Oxygen minimum zone: An important oceanographic habitat for deep‐diving northern elephant seals,Mirounga angustirostris

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.

Inferring prey size variation from mandible acceleration in northern elephant seals

“…A previous study using animal‐borne video logger showed that northern elephant seals foraged on immobile large‐sized prey (ragfish, Icosteus aenigmaticus ) at the deep depths associated with the oxygen minimum zone (Naito et al . ), supporting our interpretation. Alternatively, the seals might have encountered a higher proportion of larger prey at deeper depths.…”

“…, Naito et al . ). This negative effect on the ability of prey to escape may reduce the values of acceleration signals, which would unlikely overturn our results that seals foraged on larger prey at deeper depth, supporting the potential usage of accelerometry to investigate prey size variation.…”

“…The mandible accelerometers measured longitudinal ( i.e ., cranial‐caudal or surge) acceleration at 32 Hz, which were used to compute the occurrence of feeding events based on a data‐processing algorithm (Naito et al . , ; Adachi et al . ).…”

“…We hypothesized that inferred prey size is larger at deeper depths based on our previous study that used animal‐borne video logger, showing seals foraged on large‐sized prey at deep depth (Naito et al . ).…”

Energy-Rich Mesopelagic Fishes Revealed as a Critical Prey Resource for a Deep-Diving Predator Using Quantitative Fatty Acid Signature Analysis

“…In contrast, juvenile elephant seals appear to have a more diverse diet, mostly feeding over the continental shelf on a range of intertidal and neritic organisms (Sinclair, 1994). Recent studies using jaw motion analysis and videography suggest that small mesopelagic nekton, such as lanternfishes (F. Myctophidae), could be an important component of adult seal diet (Naito et al, 2013(Naito et al, , 2017, but such studies are either indirect (i.e., jaw motion event frequency as a proxy for prey size) or subject to the same biases as stomach content analyses. Though mesopelagic fishes have not been confirmed in the diet of northern elephant seals, multiple lines of evidence indicate that their Southern Ocean congeners, southern elephant seals (Mirounga leonina), feed predominately on deep ocean fishes (Bradshaw et al, 2003;Cherel et al, 2008;Banks et al, 2014).…”