Biological parameters in a declining population of narwhals (Monodon monoceros) in Scoresby Sound, Southeast Greenland

The narwhal (Monodon monoceros Linnaeus, 1758) is an important game animal in Greenland and is hunted by Greenlanders almost throughout its distribution (Hobbs et al. 2019). The main hunting product is the highly prized mattak (skin) of the whales, which reached a retail price of ~$75/kg in 2021 (E. Garde, personal observation, November 2021). The male tusk is also of economic value, and the meat is sometimes used for local consumption. The large demand and high prices for narwhal products makes narwhal by far the economically most valuable game animal in Greenland and is a significant cash income for the hunters (Flora et al. 2018; Nielsen and Meilby 2013). In addition to the economic value, the hunt of narwhals is also culturally significant with traditional utilization and trade through centuries (Reeves and Heide-Jørgensen 1994). The North Atlantic Marine Mammal Commission (NAMMCO) and the Canada–Greenland Joint Commission of the Conservation and Management of Narwhals and Belugas (JCNB) assess the sustainability of the hunt. The hunt in Greenland is managed by the Greenland Government and quotas in Southeast Greenland have regulated the narwhal hunt since 2011.

In East Greenland, narwhal hunting takes place from the communities of Tasiilaq and Ittoqqortoormiit located in Southeast Greenland (Fig. 1; Garde et al. 2019). No narwhal hunting takes place in Northeast Greenland (north of Scoresby Sound). The narwhal population that is available to the hunters from Ittoqqortoormiit spends the summer months traversing the Scoresby Sound fjord system and winters off the East Greenland coast, on the shelf southeast of the fjord mouth (Heide-Jørgensen et al. 2015). When the ice breaks from late June to early July, the whales enter Scoresby Sound along the southern coastline and stay in the large fjord system until the ice forms again in October or early November. Data from narwhals tagged with satellite transmitters show a consistent migration pattern between summer and winter grounds separating this population from others on the approximately 3000 km long coastline of East Greenland (Heide-Jørgensen et al. 2020a). A decreasing trend in abundance during the past decade has been identified for the narwhal population in Scoresby Sound (hereinafter the Scoresby Sound population) by the NAMMCO Scientific Committee based on a series of aerial surveys (NAMMCO 2019). The abundance dropped from 1991 narwhals (95% confidence interval (CI): 709–5590) in 2008 to 421 (95% CI: 198–895) in 2016 (NAMMCO 2020). The most apparent reason for this decline is the level of catches that since the introduction of quotas by the government of Greenland in 2011 and to 2019 have ranged between 30 and 74 individuals annually with a mean of 50 (landed animals not including struck-but-lost whales; Garde et al. 2019; Table 1). From 1993 to 2010, before the introduction of quotas, the catches ranged from 6 to 93 with a mean annual catch of 30 narwhals. In 2017, the NAMMCO Scientific Committee recommended a catch level of not more than 10 narwhals (NAMMCO 2017) and in 2019 this was reduced to a recommendation of a moratorium on narwhal catches, implying that any continued catch level would be unsustainable (NAMMCO 2019). Despite this recommendation, the quota for 2020, 2021, and 2022 was set to 40, 25, and 20 whales (naalakkersuisut.gl), respectively, jeopardizing the long-term existence of this isolated population and the prospects of continued harvest of narwhals in the local communities (NAMMCO 2019). It is vital that the scientific advice for regional populations of narwhals is accepted and populations managed responsibly (Heide-Jørgensen et al. 2020b), especially because there is evidence for a lack of strong recovery in heavily exploited odontocete populations (Wade et al. 2012). A low resilience to overexploitation and a lack of recovery is explained by odontocete life history; a relatively old age at first reproduction and low calving rate. Overexploitation can lead to social disruption, fragmentation of social units, and loss of key individuals and cultural knowledge, which can result in a decrease in birth rates because of a deficit of adult females and (or) males and disruption of mating systems (see review by Wade et al. 2012). In addition, narwhals show extreme site fidelity to their summer grounds and the limited plasticity in movements makes it unlikely that they can re-colonize areas that they have been extirpated from (Heide-Jørgensen et al. 2015, 2020b).

Alongside hunting, the Scoresby Sound population may also be affected by ongoing changes in the marine environment. Suitable habitat for the whales seems to be declining as sea surface temperatures rise (Louis et al. 2020; Chambault et al. 2020). The Scoresby Sound population have recently been shown to forage in a narrow temperature range and a continued increase in sea water temperatures could reduce the feeding habitat available for the whales (Heide-Jørgensen et al. 2020a). Also, altered prey species availability, competition from migratory species, and potentially new diseases could render the current habitat less suitable for these cold-adapted whales (Chambault et al. 2020). A consequence of such accumulated effects caused by climate change could be population displacement towards higher latitudes with colder sea water temperatures (Louis et al. 2020) or affect the body condition of the whales as a result of poor nutrition (Burek et al. 2008; Laidre et al. 2008). Even though a rise in sea water temperatures in Southeast Greenland have been documented (Alexander et al. 2018), the Scoresby sound population show no signs of either population displacement (Heide-Jørgensen et al, 2015, 2020a) or malnutrition (Heide-Jørgensen et al. 2014; Garde et al. 2015), and the anticipated effects of climate change on this small population of narwhals have not yet been detected. Concurrently with a reduction in sea ice caused by climate change, anthropogenic activities have been on the rise as an increasing number of cruise ships, sail boats, and other marine activities are present in the narwhal habitats (Reeves et al. 2014; Hauser et al. 2018). The anthropogenic activities in Scoresby Sound are, however, still at a low level with only a few tourist ships operating in the area from July through August and a minor number of local speed boats (<10) from Ittoqqortoormiit that are present in the fjord during the open water season. Narwhals are skittish animals, known to avoid humans, and disturbance caused by an increasing vessel fleet or other human activities, e.g., oil and gas exploration, can potentially have serious negative effects on the whales, as shown for various marine mammals including the narwhals (Richardson et al. 1995; Booth et al. 2020; Heide-Jørgensen et al. 2021; Tervo et al. 2021). Continued monitoring of life history parameters and population dynamics of this rapidly declining population is essential for future assessments of the consequences of these accumulated threats.

In this study, we estimated life history parameters and investigated the biological effects of a high harvest pressure on a small and isolated population of narwhals in Scoresby Sound, Southeast Greenland. We hypothesize that the level of exploitation is reflected in temporal changes in vital biological parameters as sex and age distributions, somatic growth, and reproduction. We examined this using data collected over a decade: biological samples from the Inuit hunt of narwhals (2007–2019), somatic measurements from satellite-tagged narwhals (2010–2019), and official hunters’ records (2008–2019). The relationship between somatic parameters and narwhal swimming speed was examined to explain the consequences of the exploitation on the sex and age class composition of the population.

Estimation of life history parameters, e.g., rates of growth and reproduction, of exploited populations of wildlife provide diagnostic information about body condition and changes in density-dependent parameters that are essential to the management and conservation of species (Chivers 2009; Murphy et al. 2009). Growth in length and especially mass-at-length are robust indicators of body condition, ecosystem changes, and changes in carrying capacity (Castrillon and Nash 2020). In this study, there were no signs of reduced growth or deteriorating body condition of the narwhals in Scoresby Sound (Murphy et al. 2009). There were too few samples to detect changes in age at sexual maturity for both sexes and values were similar to estimates from Garde et al. (2015).

A decreasing trend, although not significant, in pregnant females of the Scoresby Sound narwhals was observed for the period 2007–2019 coinciding with a significant and steady decrease of females. These observations were based on data from both biological samples from a minor portion of the population and from the Special Reports covering all landed narwhals, which comprise a substantial proportion of the small Scoresby Sound population. In addition to the decreasing trend of females and pregnant females, there was an increasing under-representation of young males in the catches. All evidence suggests that there is a ∼1:1 sex ratio at birth of narwhal calves and population models predict that about 1/3 of the population should be immature whales in a stable population (Hay 1984). In the present study, the catches in recent years were mainly composed of old males. We have no evidence that old males were preferentially targeted by the hunters in recent years. On the contrary, if the harvest was selective, older and bigger animals with large tusks would have been preferred by the hunters throughout the study period because they represent a higher economic benefit but instead, young animals were taken earlier in the exploitation history implying opportunistic hunt instead of a selective hunt. Furthermore, the hunter gains ∼$3900 for the mattak of an average-sized adult narwhal (130 kg mattak at ∼$30/kg), which is considerably more than they obtain for the less valuable tusk sold for ∼$200/kg (a tusk ∼200–240 cm long weighs ∼3–6 kg; Garde et al. 2012). Although we have biological samples from only a portion of the total harvest, to compensate for low sample sizes we also analyse data from the hunters’ own records, the Special Reports, where all narwhals taken in the hunt have been logged, a legal requirement of the Greenlandic government. Analyses from data contained in the Special Reports relies on accurate information from hunters. We have no reason to believe that hunters would inaccurately report this information and have no reason to think hunters have a preference for narwhal sex/size or use selective hunting practices. We do recommend future work to evaluate the proportion of males/female/calves to support this work and provide further evidence that the resulting Scoresby Sound population is not composed of 1/3 juveniles, and 50/50 males and females.

The narwhals in Hjørnedal feed regularly on squid, shrimps, polar cod, and Arctic cod, but squid seems to be the most common prey item during the summer months. Since 2016, capelin have become increasingly common in the Scoresby Sound fjord system, where it has been observed and caught in the fjord by the local people (E. Garde, personal observation, August 2019). Capelin was found in the stomachs of narwhals in 2016 and 2017, and perhaps as early as in 2014, but were not among the prey items collected from 2010–2013 and in 2015 (this study; Heide-Jørgensen et al. 2014). Nothing is known about prey densities in the East Greenland waters where the narwhals are found, but the stomach contents does not support a major shift to new prey items. No other habitat changes can be identified as drivers for the observed changes in life history parameters.

The asymptotic body lengths predicted in this study are similar to previous estimations of asymptotic lengths in narwhals, whereas the predicted tusk length in this study are somewhat higher compared with previous estimates, indicating that the sampled males are larger in recent years (Garde et al. 2015). Altogether, the body condition seems to be unchanged or even improved in the recent years and nothing suggests that the narwhals in East Greenland are nutritionally stressed. The age structures and growth patterns for this population indicate that the catches of males include increasingly older and larger animals and that a decreasing proportion of females and especially pregnant females are included in the catches.

There was no hunting of narwhals in Scoresby Sound before the establishment of the main hamlet (Ittoqqortoormiit) in 1925. Larger catches (>10 whales) were uncommon before 1979 but with increasing prices for the hunting products, catches have increased substantially. The official catch records only report on landed animals and do not include whales that were struck-but-lost before they could be retrieved, which is an additional 15%–20% of the hunt (Garde et al. 2019). The hunt for narwhals in Scoresby Sound is primarily an opportunistic open water hunt from fast moving speedboats with outboard engines, where hunters will target any spotted whale, or a hunt where single or groups of whales are driven into ∼50 m long set nets deployed from shore. These methods have been used for decades and nothing indicates that the hunters have changed their hunting practices in recent years (Garde et al. 2019). The most parsimonious explanation for the declining proportion of females and young males is the effect of hunting, where these age and sex categories are the easiest to catch. The most likely reason for this is the general slower mean swimming speed as documented for smaller whales, but other factors like experience of older whales, poor condition of females due to the nutritional demands of their calves (Hay 1984), and pregnancy, which reduces speed as shown for bottlenose dolphins (Tursiops truncatus (Montagu, 1821); Noren et al. 2011), may also play a role. The whales usually react to being chased, e.g., by killer whales or boats, by long dives or by moving towards shore (Breed et al. 2017; Heide-Jørgensen et al. 2021). Larger animals are capable of longer dives (Schreer and Kovacs 1997; Kooyman 1989) and faster swimming speeds than smaller conspecifics, as also documented here for the narwhals in Scoresby Sound. Larger animals may also be more experienced — large animals have been observed to change direction fast and often, and to dive underneath hunters’ boats as a response to being chased (M.H.S. Sinding, personal observation, August 2019). Less experienced smaller whales will seek shelter along the coast, where they are more easily targeted by the hunters that also can drive the coastal whales towards whale nets. The effect is that older, larger and more experienced whales are more likely to escape the hunt, whereas younger, less experienced whales are the easiest to catch and will be first to be targeted by the hunt. Females are generally smaller than males and are sometimes accompanied by calves, which is probably slowing them down in a hunting situation, and they are therefore more likely to be caught. The effects are seen in the changes in the age and sex structure over the period with intensive hunting. The increased age of the females may have reduced the pregnancy rates either because older females do not give birth as frequently as younger females or because of an Allee effect in a small declining population, where reproductively active females are less likely to encounter males during the ovulation period (Drake et al. 2019). The combination of Allee effect and human-caused mortality, e.g., hunting, have the potential to substantially increase extinction risk for critically depleted populations (Wade and Slooten, preprint 2020).

In conclusion, the effects of continued overharvest of a small local population of narwhals are shown in the population dynamics as a decreased proportion of females, a decreasing trend in pregnancy rate, an overrepresentation of old males and a lack of calves and juveniles. It is hypothesized that the disappearance of young whales and females in the catches is a result of opportunistic hunting practices that initially target any spotted whale but eventually include the slow and less experienced whales. This is supported by the relation between speed and size that predicts that bigger whales are faster and thereby more difficult to catch. Not all trends are statistically significant, but the population implications of the trends are in any case detrimental. For exploited species and populations that are notoriously difficult to study and monitor, like High Arctic narwhals, even minor signals in behavior and life history parameters need to be taken seriously to avoid irreversible population declines. By the time statistically robust data are acquired the populations may be gone.

The regression analyses of the temporal trends in growth, sex distribution, and pregnancy were performed again between acceptance of the manuscript (June 2021) and receiving the proofs (late December 2021). This was due to a change from “catch location” to “catch date” as a criterion for separating the Scoresby Sound narwhal stock and the putative northern stock (Table 1; NAMMCO 2021). This resulted in a larger data set; however, the changes did not affect the major conclusions.

We thank the hunters from Ittoqqortoormiit for providing narwhal samples for this study, A. Moos for design and construction of the scale used for weighing narwhals in the field, and K. Andersen for invaluable assistance in the laboratory. We also thank the two reviewers for their thorough and detailed reviews, which have improved the final manuscript. Import of samples into Denmark from Greenland was authorized by Convention on International Trade in Endangered Species of Wild Fauna and Flora permits IM 0721-199/08, IM 0330-819/09, and IM 0905-590/17. Funding for this study was obtained from the Greenland Institute of Natural Resources, the Carlsberg Foundation, the Northeast Greenland Environmental Study Program, and the Danish Cooperation for the Environment in the Arctic. Access and permits to use land facilities and live-tagging of the narwhals in Scoresby Sound were provided by the Government of Greenland. No protected species were sampled.