Some like it cold: Temperature‐dependent habitat selection by narwhals

Abstract

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.

Behavioral Response Study on Seismic Airgun and Vessel Exposures in Narwhals

“…This phenomenon may greatly enhance the propagation of signals, making them audible to the whales over vast distances. While this has not been observed directly in this study, it may occur, as thermo-and haloclines exist at <10 m depth and, albeit weaker, at 100 m depth in Scoresby Sound (Heide-Jørgensen et al, 2020).…”

“…However, there was large variation from whale to whale, not only because whales are different but also because each whale was only observed for 3-8 days, and not all their natural and unexposed behavior might be displayed in that time period. To make the threshold value more robust to that of a general narwhal population, five reference whales from the same population that provided data on distance to shore in the same area in -2016(Heide-Jørgensen et al, 2020 were included to determine the threshold for unexposed whales. For each whale, the 5% quantile was determined, and then, a weighted average of 5% quantiles was calculated, where the weight was given by the length of the observation time of each whale.…”

“…In each of the two study years, two additional narwhals were instrumented with Fastloc CTD satellite transmitters that, in addition to depth, also recorded and transmitted data through the Argos Location and Data Collection System from two daily depth profiles of water temperature and salinity that were sampled at 1 Hz (Wildlife Computers Scout-CTD-370D, see Heide-Jørgensen et al, 2020;Teilmann et al, 2020). These tags were mounted in a similar way as the Fastloc transmitters, and each cast had an associated Fastloc position, resulting in only two positions per day from these tags.…”

Bowhead whales overwinter in the Amundsen Gulf and Eastern Beaufort Sea

“…It is not clear if this pattern of movement was attributable to warmer water temperatures or related to the 2018-2019 overwintering event. Such strong avoidance of warm waters have also been recently demonstrated in narwhals in the eastern Arctic [51,52]. If the avoidance of warm ocean temperatures were the primary driver of this anomalous behaviour, it may be a significant warning sign for bowhead whales.…”

Hunting by the Stroke: How Foraging Drives Diving Behavior and Locomotion of East-Greenland Narwhals (Monodon monoceros)

“…Even if securing core body temperature to avoid hypothermia is essential for all aquatic mammals, the ability to lose excess heat in connection with exercise is equally important. Bursts of high levels of exercise by narwhals that are built for slow speed swimming (Williams et al, 2011) may constrain their ability to remove excess heat, especially in a warming ocean (Chambault et al, 2020;Heide-Jørgensen et al, 2020).…”