Humboldt Squid found to spend many hours in a band of oxygen-poor water at a depth of more than 500 m in the California current.
One reason squid may be going down there at night is because water near the surface is really warm—up to 28 C (82 F)—which may stress them.
At depths of 250 m (820ft) or more, where it's very cold and oxygen levels are extremely low, the squid's reaction time, visual acuity and swimming speed may be significantly impaired because of an inadequate supply of oxygen—a condition known as hypoxia.
However, a team of researchers from Stanford University led by Bill Gilly have found squid do not only tolerate this hypoxia, they remain in this hostile environment all day long and actively feed there. At present they do not know exactly how they accomplish this physiologically but from attempts to keep squid in aquariums, Gilly knows that the squid are able to survive in low-oxygen by slowing down their metabolism.
But other predators, including sperm whales, elephant seals, sharks and large tunas probably wish they could duplicate the feat--they all dive to the daytime squid depth but can only remain there for much shorter times.
During the day, whales and squid spent about 75 percent of the time at depths ranging from 185 to 400 m (600 to 1,300 feet), which is "consistent with the idea that the whales were foraging where the probability of encountering squid was highest," the authors wrote.
At night, however, the tagged squid spent at least half of their time in shallower waters above 185m and the remainder at 185 to 400m. One likely explanation for this vertical movement is that the squid were following small fish and other prey that migrate toward the surface at night and then return to deeper waters during the day.
Unlike squid, however, the sperm whales did not alter their diving pattern at night. Instead, they continued to spend about three-fourths of their time at depths of 185 to 400m, according to nocturnal tagging data.
"These data show that sperm whales don't change their feeding behavior, day or night," explains Randall Davis, professor of marine biology at Texas A&M University-Galveston. "Instead, they keep going down to about 1,300 feet, whether squid are there or not. Perhaps it's the only way they can catch them, but no one has ever seen a sperm whale feeding in the wild, so nobody really knows how they capture their food."
Jumbo squid populations have been increasingly migrating farther north than their normal range, in some cases as far north as Alaska. This is of concern to scientists who fear that increased ocean temperatures possibly due to global warming are to blame and/or perhaps overfishing of the jumbo squid's predators, which has allowed them to expand their range.