In a rare moment of calm, Evan Kovacs takes a breath to admire dusk from <em>Catapult</em>’s helm. Red lighting is used aboard the ship to limit interference from artificial light while Mesobot collects radiometer data below. Credit: Jennifer Berglund
In a rare moment of calm, Evan Kovacs takes a breath to admire dusk from Catapult’s helm. Red lighting is used aboard the ship to limit interference from artificial light while Mesobot collects radiometer data below. Credit: Jennifer Berglund

Following the previous day's close encounter with a humppack whale, the OTZ Team decided it was time for Mesobot to catch the evening migration of animals from the twilight zone up to the surface. The plan was to deploy Mesobot in plenty of time for the beginning of the deep scattering layer’s movement. It would dive to about 500 meters and hold its position through sunset, gathering data as the layer of twilight zone life moved past. 

“This is one of the themes of this expedition—we’re trying to vet our new technologies, and we’re trying to show their scientific applications,” said WHOI Senior Scientist, Dana Yoerger.  If Mesobot followed orders and captured data, the team would do just that.

Mesobot floats at the surface shortly after its deployment from the R/V Catapult. (Photo: Evan Kovacs)
<em>Mesobot</em> starts its descent into the waters of the twilight zone just a few miles offshore from Bermuda. (Photo: Evan Kovacs)
Mesobot starts its descent into the waters of the twilight zone just a few miles offshore from Bermuda. (Photo: Evan Kovacs)

Once deployed, the team watched Mesobot descend to a location just above the layer, and they waited for the Sun to set. Evening hues of orange and yellow soon painted the horizon, and the team's sonars began to detect stirrings in the deep scattering layer.

As the Sun dips low in the sky, evening hues of orange and yellow paint the horizon. Deep below the ocean's surface, millions of organisms are beginning their nightly migration from the twilight zone up to shallower waters. (Photo: Jennifer Berglund
As the Sun dips low in the sky, evening hues of orange and yellow paint the horizon. Deep below the ocean's surface, millions of organisms are beginning their nightly migration from the twilight zone up to shallower waters. (Photo: Jennifer Berglund
“This is exciting stuff,” said WHOI Senior Scientist Dana Yoerger (pictured at right). “Of course, from up here, we always say that watching people do an AUV dive is a bit like watching people watch paint dry, but for us it’s very exciting,” he said. Photo: Jennifer Berglund

Sunset is an important cue for twilight zone animals to begin their evening migration to the ocean’s surface. Indeed, as the light started to fade, the sonars revealed Mesobot holding its depth while twilight zone animals migrated past, heading toward the surface.  

After 3 hours in the water, Mesobot returned to the surface, right on schedule.  “We had a beautiful recovery with all the lights blinking and flashing, and all the beautiful ship maneuvering and deck work,” said Yoerger.  

The team headed home after dark to rest up for the next day.  The next mission?  Track the deep scattering layer as it migrates back to the twilight zone. Stay tuned for more adventures into the depths...

—Jennie Berglund, OTZ Field Correspondent

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