| Summary: | Successful foraging is essential for individuals to maintain the positive energy balance required for survival and reproduction. Yet, prey capture efficiency is poorly documented in marine apex predators, especially deep-diving mammals. We deployed acoustic tags and stomach temperature pills in summer to collect concurrent information on presumed foraging activity (through buzz detection) and successful prey captures (through drops in stomach temperature), providing estimates of feeding efficiency in narwhals. Compared to the daily number of buzzes (706.9 ± 368), the daily rate of feeding events was particularly low in summer (19.8 ± 8.9), and only 8–14% of the foraging dives were successful (i.e., with a detectable prey capture). This extremely low success rate resulted in a very low daily food consumption rate (< 0.5% of body mass), suggesting that narwhals rely on body reserves accumulated in winter to sustain year-round activities. The expected changes or disappearance of their wintering habitats in response to climate change may therefore have severe fitness consequences for narwhal populations. Funding provided by: H2020 Marie Skłodowska-Curie Actions Crossref Funder Registry ID: http://dx.doi.org/10.13039/100010665 Award Number: 48068 Funding provided by: Danish Cooperation for the Environment in the Arctic* Crossref Funder Registry ID: Award Number: Funding provided by: Carlsberg Foundation Crossref Funder Registry ID: http://dx.doi.org/10.13039/501100002808 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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