Cloudy Seas

When the clouds begin to thicken up, many of us head for cover. And small marine organisms may do the same thing. Some recent research found that they move up or down as cloud cover changes—a way to keep safe as the amount of light changes.

Many fish and other organisms move up and down in the water as light levels vary. When it gets dark, they move up to feed. And as day begins to break, they move down to avoid being eaten themselves. They also change their depth in response to moonlight—trying to find just the right amount of light to survive another day. That process is called “vertical migration.”

In 2018, researchers studied this up-and-down cycle in the northeastern Pacific Ocean. They watched the light levels, and compared them to the migrations of groups of tiny fish and other organisms. In particular, they used sonar to look at a layer of organisms that stayed about a thousand feet deep during the day.

It was cloudy during the entire research cruise. But the thickness of the clouds varied, so the amount of sunlight varied as well. Scientists found that when the clouds got thicker, the tiny organisms moved up. And when the clouds thinned out, they moved down. It took a few minutes for the critters to respond to the change. And there had to be a pretty good shift in the light level to trigger a move. But the organisms often moved up and down many times during the day, and by a good distance each time—keeping the light just right below the ocean surface.