Naito Yasuhiko, Costa Daniel P, Adachi Taiki, Robinson Patrick W, Peterson Sarah H, Mitani Yoko, Takahashi Akinori
National Institute of Polar Research Midori-cho Tachikawa, Tokyo Japan.
Long Marine Laboratory Center for Ocean Health Institute of Marine Sciences University of California Santa Cruz CA USA.
Ecol Evol. 2017 Jul 3;7(16):6259-6270. doi: 10.1002/ece3.3202. eCollection 2017 Aug.
Little is known about the foraging behavior of top predators in the deep mesopelagic ocean. Elephant seals dive to the deep biota-poor oxygen minimum zone (OMZ) (>800 m depth) despite high diving costs in terms of energy and time, but how they successfully forage in the OMZ remains largely unknown. Assessment of their feeding rate is the key to understanding their foraging behavior, but this has been challenging. Here, we assessed the feeding rate of 14 female northern elephant seals determined by jaw motion events (JME) and dive cycle time to examine how feeding rates varied with dive depth, particularly in the OMZ. We also obtained video footage from seal-mounted videos to understand their feeding in the OMZ. While the diel vertical migration pattern was apparent for most depths of the JME, some very deep dives, beyond the normal diel depth ranges, occurred episodically during daylight hours. The midmesopelagic zone was the main foraging zone for all seals. Larger seals tended to show smaller numbers of JME and lower feeding rates than smaller seals during migration, suggesting that larger seals tended to feed on larger prey to satisfy their metabolic needs. Larger seals also dived frequently to the deep OMZ, possibly because of a greater diving ability than smaller seals, suggesting their dependency on food in the deeper depth zones. Video observations showed that seals encountered the rarely reported ragfish () in the depths of the OMZ, which failed to show an escape response from the seals, suggesting that low oxygen concentrations might reduce prey mobility. Less mobile prey in OMZ would enhance the efficiency of foraging in this zone, especially for large seals that can dive deeper and longer. We suggest that the OMZ plays an important role in structuring the mesopelagic ecosystem and for the survival and evolution of elephant seals.
对于深海中层带顶级捕食者的觅食行为，我们了解甚少。尽管在能量和时间方面潜水成本很高，但海象仍会潜入生物量少、氧气含量极低的区域（深度>800米），但它们如何在该区域成功觅食在很大程度上仍不为人知。评估它们的摄食率是理解其觅食行为的关键，但这颇具挑战性。在此，我们通过下颌运动事件（JME）和潜水周期时间评估了14只雌性北象海豹的摄食率，以研究摄食率如何随潜水深度变化，尤其是在低氧区。我们还从海豹佩戴的视频中获取了视频片段，以了解它们在低氧区的觅食情况。虽然JME的大多数深度都呈现出明显的昼夜垂直迁移模式，但在白天会偶尔出现一些超出正常昼夜深度范围的极深潜水。中层带中部区域是所有海豹的主要觅食区。在迁徙过程中，体型较大的海豹往往比体型较小的海豹表现出更少的JME次数和更低的摄食率，这表明体型较大的海豹倾向于捕食更大的猎物以满足其代谢需求。体型较大的海豹也频繁潜入深层低氧区，这可能是因为它们的潜水能力比体型较小的海豹更强，表明它们对更深层区域食物的依赖性。视频观察显示，海豹在低氧区深处遇到了很少被报道的囊头鲉，这种鱼对海豹没有表现出逃避反应，这表明低氧浓度可能会降低猎物的活动能力。低氧区中活动能力较弱的猎物会提高该区域的觅食效率，尤其是对于能够潜入更深、时间更长的大型海豹来说。我们认为，低氧区在构建中层带生态系统以及海象的生存和进化方面发挥着重要作用。
