DOI: 10.5061/dryad.zcrjdfn81
title: Data from: Temperature dependent habitat selection by narwhals

description: 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 &gt;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.
description: 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 <i>in
 situ</i> 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<i> </i>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.