Abstract
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 (Icosteus aenigmaticus) 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.
Highlights
While many large predators depend on mesopelagic prey, it is apparent that the biota of this zone is strongly affected by the amount of dissolved oxygen which varies largely according to depth (Robison, 2004)
The shallower part of the mesopelagic zone is rich in biota, but the deeper part is biota-poor as an oxygen minimum zone (OMZ) generally develops forming a habitat-restrictive anoxia zone (Robinson, Steinberg, et al, 2010; Robison, 2004)
Measuring foraging success based on gain and cost ratios in the mesopelagic zone is extremely difficult, and has far prevented an understanding of how deep divers maintain foraging success in the OMZ when challenged with the increased costs of deep diving
Summary
The mesopelagic zone, a vast oceanic region typically between depths of 200–1,000 m, is one of the largest ecosystems on earth (Irigoien et al, 2014; Robinson, Steinberg, et al, 2010; Robison, 2004) and forms a critical foraging habitat for a variety of large predators, that is, fishes, turtles, penguins, toothed whales, and seals (Block et al, 2011; Charrassin et al, 2001; Cherel, Ducatez, Fontaine, Richard, & Guinet, 2008; Eckert, Eckert, Ponganis, & Kooyman, 1989; Miller, Johnson, & Tyack, 2004; Potier et al, 2007). We aimed to reveal how female northern elephant seals manage their foraging success in the deep mesopelagic zone, in the biota-poor OMZ, despite the high cost in time of deep diving during the limited period of their postbreeding migration For this purpose, we monitored the variability in the seal’s feeding rates based on jaw movement events (JME) using jaw motion recorders (Naito et al, 2013) attached to 14 seals. We discuss the importance of the OMZ as a foraging habitat for elephant seals and other deep-diving marine mammals
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
