptotic body mass was estimated to be 904 kg for females and 1,645 kg for males. Using the von Bertalanffy growth model, age at sexual maturity was estimated to be 6–7 years for females and 9 years for males.

The Arctic is warming at an unprecedented rate, with unknown consequences for endemic fauna. However, Earth has experienced severe climatic oscillations in the past, and understanding how species responded to them might provide insight into their resilience to near-future climatic predictions. Little is known about the responses of Arctic marine mammals to past climatic shifts, but narwhals (Monodon monoceros) are considered one of the endemic Arctic species most vulnerable to environmental change. Here, we analyse 121 complete mitochondrial genomes from narwhals sampled across their range and use them in combination with species distribution models to elucidate the influence of past and ongoing climatic shifts on their population structure and demographic history. We find lowlevels of genetic diversity and limited geographic structuring of genetic clades. We showthat narwhals experienced a long-termloweffective population size, which increased after the Last Glacial Maximum, when the amount of suitable habitat expanded. Similar post-glacial habitat release has been a key driver of population size expansion of other polar marine predators. Our analyses indicate that habitat availability has been critical to the success of narwhals, raising concerns for their fate in an increasingly warming Arctic.