The impact of rising sea temperatures on an Arctic top predator, the narwhal (original) (raw)
Arctic top predators are expected to be impacted by increasing temperatures associated with climate change, but the relationship between increasing sea temperatures and population dynamics of Arctic cetaceans remains largely unexplored. Narwhals (Monodon monoceros) are considered to be among the most sensitive of Arctic endemic marine mammals to climate change due to their limited prey selection, strict migratory patterns and high site fidelity. In the context of climate change, we assume that the population dynamics of narwhals are partly influenced by changes in environmental conditions, with warm areas of increasing sea temperatures having lower abundance of narwhals. Using a unique large dataset of 144 satellite tracked narwhals, sea surface temperature (SST) data spanning 25 years (1993-2018) and narwhal abundance estimates from 17 localities, we (1) assessed the thermal exposure of this species, (2) investigated the SST trends at the summer foraging grounds, and (3) assessed the relationship between SST and abundance of narwhals. We showed a sharp SST increase in Northwest, Mideast and Southeast Greenland, whereas no change could be detected in the Canadian Arctic Archipelago (CAA) and in the Greenland Sea. The rising sea temperatures were correlated with the smallest narwhal abundance observed in the Mideast and Southeast Greenland (< 2000 individuals), where the mean summer sea temperatures were the highest (6.3 °C) compared to the cold waters of the CAA (0.7 °C) that were associated with the largest narwhal populations (> 40,000 individuals). These results support the hypothesis that warming ocean waters will restrict the habitat range of the narwhal, further suggesting that narwhals from Mideast and Southeast Greenland may be under pressure to abandon their traditional habitats due to ocean warming, and consequently either migrate further North or locally go extinct. Climate change is affecting almost all regions and ecosystems, with the Arctic demonstrating the greatest, irreversible consequences on marine life. Rapid sea-ice loss and increasing temperatures 1,2 are altering the distribution and abundance of low trophic-level organisms, generating cascading effects through the entire food chain from phytoplankton to mammalian predators 3. Arctic marine mammals have the potential to move over long distances to adapt to changing and erratic resource availability. However, their capacity to adjust long-term adaptations, like site-fidelity and fixed migratory patterns, to climate-induced perturbations remains poorly known 4,5. Among Arctic cetaceans, the narwhal (Monodon monoceros) has a large geographic range extending from the Canadian Arctic Archipelago (CAA) to East Greenland, Svalbard and the western part of the Russian Arctic 6-11. Narwhals are known to exhibit a high degree of site-fidelity and to be closely associated with specific migratory corridors during spring and fall movements between summer and winter grounds 12. During winter, narwhals are mainly found in offshore areas over deep water and often in areas completely covered with pack-ice with only leads available for breathing. In spring the narwhals move towards coastal areas. The inshore summer foraging grounds, where they congregate during the open water season, are considered the primary basis for identifying separate population units of narwhals 13 .