My master’s thesis at the School of Aquatic and Fisheries Sciences at the University of Washington, was focused on the midwater ecology of Monterey Bay, California. I did not observe it directly, however—my view came from a sonar echosounder located on the seabed in the outer bay, in almost nine hundred meters of water at the edge of the Monterey Submarine Canyon.
The instrument was the Deepwater Echo Integrating Marine Observatory System (DEIMOS), and it was plugged in to MARS, the Monterey Accelerated Research System. MARS is a car-sized metal box at the end of a 50 km cable. It has a number of ports where scientific instruments can be plugged in to electrical power and data connections, allowing experiments to be run remotely from the shore. See MARS’s website for the current cast of instruments there.
DEIMOS itself was a Simrad EK60 scientific echosounder, modified to operate at depth. It emitted a short pulse of sound every five seconds, and then listens to the echo. Since sound travels at a known speed, by timing how long it takes to for the echoed sound to return, it can calculate the range to whatever the sound bounced off of. By recording how loud the echo is, it can get a sense of how big or thick the object or objects are. And from there, we’re off into the wonderful world of sonar analysis and acoustic inference…
What does this data look like? Below is a 42-hour echogram from Feburary 28 to March 1, 2009. Older observations are on the left, and newer ones are towards the right. The ocean bottom is at the bottom, with the surface at the top.
The colored smudges and squiggles show areas of the water column that scattered some sound back to the echosounder—that is, had some kind of stuff floating in them. For the most part, this stuff is micronekton: small, weakly-swimming animals, including lanternfish, shrimp, krill, squid, jellies, and others. Every night, these animals migrate up to hundreds of meters from the deep up to the surface to feed, returning back down at dawn to avoid being seen by predators. At the left edge of this echogram, it is nighttime, and the animals are concentrated at the surface. Several hours later, though, you can see them migrating down and splitting into several layers, where they remain until dusk.