Original subtitles

(synth music)

- [Narrator] The sun-kissed Florida coastline.

Every autumn a mysterious shadow gathers

at the water's edge.

Millions of fish

synchronized in shoals

larger than football fields,

moving like a gigantic, swirling tornado,

pitting themselves against a frenzied assault from above

and beneath the waves.

This peaceful shoreline is transformed into a battlefield.

(dramatic music) (water splashing)

With a primordial instinct to gather in their millions

for just a short window each year,

these fish leave themselves vulnerable to hurricanes,

toxic algae,

and predatory attacks

from some of the biggest fish around.

Nevertheless, survival of their species depends on it.

(dramatic adventurous music)

(lion roaring)

Early September.

It's still a sticky 30 degrees Celsius here in Florida,

but there are subtle signs that the season is changing.

(waves crashing)

This is Singer Island,

on the southeastern edge of the Florida peninsula.

This eight-kilometer sandy stretch,

accompanied by high rise buildings,

is an unlikely setting for a wildlife mystery.

(waves crashing softly)

But it is here that wildlife photographer

Michael O'Neill first captured a strange phenomenon.

- The first time I saw this giant school of fish

coming down the beach, close to the shoreline,

I was extremely curious because I had never seen it before.

It looked like a giant serpent swimming down the coastline.

I couldn't wait to study it

and photograph it and find out what it was.

- [Narrator] This giant shoal only appears between September

and October, with a window of just a few days.

Now is the right time to see it again,

and the crew must be ready for every opportunity.

It has been witnessed numerous times along the coastline

from Singer Island, ranging 120 kilometers to the north.

With Singer Island as the starting point,

Michael and the film crew embark

on their journey of discovery.

(dramatic music)

Their aim?

To travel northwards and intercept the shoal

as the fish move in the opposite direction to the crew.

With diving equipment, underwater cameras,

and an aerial drone at the ready,

they are fully prepared to capture the spectacle

from the water and from the air.

(dramatic music)

- All right, it looks like we have a big patch.

- [Narrator] The team have spotted something

just kilometers from their starting point.

(suspenseful music)

A black figure, swimming close to the surface.

(suspenseful music)

(water splashing)

Not a school of fish, but a manta ray.

At two and a half meters in length,

this manta is still a juvenile,

but it will grow to be one of the biggest rays in the world.

Adults grow to be in excess of four meters.

These solitary rays are commonly seen

close to the coastline where nutrients

from upwellings and coastal rivers provide them with food.

(soft mystical music)

This one is in search of high concentrations

of zooplankton near the surface.

And it's not alone.

Bottlenose dolphins breed here.

They are found in most oceans around the world,

but favor the shallow seas of continental shelves

just like Singer Island.

- [Michael] See? A momma and her baby.

- [Narrator] The mother appears to be showing

the baby how to hunt.

She's focused on following and capturing a solitary fish.

(soft mystical music)

The team have found a manta ray and dolphins,

but not the school of fish they're after.

They continue their search.

It isn't long till something else catches

the crew's attention.

- [Michael] Small school.

Must, I would get ready right now.

Moving very fast.

(soft dramatic music)

- [Narrator] Using the drone,

the crew have picked up a black mass

that has congregated close to the shoreline.

It's just a small shoal,

but it's similar to the one in the photo.

(soft dramatic music)

The team approach cautiously,

but the fish don't appear to feel threatened.

They ignore the boat as it moves in closer.

Time to enter the water for a closer look.

(water bubbling)

A shoal of mullet.

There are approximately 80 species of mullet

found in waters around the world.

Florida is home to four of them.

(water swishing)

These are striped mullets.

Individuals can grow up to 50 centimeters long,

and here there are around a thousand of them

gathered in one spot.

(soft mystical music)

They appear to gradually edge their way along the coast,

hugging the shoreline.

But this shoal is nowhere near the size of that

in Michael's photo.

The team continues northwards in search

of something bigger.

The crew are now a hundred kilometers north

of their starting point of Singer Island.

(soft dramatic music)

(birds squawking)

Birds are congregating close by.

A not so subtle sign that there is something happening

below the water's surface.

Another shoal of mullet.

This time, much bigger.

Up to 20,000 of them.

Edging along southwards slowly but surely,

still packed tightly to the shoreline

(waves crashing)

and seemingly unaffected by the waves

that crash in on the beach.

(dramatic music)

Brown pelicans are on the approach.

(dramatic music)

They dive in, one after another.

(dramatic music)

The mullets speed up in defense.

(dramatic music)

Despite a barrage of attacks, the mullet are unwavering.

They continue their mysterious journey.

(waves crashing)

And their speed is increasing.

(soft dramatic music)

(speaking in foreign language)

- [Narrator] As they head south,

smaller schools join the aggregation.

(soft dramatic music)

Some of these fish have traveled over

a hundred kilometers from the north.

As the season changes, they seek the warmer water down south

and congregate here on the peninsula.

The whole area now boils with thousands of fish.

(soft dramatic music)

The shoal has increased to 200,000,

with more and more joining.

(soft dramatic music)

This behavior is only seen for just

a short time each autumn,

with the rest of the year spent in rivers or lakes inland.

(soft dramatic music)

The mouth of the river is where freshwater

and seawater mix.

The brackish water forms in pockets

around the peninsula,

and an animal symbolic to Florida's coastline

travels down rivers into brackish waters to feed.

Easy to spot as it moves,

it raises a cloud of mud and silt.

The manatee.

With a body length of three and a half meters

propelled along by a fan-like tail,

manatees are peaceful creatures that keep to the shallows.

(soft mystical music)

They spend up to eight hours a day

grazing on aquatic plants,

and these calm, warm waters provide ideal growing conditions

for sea grasses and other vegetation.

Algae even colonizes on the skin of the manatee.

Mangroves are a common feature of this coastline.

They serve as a buffer,

protecting land from hurricanes and coastal storms,

and form a vital habitat for the animals

that live above water

and below it.

(water swishing)

Their tangled roots provide shelter and safety

for the mullet, as well as filtering out toxins

in the water.

(soft melodic music)

Florida boasts 30,000 lakes,

big and small,

all surrounded by thick forest.

(soft melodic music)

Upstream, the brackish water turns to freshwater.

(soft melodic music)

The mullet can live here in relative safety.

There are no big predatory fish here.

(soft melodic music)

Thanks to special regulatory mechanisms,

mullets can tolerate both salt and fresh water.

(water swishing)

These crystal clear waters provide a tempting feeding spot.

(water swishing)

Mullets are omnivorous.

(water swishing)

Their diet consists of algae, detritus, and zooplankton.

(water swishing)

The seabed is thick with algae,

which is fed by the nutrients of the surrounding forests,

so these mullets have plenty to eat.

And at this time of year,

they appear to gorge themselves,

taking inviting nutrients in preparation

for what is to come.

(soft mystical music)

Their feeding method is unique.

Using a row of tiny teeth on its upper jaw,

the mullet moves its head hurriedly back and forth

across the rocks like a broom,

breaking off pieces of algae

less than a millimeter in length,

leaving marks on the rock surface.

Where there's no rock, it's lower jaw,

shaped like a shovel, comes in handy for scooping up sand

and filtering out organic matter.

(soft mystical music)

Plenty of food and no enemies,

this inland waterfront is paradise for mullets,

and ideal conditions for young mullets to develop and grow.

There are around 220 species of freshwater fish in Florida.

These include the bluegill,

a fish that feeds on aquatic insects and other small fish.

And the channel catfish.

It might look charming, but it is in fact

a savage carnivore with a diet of snakes and frogs.

This Florida gar is a primitive fish that hasn't changed

in over 110 million years.

It's distinguishing features are it's long, thin mouth

and diamond-shaped scales that cover its body.

It's gill respiration is weak compared to other fish.

It must take in oxygen from air at the water's surface.

(soft mystical music)

Striped bass journey upriver in search of fresh water

to rid themselves of parasites.

They harbor parasites that survive only in saltwater.

Fortunately for the bass, they, like the mullets,

can survive in both.

(water swishing)

They congregate around the spring water upwellings.

Rainwater, stored underground, flows through holes

in limestone rock.

(water gurgling)

Florida's aquifers are considered to be one of the most

productive underground water sources in the world,

ranging from little more than a trickle

to others that discharge millions of cubic meters per day.

(water lapping softly)

But for the adult mullets in this lake,

a natural instinct has been triggered.

Despite this mullet's safe haven,

the mullets seem to know that they cannot stay here.

(soft dramatic music)

Across the peninsula, mullets are leaving

the freshwater of their summer haunts,

making their way down rivers and gathering at river mouths

before moving south in unison towards the open ocean.

(soft melodic music)

(water splashing)

The changing season appears to have sparked

a mass movement that goes against safety and security,

and the crew are on hand, watching their every move.

- Oh, huge school, guys.

Unbelievable.

- [Narrator] A huge swirling mass

just 20 kilometers north of Singer Island.

This is what the team have been waiting for.

(soft dramatic music)

It's not long before pelicans move in once

to take advantage of this easy meal,

(soft dramatic music)

causing chaos in the water.

The mullets make frenzied leaps into the air.

(dramatic music)

An osprey's presence adds to the turmoil.

These raptors feed exclusively on fish.

Their vision is adapted to hone in on their target

from the air.

Backwards-facing scales on its talons

enable it to hold on firmly to its catch.

This is a shoal in crisis.

(soft dramatic music)

Suddenly, Michael spots something.

- [Michael] Shark.

- [Narrator] Blacktip sharks.

(dramatic music)

More than 40 of them lying in wait.

Their hunting strategy is to trap the mullets

in the shallow water before charging

through the mullets' formation.

Trevally, too, seize upon the opportunity.

This meter long carnivorous fish is known locally

as the crevalle jack.

(dramatic music)

The mullets change their formation in response

to surrounding pressure.

(waves crashing)

What was initially an elongated ribbon

is becoming a more tightly bound mass.

(dramatic music)

The close formation offers some protection

to those in the middle.

(dramatic music)

As sharks and trevally charge in,

the mullets do their best to part,

forming patches of clear water.

(waves crashing) (dramatic music)

The sharks and trevally often miss their target,

but the darting response of the escaping mullet

throws the shoal into more confusion.

(waves crashing) (dramatic music)

In these normally gentle waters,

life hangs in the balance.

(water gurgling)

(waves lapping)

And suddenly there's a new arrival.

(dramatic music)

Tarpon.

At two and a half meters long

and weighing in at 150 kilograms,

these are some of the largest, strongest fish in the sea.

(dramatic music)

The mullets attempt to maintain their distance

just as they did with the sharks,

(dramatic music)

but the tarpon are equally as persistent.

The mullets swim closer to the shoreline,

out of reach from the larger fish,

but the tarpon change tact,

swimming on their sides to reach the shallows.

(dramatic music)

Then with one sudden movement,

the tarpon makes a lunge towards the shore,

(water splashing)

throwing its large body against the water's surface.

(dramatic music)

The tarpons' impact on the water stuns the mullets.

(dramatic music)

By moving along on their sides,

the tarpon have another advantage.

With one eye looking up,

they can scour for mullets near the surface

and can watch the movement of the school carefully

before making their next move.

(water splashing) (dramatic music)

The tarpon make leaps that the mullet attempt to dodge.

(water splashing) (dramatic music)

The battle continues for nearly one hour.

(water splashing) (dramatic music)

The mullet swim a fine line between swimming as close

as possible to the shore and stranding themselves

as the big waves hit.

(waves crashing)

(soft somber music)

The bodies of the dying can be seen in the shallows,

clinging on to life.

(soft somber music)

But the shoal survives to continue its journey.

It's a migration of life and death,

but the survivors must now contend with other threats.

(thunder clapping)

It's the end of September.

Lightning and tornadoes are frequent at this time of year.

And with hurricanes on the approach,

the sea is too rough for a boat.

(waves crashing)

New dangers are also emerging.

A red tide.

A toxic algal bloom,

poisoning fish,

and de-oxygenating the water.

(suspenseful music)

(waves crashing)

Seaweed washes up on the beach, bringing death with it.

(waves crashing)

(crow squawking)

The team begin to wonder if the giant schools

of mullet they saw just weeks ago have survived.

Two weeks later, calm has returned.

And on Singer Island, so have the beach goers.

(waves crashing)

And the mullets.

(waves crashing)

They have overcome hurricanes, red tides,

and predatory attacks.

But the sharks and tarpon are now nowhere to be seen.

(soft gentle music)

This school swims calmly and slowly.

They are no longer journeying south.

It's as if they've arrived.

These fish have traveled from inland waterways,

some more than a hundred kilometers from the north,

and they have come together to form a giant shoal

off the beach at Singer Island.

Such a large gathering is a rare sight,

even for the locals.

(soft gentle music)

Until the 1980s, mullets were fished excessively

for use as crop fertilizer and bait for fishing.

As a result, the mullet population dramatically fell

to near extinction at the time,

but strict measures have ensured their comeback.

And now they are thriving.

- State officials banned the use of large nets

some 20 years ago, 25 years ago.

The mullet have made a remarkable recovery,

and I never get tired of seeing these huge schools of mullet

as they come down the beach every September and October.

(waves crashing softly)

- [Narrator] The next day, something unexpected happens.

The crew discover the giant shoal of mullet

have vanished overnight.

(soft gentle music) (waves lapping)

Schools of mullet have been witnessed

at the beach for several years now,

but it has not been known where they disappear.

A local diver shed some light on the matter

after returning from an open sea dive.

- There was a lot of boiling.

- Boiling.

- Yeah, appears to be boiling.

- Nice.

- With, you could see the mullet,

obvious mullet jumping and you see a lot

of bird over them, hovering.

- [Narrator] Mullets been spotted three kilometers

away from the coast.

(suspenseful music)

But as the team arrives at the location,

there is no sign of them.

They head further out to the open sea.

(birds squawking)

Birds circle overhead like sentinels.

(birds squawking)

And in the water, mullets.

(soft gentle music)

The crew decided to dive in.

(soft mystical music)

The mullets are moving much faster this time.

The divers can't keep pace.

Soon they've overtaken the boat

and are heading further out to sea.

But when the boat catches up, the team are rewarded.

They have followed the mullets' migration

from rivers and lakes in the north,

down the coast, all the way to Singer Island,

and now out to sea.

They are the first crew to have captured

the behavior of such a large number

of mullet on film off the coast of Florida.

- [Man] Big water ahead.

- It's spectacular.

And I can't believe you found it.

(soft mystical music)

- [Narrator] The crew continue to follow them

until the mullets descend, disappearing out of sight.

A study has shown that at depths of 200 meters

off the coast of the southern tip of Florida,

young fish, including young mullets, can be found.

Scientists have discovered that adult mullets

migrate offshore and lay their eggs

in the salty depths of the ocean.

If the mullets are spawning,

the crew may be able to find newly hatched mullet

and film the next important stage of the mullets' journey.

But for this, they must wait till dark.

(soft dramatic music)

They leave the bright lights of the marina

and head out into the blackness.

(soft dramatic music)

(water splashing)

(soft mystical music)

A strong light is shown into the sea

to attract the young fish

and increase their chances of finding mullets.

(soft mystical music)

The plankton in the water is a good sign.

They become nutritious food for the young fish,

like this young mackerel

(soft mystical music)

and a baby slipper lobster, riding on a jellyfish.

(soft mystical music)

This larvae is a species of tongue fish.

And this one, a grouper.

Its spines will offer protection as it grows.

A young flying fish.

(soft mystical music)

And something else not often seen:

a young tarpon,

distinctly lacking the strength and ferocity of its parents.

Plenty of young fish, but no young mullets.

But why do these mullets gather at Singer Island

before heading to the open ocean?

The season may be changing, but the mullet make the most

of the warmest waters by traveling out of their lakes

down the coast and southwards,

along the peninsula to its tip.

Four kilometers off the coast,

the seabed depth plunges to 100 meters.

A rich stream of plankton flows in off the Gulf Stream,

providing the perfect spawning conditions for lots of fish.

And the mullets take the shortest route to get to this depth

and take advantage of the nutrients.

The school of mullets,

by using their geographical knowledge of the seabed,

to make their migration as efficient as possible.

(soft dramatic music)

It is the end of October.

In the brackish waters, close to the coast,

a school of young mullets.

After hatching in the open ocean,

they arrive at the coast,

having ridden the current for a month.

Further inland, these larger mullets are now a year old.

Their jumps, even at this age,

resemble the jumps of mature mullets

when dodging their enemies.

(soft gentle music)

They will spend a total of four years in the safety

of Florida's inlets before running the gauntlet,

embarking on their own epic journey

out to the open ocean to have young of their own.

These young mullets will have to overcome the threat

of sharks, trevally and tarpon,

(thunder clapping)

red tides and hurricanes,

and the perils of spawning in the depths of the open ocean,

just like their parents did.

(soft gentle music)

But safety in numbers will see them through.

(soft gentle music)

(soft adventurous music)

(synth music)