Data from: Temperature dependent habitat selection by narwhals

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 >300m. 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 are 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.

Methods

Wildlife Computers (Redmond, Seattle, WA, USA) Fastloc-GPS-receivers and Argos transmitters (SPLASH10-BF-2380) were mounted on the back of twelve whales across 2017 and 2018 with three 8 mm delrin nylon pins secured with nylon washers and bolts on each end, following instrumentation techniques used in similar studies in Canada and West Greenland. The transmitters were programmed to collect an unrestricted number of FastLoc snapshots through August and September. In 2017 the transmitters were restricted to provide data only every seventh day in September. The Fastloc snapshots were transmitted to and relayed through the Argos Location and Data Collection System (www.argos-system.cls.fr). 

In 2017 and 2018 two narwhals were instrumented with satellite transmitters that in addition to depth also recorded and transmitted data on in situ water temperature and salinity (Wildlife Computers Scout-CTD-370D, 12´6´3.5 cm, 316 g). The CTD (Conductivity-Temperature-Depth) tags were electrode-based, were powered by four AA lithium batteries and had a temperature range of -3 to 40°C and a salinity range of 0–50 PSU. The resolution was 0.001 for salinity and temperature, and 0.5 m for depth. The minimum requirement for a dive was 50 m and the minimum interval between dives was set to 15 min. For each qualifying dive, the deepest point was detected, after which sampling of the various parameters was continued at 1 Hz until reaching the surface. The first dive set the baseline or the minimum depth for a dive to be recorded and the next dive had to be ten percent deeper than the baseline dive to overwrite the data. This continued until the end of the summary period, when the CTD data and the closest position data were processed into Argos messages for transmission. When the tag was at the surface, the measured environmental data were transmitted through Argos Data Collection and Location System. The tags were programmed to capture the deepest profile in every 12-hour period, i.e. a total of two profiles per day, and was set to transmit the profile repeatedly 12 times to increase the chance that a given profile would be received by an Argos satellite during the following 12-hour period. The tags were mounted in a similar way as the Fastloc-GPS transmitters mentioned above. Data from the CTD casts were collected at standard depths following Levitus World Ocean Atlas 1994 (WOA94, 0, 10, 20, 30, 50, 75, 100, 125, 150, 200, 250, 300, 400, 500, 600, 700, 800, 900, 1000 m, etc.) standard depths and each cast had an associated Fastloc-GPS snapshot, providing only two positions per day from these tags.

Sixteen narwhals were instrumented with Acousonde™ acoustic tags (Table 1, www.acousonde.com), whose float had been modified to accommodate an Argos transmitter (Wildlife Computers SPOT5) in addition to a VHF transmitter (ATS Telemetry). The Acousonde recorders were attached to the skin with suction cups, on the rear half of the animal, to the side of the dorsal ridge, and released from the whales after four to ten days. They were subsequently located and picked up at sea with Argos and VHF transmitters. A custom-written buzz detector (Matlab, The MathWorks, Inc., Natick, MA, USA) was used to identify buzzes in the records and all potential buzzes were verified manually in 13 of the Acousonde recordings with sufficient quality of the acoustic data. As was done for the GPS positions, the first 24 h of acoustic data were excluded. In addition to acoustic sampling the Acousonde also provided data on depth (precision 0.5 m) and temperature (precision of 0.01°C) every one second. Since the temperature sensor on the Acousonde is embedded in epoxy there is a delay in the temperature readings relative to the depth.