So large that the military used to hide submarines in it.
In a first, Marine biologist and filmmaker Rick Rosenthal dived into and recorded the largest migration on Earth. His documentary, called Ocean Magic at Night, aired January 7 on CBC-TV’s The Nature of Things.
This "vertical migration" is the movement of billions of ocean animals, including fish, krill, jellyfish, and more, that swim upward more than half a kilometer from the depths of the ocean to feed on plankton near the surface — and this happens every single night, Rosenthal told CBC.
Vertical migration is a very important part of the food chain because it allows larger predators — who are unable to dive deep — to feed on food like squid when they come to the surface.
However, scientists do not really know all that much about this migration since it only began to be studied by the military in the mid-20th century. This migration is sometimes so thick that the military even used it to hide submarines. It is also known to change the direction of ocean currents.
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Rosenthal used digital cameras that allowed him to record at night with very little light. Light changes the behavior of the animals — which is something Rosenthal wanted to avoid.
Night diving is very dangerous since you can get very disoriented quickly — not knowing which way is up and down. Also, ocean currents are very strong and you could easily drift away in just a few minutes. As a safety measure, Rosenthal was equipped with a beacon that would send an emergency signal to the nearest boat if he got lost. Luckily, he never had to use it.
Rosenthal did most of his filming in the waters of Hawaii, Costa Rica and Panama. He needed the water to be warm since he would be spending a lot of time in it, but he also needed it to be relatively clear so he could capture the migration at a distance.
It was still very difficult to capture these animals on camera because animals stay where the brightness is about one percent of full daylight. Unfortunately, there were a lot of wildlife below him that he just could not capture on camera. Also, he is certain many of the larger animal species were aware of the boat and his presence, so they kept their distance.
Rosenthal’s next project is a documentary on the effects and impacts of El Niño.
Here is a video by National Geographic explaining this mysterious vertical migration.