DOI: 10.5061/dryad.zpc866tdh
title: Data from: The nightscape of the Arctic winter shapes the diving behavior of a pelagic predator

description: Predator-prey interactions in marine ecosystems are dynamically structured
 by light, which is exemplified by diel vertical migrations of low-trophic
 level organisms. At high latitudes, the long winter nights provide
 foraging opportunities for marine predators targeting vertically migrating
 prey closer to the surface at night while minimizing energy expenditure,
 but there is limited documentation of such diel patterns under extreme
 light regimes. To address this knowledge gap, we recorded the diving
 behavior of 17 harbour porpoises just south of the Arctic circle in West
 Greenland, from summer to winter. Unlike classical diel vertical
 migration, the porpoises dove three times deeper at night and the
 frequency of deep dives (&gt;100 m) increased tenfold as they entered
 the darkest months. The daily mean depth was negatively correlated with
 daylength, confirming this reverse diel migration and suggesting an
 increased activity—presumably to target prey at greater depths—when
 approaching the polar night. Our findings illustrate a light-mediated
 strategy in which harbour porpoises would maximize energy gain during long
 periods of darkness while minimizing energy expenditure by accessing
 vertically migrating prey, which are otherwise inaccessible in deep
 waters. Extreme light regimes observed at high latitudes are therefore
 critical in structuring pelagic communities and food webs.
description: (a) Animal
 instrumentation Between 2012 and 2014, 17 harbour porpoises were
 live-captured on the continental shelf approximately 50 km south-west of
 Maniitsoq, West Greenland (Fig. 1). Following Nielsen’s <em>et
 al</em>.’s procedure19, the individuals were instrumented with two
 types of satellite transmitters (SPLASH and MK10, Wildlife Computers,
 Redmond, WA, USA). Details on the tagging procedure and tag’s
 specifications can be found in Nielsen <em>et al</em>.
 (2019). The study was performed with permission from
 the Government of Greenland, permit no. 2012-069733, Doc.
 1265044. (b) Identification of diel
 patterns All analyses were conducted using R software
 version 4.3.0 [39]. To investigate diel patterns in the diving
 behavior, sunlight phases (times of sunrise, dusk, day, dawn and sunset)
 were extracted at each individual’s location using the suncalc package
 [40] in R . For the five tags that were retrieved, no
 coordinates were stored in the tags. Satellite-relayed Argos coordinates
 (from the low-resolution datasets) were therefore matched to the
 high-resolution data based on the closest date and time for each
 individual. Diving behavior variables (mean dive depth, maximum dive
 depth, median dive depth and dive duration) were then compared between the
 different phases (dawn, day, dusk and night). During midnight sun in
 summer, dawn and dusk are not always identifiable due to rapid sunsets and
 sunrises during long periods of daylight. For this reason, diurnal and
 nocturnal dives were instead compared by merging dawn, dusk and night into
 the same “nighttime” category. Times of sunrise and sunset were then used
 to derive the daylength for each tracking day and each
 individual. To map the day-night regimes in relation to the
 porpoises’ locations in West Greenland, solar elevation was extracted
 individually at each location and for the corresponding date. Based on the
 solar elevation angle (i.e., position of the sun in relation to the
 horizon), three phases were identified: day (solar elevation &gt;0°),
 twilight (between 0 and -18°) and night
 (&lt;0°).  (c) Investigation of the relationship between
 daylength and dive depth A series of Generalized Additive Models (GAM)
 were performed to relate the daylength to the daily dive depth or the
 harbour porpoises using the <em>mgcv </em>package. Dives were
 defined as deeper than 5 m and longer than 20 sec. Three separate models
 were tested using three diving behavior metrics summarized daily: mean,
 median and maximum depth. The short tracking duration of four individuals
 precluded the identification of seasonal patterns and they were therefore
 excluded from this analysis. Pairwise correlation between all covariates
 were first investigated (SI Fig. 1). To account for the inter-individual
 variability, individual’s ID was added as a random factor on both the
 slope and the intercept. Individual response curves were then generated
 for each harbor porpoise. The residuals were then inspected using
 diagnostic plots for each model.