Remote polar and deepwater fish faunas are under pressure from ongoing climate change and increasing fishing effort. However, these fish communities are difficult to monitor for logistic and financial reasons. Currently, monitoring of marine fishes largely relies on invasive techniques such as bottom trawling, and on official reporting of global catches, which can be unreliable. Thus, there is need for alternative and non-invasive techniques for qualitative and quantitative oceanic fish surveys. Here we report environmental DNA (eDNA) metabarcoding of seawater samples from continental slope depths in Southwest Greenland. We collected seawater samples at depths of 188–918 m and compared seawater eDNA to catch data from trawling. We used Illumina sequencing of PCR products to demonstrate that eDNA reads show equivalence to fishing catch data obtained from trawling. Twenty-six families were found with both trawling and eDNA, while three families were found only with eDNA and two families were found only with trawling. Key commercial fish species for Greenland were the most abundant species in both eDNA reads and biomass catch, and interpolation of eDNA abundances between sampling sites showed good correspondence with catch sizes. Environmental DNA sequence reads from the fish assemblages correlated with biomass and abundance data obtained from trawling. Interestingly, the Greenland shark (Somniosus microcephalus) showed high abundance of eDNA reads despite only a single specimen being caught, demonstrating the relevance of the eDNA approach for large species that can probably avoid bottom trawls in most cases. Quantitative detection of marine fish using eDNA remains to be tested further to ascertain whether this technique is able to yield credible results for routine application in fisheries. Nevertheless, our study demonstrates that eDNA reads can be used as a qualitative and quantitative proxy for marine fish assemblages in deepwater oceanic habitats. This relates directly to applied fisheries as well as to monitoring effects of ongoing climate change on marine biodiversity—especially in polar ecosystems.

Variation in resource selection among sub-populations may elucidate differences in fitness and life history strategies. Specifically for top marine predators, differences in movements and behavior may result from responses to variation in a patchy, dynamic environment. Satellite-linked time-depth recorders (SLTDRs) were used to examine differences in narwhal Monodon monoceros diving behavior and habitat selection among 3 sub-populations in Canada and West Greenland (n = 16 individuals). The number of dives to different depths and time allocation within the water column was investigated in 3 seasons, with a focus on 2 discrete wintering grounds in Baffin Bay. Diving parameters were calculated from binned dive data and analyzed using repeated-measures mixed models accounting for temporal autocorrelation and individual variability. The number of surface dives (0 to 50 m) and time at the surface declined between summer and winter. Clear differences were observed between 2 wintering grounds. Whales occupying one wintering ground spent most of their time diving to between 200 and 400 m (25 dives per day, SE 3), confirmed by both depth and temperature recording tags. In contrast, narwhals in a separate wintering ground spent less time at shallow depths and most of their time diving to at least 800 m (13 to 26 dives per day, SE 1 to 3). A model of occupancy time at depth showed that whales making multiple daily deep dives spent over 3 h at > 800 m (SD 0.6) and traveled 13 min (SD 1) per round trip to reach this depth.

Comparison of behavioural similarities between subpopulations of species that have been isolated for a long time is important for understanding the general ecology of species that are under pressure from large‐scale changes in habitats. Narwhals (Monodon monoceros) east and west of Greenland are examples of separated populations that, in different ocean parts, will be coping with similar anthropogenic and climate‐driven habitat alterations. To study this, 28 narwhals from the Scoresby Sound fjord system were tracked by satellite in 2010–2013. The average duration of contact with the whales was 124 days with one tag lasting 305 days and one whale recaptured <1 km from its tagging site 366 days later. All whales exhibited the same migratory pattern. The whales departed from the summering grounds in Scoresby Sound in September and arrived at the edge of the continental shelf by November. Here, they stayed through May–June and conducted daily dives to the mesopelagic zone at ∼1000 m depth. Despite the isolation by the landmass of Greenland and the genetic differentiation from other narwhal populations, there is still a remarkable similarity not just in behavioural traits like phenology of migrations and movements in relation to sea ice formation, but also in site fidelity, diving behaviour, feeding ecology, habitat selection, daily travel speed and even potential conflicts with fisheries for Greenland halibut (Reinhardtius hippoglossoides). Greenland halibut are likely target prey during the deep dives in winter but capelin (Mallotus villosus) may, with ocean warming, become of increasing importance. The ocean‐wide predictability in culturally inherited migration patterns, size of wintering grounds and habitat selection among narwhal populations is certainly different from other Arctic cetaceans and renders narwhals more vulnerable to large‐scale changes in their restricted and specialized habitats.

The bathymetry of Baffin Bay, with shallow sills both to the north and south, creates a relatively isolated body of deep polar water, unique among the Arctic Seas. During 263 trawl hauls completed during October 1999 and September to November 2001, 116 fish species were collected in Davis Strait and the southern Baffin Bay (61°44.1′ N-73°52.8′ N, depths of 145-1484 m). The abundance data for the 80 benthic species were used for analyses of the fish fauna diversity and fish assemblages. As a first step, seven assemblages were found by a standard type of cluster analysis. A Bayesian multinomial logit model was then applied to calculate vectors of probabilities defining the likelihood of each haul belonging to each of the seven clusters. The spatial distribution of the conditional probabilities for each cluster (assemblage) was mapped by means of a geostatistical tool. Each assemblage was further defined by indicator species, depth, and temperature. Four of the assemblages were found in Baffin Bay, two in Davis Strait and one mainly in Davis Strait but scattered into Baffin Bay.Résumé : La structure bathymétrique de la baie de Baffin avec des seuils peu profonds vers le nord et le sud forme un bassin relativement isolé d'eau polaire profonde, une situation unique dans les mers arctiques. En octobre 1999 et de septembre à novembre 2001, 263 récoltes au chalut ont capturé 116 espèces de poissons dans le détroit de Davis et le sud de la baie de Baffin (61°44,1′ N-73°52,8′ N, à des profondeurs de 145-1484 m). Les données d'abondance de 80 espèces benthiques ont servi à des analyses des peuplements de poissons et de la diversité de la faune des poissons. Dans une première étape, une analyse standard de groupement a identifié sept communautés. Nous avons alors appliqué un modèle logit multinomial bayésien afin de calculer les vecteurs de probabilité qui définissent la probabilité que chacune des récoltes appartienne à chacun des sept groupements. À l'aide d'un outil géostatistique, nous avons cartographié la répartition spatiale des probabilités conditionnelles de chaque groupement (communauté). Nous avons ensuite caractérisé chacune des communautés à l'aide des espèces indicatrices, de la profondeur et de la température. Quatre des communautés se retrouvent dans la baie de Baffin, deux dans le détroit de Davis et une concentrée principalement dans le détroit de Davis, mais aussi éparpillée dans la baie de Baffin.[Traduit par la Rédaction] Jørgensen et al. 1852