| Summary: | 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 (>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. Funding provided by: HORIZON EUROPE Marie Sklodowska-Curie Actions Crossref Funder Registry ID: http://dx.doi.org/10.13039/100018694 Award Number: 48068 Funding provided by: Greenland Ministry of Education, Church, Culture & Gender Equality* Crossref Funder Registry ID: Award Number: Funding provided by: Danish Cooperation for the Environment in the Arctic* Crossref Funder Registry ID: Award Number: Funding provided by: Greenland Institute of Natural Resources Crossref Funder Registry ID: http://dx.doi.org/10.13039/100008075 Award Number:
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