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

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.

成功的觅食行为对于个体维持生存与繁殖所需的正向能量平衡（positive energy balance）至关重要。然而，学界对海洋顶级捕食者（marine apex predators），尤其是深潜哺乳动物（deep-diving mammals）的猎物捕获效率的记载却极为匮乏。本研究于夏季部署声学标签（acoustic tags）与胃温丸（stomach temperature pills），同步采集疑似觅食活动（通过觅食蜂鸣检测（buzz detection））与成功猎物捕获事件（通过胃温下降监测）的数据，以此估算独角鲸（narwhals）的摄食效率。与每日蜂鸣次数（706.9 ± 368）相比，夏季每日觅食事件发生率极低（19.8 ± 8.9），且仅8%~14%的觅食潜水（foraging dives）成功捕获猎物（即可检测到猎物捕获行为）。这一极低的成功捕食率使得独角鲸的每日食物摄入量（daily food consumption rate）极低（不足体重的0.5%），这表明独角鲸依赖冬季积累的身体储备（body reserves）来维持全年的生命活动。因此，气候变化导致的其越冬栖息地（wintering habitats）变化乃至消失，可能会对独角鲸种群的适合度（fitness）造成严重影响。