Elephant Seals' Extreme Diving Allows Them to Exploit Deep Ocean Niche

Using data captured by video cameras and smart accelerometers attached to female elephant seals, Taiki Adachi and colleagues show that the animals spend at least 80% of their day foraging for fish, feeding between 1,000 and 2,000 times per day. The unique glimpse at elephant seal foraging strategy shows how these large marine mammals exploit a unique ocean niche filled with small fish. The findings also may offer a way to monitor the health of the mesopelagic zone, the dark and cold ocean "twilight zone" ecosystem at 200 to 1,000 meters deep. Small mesopelagic fish dominate the world's ocean biomass, but little is known about the mesopelagic zone or how it is affected by global climate change. Large marine mammals can dive deep to find food in this zone, but the bigger they are, the more food they need to sustain themselves. Massive marine mammals like sperm whale solve this problem by diving into the mesopelagic zone to eat large prey like jumbo squid. The data collected by Adachi et al. suggest that the smaller elephant seals solve the problem by eating abundant small mesopelagic fish, but the smaller mouthfuls mean they must dive repeatedly to maintain a positive energy balance. The researchers say that elephant seal foraging activity could be used to trace the future health of mesopelagic fish populations, as the mesopelagic zone is expected to undergo substantial changes in temperature and oxygenation by the end of the century.

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