Population structure and divergence time among East Greenland and West Greenland/Eastern Canadian Arctic narwhals, Monodon monoceros

The narwhal (Monodon monoceros) is an Arctic endemic odontocete that is particularly sensitive to climate change. Narwhals are also a key species in subsistence hunts in both Canada and Greenland. Understanding the genetic population structure is crucial to help management authorities set sustainable harvest quotas to avoid overexploitation of vulnerable narwhal groups.
Additionally, estimates of population divergence times and their correlation with potential environmental drivers may be informative regarding the effects of environmental change. Herein, 2236 genome-wide single-nucleotide polymorphisms
from 40 narwhals were used to infer population structure and divergence times. Samples were collected in six localities, one in East Greenland, four in West Greenland and one in the Eastern Canadian Arctic. The highest degree of genetic differentiation
was observed between narwhals from Kangertittivaq (East Greenland) and Tasiujaq (Eastern Canadian Arctic), with a θ of 0.021 (95% confidence interval (CI): 0.014–0.028). While some locations in West Greenland also exhibited significant levels of differentiation (e.g., Uummannaq vs Qeqertarsuaq, θ = 0.011, 95% CI: 0.004–0.020), the East Greenland narwhals were most distinct based on both θ (varying from 0.01, 95% CI: 0–0.01 with Uummannaq to 0.021 95% CI: 0.014–0.028 with Tasiujaq) and clustering analyses. Our results are relevant to the management of narwhals in East Greenland, where current hunting levels are likely unsustainable. The divergence time estimated between East Greenland and West Greenland/Eastern Canadian Arctic suggests that narwhals in these two areas became separated during the Last Glacial Maximum, offering additional insight into the long-term population dynamics during climate change.