The Mystery Behind Earth's Most Epic Migration.

The Mystery Behind Earth’s Most Epic Migration.

During World War II, submarine Sonar recorded these strange

dense signals rising from the deep

as if parts of the ocean floor were moving up and down

by as much as 3,000 feet.

The sea floor wasn't moving.

The sonar was actually detecting huge masses of tiny animals

known as zooplankton

ascending from the depths to the surface every night

and returning down again.

Turns out this happens in every ocean, every night

and scientists were completely bewildered.

I mean, why do these nearly microscopic plankton

make such an incredible daily journey?

Turns out the answer could be linked to phenomena

as seemingly unrelated as biological clocks

and even climate change.

Hey, smart people, Joe here.

This is the strange story

of Earth's largest and most mysterious migration.

(bright upbeat music)

- Vertical migration in the ocean is the largest

net animal movement on our planet.

It's really remarkable.

I'm Kelly Benoit-Bird, a senior scientist at MBARI,

the Monterey Bay Aquarium Research Institute

where I use sound to study lives of ocean animals.

- So first, you really need to appreciate

how tiny zooplankton are.

Like smaller-than-the-tip-of-a-crayon tiny

but the distances they move in the ocean

are absolutely immense, for them anyway.

- If we were to scale the migrations to a human,

we'd be talking about you doing a 10K twice a day,

once to get your breakfast and once to go to bed

but you'd have to swim at twice the speed

of an Olympic marathon runner.

It's a pretty remarkable endeavor each and every day.

- If you add up all of the vertical migration

happening in all the oceans and lakes on earth,

scientists estimate 10 billion tons of biomass,

25 times the mass of all humans on earth

is racing between the surface and the deep every night.

It's called the diel vertical migration or DVM for short.

But why go to all that trouble?

- Vertical migration is probably

one of the most common behaviors that we see in the ocean.

It happens from the smallest animals to some of the largest

and the most abundant, in terms of biomass,

migrators are typically small fish

like bristlemouths and lantern fish

that are following the vertical migrations

of the zooplankton.

It is a pretty different way of thinking,

most often we've thought about plankton,

all of the plankton as just wanderers

like the Greek word for plankton defines them

but they are are capable of making decisions.

- Zooplankton live in the twilight zone.

No, not that twilight zone,

though some do look pretty strange.

We're talking about the mesopelagic zone,

it's a region of semi-deep water

that receives only about 20% of the light

that you get up on the surface.

- Well, we know that this vertical movement is a real dance,

a balance by these animals to try to get food

which is most abundant in the surface waters

where photosynthesis lets things grow

but while they're trying to avoid becoming dinner

for something else

And so if you're trying to avoid getting eaten,

you wanna be in the dark.

So most often what we see is that animals stay

deep in the dark during the day and then as the sun sets,

they migrate up to the surface

before leaving again at sunrise.

- Responding to tiny changes in light that would prompt them

to move up the water column when the sun went down

and then back down at sunrise.

- But sometimes we see, organisms actually do the opposite.

We sometimes call reverse diel vertical migrations.

- Researchers found that zooplankton move up and down

in the water by as much as 200 feet

just from clouds passing overhead.

That means they're pretty dang photosensitive.

But scientists thought there might be more to the picture.

- Just like those early observers

of diel vertical migration, we're using sound.

Light doesn't penetrate very far in the ocean

so when we try to go down with a camera

with a lot of lights, we're lucky if we see

a few arm lengths in front of us

but sound travels both further and faster in water

than it does in air an