Extremely low seasonal prey capture efficiency in a deep-diving whale, the narwhal

Abstract

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 (707 ± 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 (less than 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.

Narwhal (Monodon monoceros) echolocation click rates to support cue counting passive acoustic density estimation

“…This might lead to a potential bias when extending this cue rate, here calculated exclusively from the month of August, into density estimates for other seasons. For example, Chambault et al (2023) show that in data also exclusively obtained during summer months, narwhal's prey capture efficiency was much lower than expected, leading them to presume that narwhals sustain their bioenergetic demands by foraging more at other times of the year. Reduced prey density, extreme prey selection and varied energetic needs at different times of year could all have contributed to a further nuanced use of the value presented here.…”

The nightscape of the Arctic winter shapes the diving behavior of a marine predator

“…The porpoises in Greenland might not forage much during daytime, remaining in shallow waters while concentrating their feeding activity mostly at night in deeper layers. However, unlike other Arctic toothed whales such as narwhals and belugas that can rely on their body reserves for long periods without feeding 35 , harbour porpoises have high energy demand 17 , and may therefore need to feed continuously to survive. Regularly reaching deep waters (> 100 m), the daily dive duration of the porpoises in Greenland rarely exceeded 4 min (less than 3% of the dives, mean: 1.6 ± 1.02 min, maximum: 7 min, Table 1).…”