He had expected this pattern of behavior, having studied long-finned pilot whales in the Mediterranean, off the west coast of Italy. It was the first time anyone had recorded the dive depths of this species in the wild.
Data were taken using time-depth recorders (TDRs). "They have a pressure sensor for determining depth, as well as a velocity meter for swim speed, and also a light sensor," Baird explained.
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During the day the whales spent their time in the top 16 meters (52 feet) of the water column. But shortly after sunset they turned tail and plummeted to depths of up to 650 meters (2,133 feet). Baird says this is when the vertically migrating deep scattering layer (DSL; a sound-reflecting layer of dense zooplankton, plus fish, squid and other organisms) comes into range as it ascends under the cover of darkness.
Depth and velocity data suggested the whales then followed their prey to the surface where they continued to feed until before daybreak.
Yet the Hawaiian pilot whales also showed diving patterns that were out of sync with their European cousins. Crittercam and TDR data showed that some whales did dive deep during the day, to more than 800 meters (2,625 ft), staying down for almost half an hour. Even more surprising was the finding that nighttime activity was typically between 300 and 500 meters (984 and 1,640 feet), with no indication the whales rose to the surface to feed.
Baird suggests this may be due to both the clarity of the water and the starry night sky out in the middle of the Pacific.
He said, "Information from the light sensors has been extremely useful in understanding why the dives at night were deeper than expected. Light levels at the surface at night were still substantially higher than at depth. So while the depth patterns indicate the whales are following DSL-associated prey around sunset, it appears the light levels are high enough to keep these prey away from the surface."
How these toothed whales detect prey deep in the night sea remains something of a mystery. Baird said, "Presumably, echolocation is most important, perhaps supplemented by detecting prey visually if they produce light; either actively, or by disturbing light-producing organisms in the water as they move."
Fast, intense clicking sounds produced by pilot whales, which have large, bulbous heads, rather like the nose of a submarine, have long been implicated in echolocation. Meanwhile, studies have shown that they often target luminous deep-sea squid species, suggesting they do detect prey by sight.
It's strange then, given these powers of pinpoint detection, that pilot whales sometimes seem unable to identify dry land. Mass strandings often occur. Whether illness, magnetic interference, or confused echolocation due to a gently shallowing seabed causes these catastrophic navigation failures is unclear, though the social cohesiveness of pilot whales probably doesn't help. It appears they would sooner follow the lead, or 'pilot', whale into mortal danger than break away from the pod.
Despite the dramatic new insights provided by Crittercam, we still have much to learn about these extraordinary animals.