[Narrator] There is a place... on this blue planet... unlike any other... ...where ice retreated, water flowed in... and life exploded.

The first people here called it "The Dawnland"-- the place where the sun rose first.

This is the story of how we used the treasure of this place... ...founded nations... ...and devastated others.

Today, this sea within a sea is warming faster than 97% of the global ocean.

So, what happens here in the Gulf of Maine could happen everywhere, for jobs, for culture, for every living thing.

But there is a hidden strength here.

Fishers, scientists and the first people here believe answers are in the sea... ...a way forward... to strike a balance between people and nature.

It will happen here, where we face the test that will define us all... ...in this place, where the sun rises first.

[♪♪♪♪♪] [Narrator] It's a place that many people think they know, a proverbial vacation land, from the sandy beaches of Cape Cod to the rugged coast of Nova Scotia.

This stretch of the Atlantic Ocean is called the Gulf of Maine.

Drawn to its raw beauty, most have only looked at its surface... ...but hidden beneath lies an ecosystem unlike any other on Earth... ...home to more than 3,000 species.

That bounty has sustained millions of people for thousands of years, but now the Gulf of Maine is threatened.

[Brian Skerry] I'm really worried about the Gulf of Maine.

So many of the places I used to go just don't have what they used to have.

[Narrator] This is the story of the scientists racing to understand the threat... [Doug Rasher] Conserving ocean resources is really essential for conserving our quality of life, even on land, whether you live close to the ocean or not.

[Narrator] ...the fishers who feel its impact... We adapt to what can we catch, do it sustainably, and make a dollar at it.

I mean, at the end of the day, we're trying to feed our families, too.

[Narrator] ...and the creatures, some thriving, some struggling, like us, all trying to adapt to a warming planet.

It's also the story of a rare expedition to a remote and dangerous place, Cashes Ledge, a seamount that could hold secrets to the survival of the Gulf.

[♪♪♪♪♪] [Robbie Lamb] Cashes Ledge is a magical place.

When you go down there, there's no land around, so you have no frame of reference.

When you start to go down, about 10, 15 feet, you start to see the tips of the kelp waving back and forth in the surge, like it's beckoning you to come down and explore.

It's just the most magical thing in the world.

But Cashes Ledge, it's not someplace that everybody goes to.

It's 80 miles offshore.

It takes a lot of infrastructure, a lot of logistical planning.

It's hard to get there, and it's hard to get down.

[Narrator] Because it's 80 miles offshore, it remains relatively unspoiled.

It also boasts the largest kelp forest on the East Coast, and features every subtidal ocean habitat in the Gulf.

It's the perfect laboratory to study the future of this place.

[♪♪♪♪♪] It's been seven years since a scientific expedition has been to Cashes, and that same team is heading out today.

-Hey, can I hand this to you?

-Yup.

It's heavy.

[Narrator] Leading them is Jon Witman.

[Jon Witman] The first time I dove on Cashes Ledge was in 1977.

You know, I was in this huge kelp forest, it was absolutely stunningly beautiful, and that was it, I was hooked.

Hundreds of cod going by in an hour, you know, it's just spectacular.

[Narrator] Joining Jon on this trip is fish ecologist, Robbie Lamb.

[Lamb] We're mostly doing an ecological assessment.

You know, how does the ecosystem look compared to when we were there seven years ago?

Because Jon Witman has been working here since the '70s, we can actually look all the way back to that point in time, and see how kelp density, and fish abundance, and fish diversity has changed in that period of time.

[Narrator] Kelp is a barometer of ocean health, and the Gulf has already lost huge swaths of it, especially close to shore.

Doug Rasher has been witnessing kelp loss for almost a decade...

Very heavy.

[Doug Rasher] I study the ecology of kelp forests, and how they're changing as a result of climate change and fishing pressure.

We've seen a lot of rapid change along the coast just in the last five years, and so I would guess that we're going to see some similar sorts of changes out on the ledge as well, but we'll see.

[Narrator] Helping document these changes is National Geographic photographer Brian Skerry.

Always helpful to have a couple more days.

[Narrator] Since Brian and the rest of the team were last at Cashes in 2016, the Gulf has endured several extreme heat waves... [Skerry] Anywhere out of the way, and I'll straighten it out later.

[Skerry] I started diving on Cashes Ledge a little over a decade ago, and it was, to me, like going back in time.

I mean, it wasn't like even anything I'd seen in the last four decades.

I know what's happening along the coast.

I just can't imagine that Cashes isn't going to be damaged as well.

[Narrator] It will take the crew all night to go the 80 miles out to Cashes, which sits in the middle of the Gulf of Maine.

The Gulf is really a sea within a sea, semi enclosed by large underwater banks that rise from the sea floor more than a thousand feet.

Just outside is where two major currents meet, the cold Labrador from the Arctic, and the warm Gulf Stream from the Gulf of Mexico.

Until recently, the Labrador kept much of the warm water out, fertilizing the Gulf with a steady flow of cold water, loaded with oxygen and nutrients.

As it circulates across the Gulf, it hits Cashes Ledge, an underwater mountain range.

All that oxygen and nutrients are then forced up to the surface, where the sun triggers photosynthesis, and life explodes.

As the planet warms and glaciers melt, fresh water is making the Labrador less salty, weakening it, and allowing the warm Gulf Stream to push in... ...but what kind of impact is that warming having on places like Cashes Ledge?

Once on site, and with the weather cooperating, the team gets in and takes their first look.

43 feet, 100 feet of line.

[Skerry] I'm really excited about this Cashes Ledge trip, but I'm nervous about what we might find there.

This is, in my mind, our last hope... ...but I don't know what I'm going to find out there, so fingers crossed.

[Narrator] The crew hopes to find an intact ecosystem, with a bounty that was created by the Gulf's own unique formation, about 14,000 years ago.

Over millions of years, massive ice sheets repeatedly expanded and contracted across the northern hemisphere, as Earth alternately cooled and warmed.

At some point, so much of the planet's water was frozen that the ice covered the entire Gulf in sheets.

In some places, those sheets were more than a mile thick.

As it contracted, the ice served as a massive grinder, chewing the bedrock into a mix of gravel and sand, then rivers of glacial melt pushed it all hundreds of miles downstream, and out to sea, shaping the Gulf's unique floor.

The conditions helped marine life thrive.

Lobsters took to the rocky crevices.

Kelp and underwater plant life took hold, providing food and shelter for the Gulf's smaller creatures.

Thousands of species drifted in and flourished, from present-day Cape Cod to the northernmost corner of the Gulf of Maine, the Bay of Fundy.

This is one of the coldest parts of the Gulf.

That, combined with unparalleled tides, powers an ecosystem that benefits many of the animals that live and visit here.

Because of its unique funnel-like shape, the world's most extreme tides flow here.

In parts of the Bay of Fundy, between tides, the difference in water levels can reach over 50 feet.

Every day, there are two high tides and low tides.

When high tides flood in, they carry about 100 billion tonnes of cold, nutrient-rich water.

As the bay empties, miles of seafloor are exposed, revealing a feast.

The creatures here have come to rely on it... ...like these periwinkles.

These sea snails are after one thing... algae.

Carried in on the tide or grown here when the sun hits the water, periwinkle numbers suggest there is plenty to feed on as they graze the mudflats.

Marine snails have a structure in their mouths, kind of like our tongue, but theirs is covered by rows of teeth, which they use to rake in and eat algae.

There's competition out here for that algae, and it comes from below.

Little mud shrimp, the size of a grain of rice, spend much of the year in the mud... ...but the summertime is mating season, so they leave the safety of the mud, and seek out partners on the surface.

Their natural drive to reproduce can carry a high cost.

Higher up the food chain, hundreds of thousands of semipalmated sandpipers descend on the Bay of Fundy.

They're coming from the Arctic, some having flown more than a thousand miles, and they're hungry.

Weighing less than a chicken egg, they're here to bulk up for the next, longer leg of their journey, a 2,000-mile flight down to South America.

To do that, they'll have to double their weight, to around two ounces.

At this critical time of their lives, only two things matter, feeding and resting.

Feed during low tide, rest during high tide.

When waters recede, sandpipers follow the tideline, and work the flats for the next six hours.

It's estimated that one sandpiper can dig out 20,000 mud shrimp in a single day.

They want to stay as close to the food as possible, so when the water comes back in, they just bunch up on the beach, even if it means sharing it with 100,000 others.

Crowding at the beach until next mealtime doesn't seem to bother them, but maybe it should, because in the Gulf of Maine, when you come for the food, at some point, you may become it.

High tide serves up an all-you-can-eat buffet.

The peregrine falcon is the fastest creature in the animal kingdom, clocking in at over 200 miles per hour when diving.

These sandpipers may not be as fast, but they are agile, and there is safety in numbers.

Like schools of fish, a flock of thousands can change direction in an instant, evading the straight-line flyer.

The sandpipers are no longer individuals, but a hypnotizing mass that contorts its shape and shade to confuse the hunter.

This group-think does come at a cost.

The energy spent here could be used for their 2,000-mile journey... ...but it's energy well-spent, because for the falcon, the fine print on this tide's buffet reads "all-you-can-catch."

Tired after a long feed, and fending off a falcon attack, the sandpipers bunch up once again to rest and wait for the next ebb tide.

The appeal of 100,000 bite-sized edibles isn't lost on another bird.

Consider the seagull, a shoreline regular, not really noticed, but always lurking in the background, watching and waiting for a beach snack.

Without the talons or blinding speed of a falcon, the seagull just blends, a predator hiding in plain sight.

Maybe the sandpipers don't take gulls seriously, maybe they're in a shrimp coma... ...but they allow a fatal distance.

The spellbinding synchronization of thousands of birds has no effect on the seagull.

Persistence pays, and the slowest sandpiper won't be leaving the Gulf of Maine.

At just one to two ounces each, it'll take a few sandpipers to satisfy the seagull.

As the tides retreat once again, the birds return to the flats to get their fill for their journey south.

Most of these seasonal tourists will get what they came for this year... ...but 300 miles south, will the crew at Cashes Ledge find an ecosystem as intact?

Fog has moved in, and all diving has come to a standstill.

This is why diving out here at Cashes is particularly dangerous.

Here, it's kind of like being on the moon, or on Mars.

It kind of all looks the same.

It's kelp everywhere you look.

That up and down mooring line is your lifeline because if something happens, you've got to get back to that line, and if you can't find it, you have to punch out, and just come up to the surface.

You're out in the thick fog.

This boat may not be able to find you.

So you have to be prepared for what Cashes can throw at you, and knowing that we may only have another day or two max means that we're trying to ramp it up and get it all done.

[Narrator] The fog delay means they've lost four dives worth of potential data, but finally, it clears enough to get back in the water.

[♪♪♪♪♪] Kelp forests are critical for keeping everything down here alive.

They're connected to the sea floor by an ingenious feature called a "holdfast".

Once these tendrils attach, they grow upward towards the sun, harnessing its energy in the same way plants and trees do.

Just like forests on land, kelp forests provide food, produce oxygen, and filter water, everything their inhabitants need to survive.

They also provide shelter from predators and rough seas.

Take this critter.

Not only do these folds of kelp buffer strong currents down here, a six-inch blade of kelp can shield a three-inch Jonah crab from a hungry lobster, and this juvenile codfish gets a covered highway to adulthood.

Without kelp, we lose pretty much everything down here... ...and that's what we are starting to see closer to shore.

When Doug Rasher began studying coastal waters almost ten years ago, he could already see kelp forests struggling-- very patchy, covered with parasites, or shot through with holes.

500 years of intense human development, fishing, and pollution had taken its toll.

Over the past five years, rapid warming has further reduced what remained.

Warmer waters opened the door to an invasive algae species which outcompetes the weakened kelp.

As the coastal kelp disappears, so goes the habitat that supports so much life.

We have healthy kelp forests in the north, collapsed kelp forests in the south, so where Cashes Ledge lies relative to the story along the coast is something I'm really excited to see.

[Narrator] If the kelp forest is strong, then this system has a chance against the changes the Gulf faces.

[♪♪♪♪♪] In 1987, Jon Witman created a data baseline here by laying down one-metre-square grids on the seafloor, and collecting the kelp and everything else in them.

Today, the team is doing the same thing, at the same coordinates, establishing a new reference point.

This expedition will add more information to Cashes' health chart, helping scientists understand what changes have taken place.

[exhaling] Well, it's as advertised.

It's amazing down there.

The things that stand out here relative to the coast is just the sheer amount of kelp in the forest, how dense it is, how tall the plants are, and the sheer amount of fish here-- and the size of those fish, it's just unbelievable.

[Jon Witman] It's another metre-square scraping of kelp for biomass and density.

It gives us a lot of information.

[Narrator] Each bag is filled with answers to one big question-- how is Cashes Ledge doing?

By calculating length and weight of kelp blades, as well as density in each square metre, they get a snapshot of Cashes' health right now.

We're looking at a kelp forest in one square yard.

It's actually one metre about this big.

These are our trees, I mean, look how many are in a small patch of underwater habitat.

This is a long single blade.

That's Saccharina longicruris, and then we've got the multi-bladed one that looks like fingers.

This is called "finger kelp".

This is Saccharina digitata.

Longicruris, because it's so much longer.

This is kind of the redwood of the forest.

[Rasher] It historically, was really important ecologically.

It was a big part of our coastal kelp forest in the past.

Not so much today.

That's why it's so heartening to see that there's a lot of it here at Cashes Ledge.

We need data.

We need hard numbers to quantify the state of the kelp forest, and the abundance of animals that live there, and that's what we're doing, whether it's algae that lives on the seafloor like kelp, or fish that use the kelp forest as habitat.

They've shown in Norway that juvenile cod actually grow up in kelp forests, so they're very important nursery habitats.

283, Longicruris.

Its mass is 161.

It's all fun.

You know, diving is great and all, but actually seeing the numbers come in, and understanding what kind of change we're seeing, or lack thereof, is equally as exciting, because we're all driven by these questions.

96.

[Narrator] At first glance, the news is encouraging... ...and as if on cue, one of the Gulf's iconic predators shows up... ...a humpback whale.

[Witman] There it is.

[Rasher] Oh, it's getting closer.

-Humpback.

Nice.

-Nice.

It's that kind of a day.

For sure.

There it is!

Do you see it, Steve?

[Narrator] The relative health they're seeing at Cashes is in large part due to something that feeds and drives the entire Gulf, its unique currents.

When water from the Labrador current enters, the Gulf's shape forces it into the Bay of Fundy first and deflects everything else west.

As the water circulates in a counter-clockwise pattern, 60 rivers' worth of fresh water and their nutrients pour into this bathtub, adding the final element to the Gulf's unique blend... ...one that created a dynamic ecosystem that supported more than 3,000 species.

It allowed those species to thrive in huge numbers, leading the global fisheries that have fed millions of people for hundreds of years, but today, warming waters are threatening many of these species, and the jobs that depend on them.

Like most creatures here, sea scallops make their living off the Gulf's currents and tides.

The tide is essentially a conveyor belt of plankton and algae, so eating can be as simple as opening wide.

All scallops propel themselves by ejecting water out of valves next to their hinge, and they do it by pumping their one single muscle.

It's that muscle that ends up on the dinner table... ...and that's what David Tarr is after.

He spends every winter diving for scallops.

In this part of Maine, he can only harvest by hand for a maximum of 70 days per year.

[David Tarr] No visibility.

[Narrator] A massive storm two days ago is making things difficult.

I can see about two or three feet down there.

We had our third pretty major southeast storm here in the last month or so.

The peak winds were probably, oh, 70, 80 miles an hour, a lot of rain with it right at high tide.

So the problem with that is not only are we dealing with the big seas churning up the bottom, we're dealing with the runoff running off the land, and then... and it takes days for it to settle down.

So that's just kind of screwed up today.

So I gotta rethink things here a little bit.

That's about 40 feet.

I'm gonna have you just set me up on a little shoal right on that rockpile.

[Marshall Lebel] Yep.

So we do it all again.

Take two.

[Lebel] That tank's no good to put back on?

[Tarr] Yeah, I got about half a tank, so no more than two bags.

[Lebel] Yep.

I probably was about that tall.

He could put me up on the bulkhead.

I think... How old was I?

Five, six?

[Tarr] His dad worked with me for years.

So Marshall has been around this boat and me for most of his life.

[chuckling] [Lebel] It's work, but I love it.

He keeps an eye on me.

I mean, I could... You know, I could die out here, so I want someone I can trust.

Yeah, I'm all set.

All right.

We'll put him on a spot.

About right here?

[Tarr] I want to just be in over that bank, right over this way.

Right on a bed of scallops, okay?

[Lebel] I'll try!

[chuckling] -50 feet?

-Yep.

Good luck!

[Narrator] David's been doing this for the past 40 years, but there's always risk when diving alone in 40-degree water.

[Lebel] I usually just keep an eye on him, watch his bubbles, see what he's doing.

Like, usually, I'll watch his bubbles.

If he's moving like that, he's looking, but if he's usually sitting in one spot, he's got something.

Pretty easy, just sit here and watch.

[chuckling] This year's just been kinda, between the weather and the tide and everything, it's been kinda hard to get a good day where you actually can find scallops.

He's moved probably almost 200 yards now.

Usually, when he's moving that much, he's looking still.

Maybe he's finding a patch here and there, but I like it when his bubbles aren't moving.

That usually is a good sign, means he landed right on 'em.

[Narrator] But he's not on 'em, and he needs to move to a different spot.

[Lebel] Ah, he's coming up.

Especially that long, that was almost 20 minutes he was down there.

Ah, that's not good.

[Narrator] They're two dives in, without much to show for it.

[Tarr] The silt that I make combined with the silt that is already there.

Part of what happens to me down here is I'm really limited.

I can only work into the tide because I need what silt I'm stirring up behind me, and it limits where I can go, even if I see something, and I think I want to go with the tide, say, I just, I can't.

I'm not giving up yet.

I've moved a little bit.

We're going to try another spot here.

If I don't find something on this tank, I may not finish it.

If I don't find something pretty soon, it's just going to be late in the day when I get done.

It's my, uh, 57th birthday today, and I got a little celebration planned.

I'm not gonna be out here that much longer.

[chuckling] 57, I forgot!

57 today.

I forgot about that.

[Narrator] Fishing regulations this time of year allow David to dive three days a week.

With storms on either side, today's his only chance.

[Lebel] Try right there!

[Narrator] This is grinding work.

It's five to eight dives down to 30-60 feet over a few short hours.

It takes a toll on the body.

David spends the other half of his week in the boat, dragging for scallops using a steel net, which comes with its own set of dangers.

He's one of thousands of people on these shores who pull their living from the Gulf.

Whether it's shellfish, groundfish, or aquaculture, the strength of this ecosystem has a direct impact on whether these folks can make a living, and David has witnessed the system changing.

[Tarr] We've got these Sea Squirts here now that are causing a lot of grief for some of these bottom dwellers like scallops.

I don't know where they come from, but we've had 'em, at least on the mid-coast, quite a lot in the last five years.

[Narrator] Sea Squirts are a kind of invertebrate, invasive in the Gulf.

Like scallops, they're filter feeders, but their babies become adults and reproduce themselves in just a matter of months, easily outcompeting scallops for food, oxygen, and real estate.

[Tarr] They can be six or eight inches deep on the bottom, and they pretty much smother out whatever else is there.

They're filter feeders just like those scallops are, so they're eating the same food the scallop wants.

Just makes it hard for them to get by.

[Narrator] For now, Sea Squirts have not had a huge impact on scallop numbers where David dives, but that could change, because they thrive in warmer waters.

[Tarr] Somehow, those things got introduced to our waters, probably from the ballast from a ship or something, and so we're stuck with them.

It's like Zebra mussels in the Great Lakes, same idea.

They can outcompete the natural population sometimes, so we're constantly adapting to what's going on in this environment, just trying to fit in with all of that, not wipe something out, but being able to make a living at it at the same time.

[Narrator] David's found a decent spot, and even filled a bag.

He hasn't moved a lot.

Okay, he's gonna need a bag.

All right, we'll clip a bag on there.

Yeah, just watch.

See where his bubbles are?

There it is.

[laughing] [Narrator] Fishing's a lot more fun when you're catching.

[Lebel] That's a full bag.

It'd make me feel really good if he gets another bag.

[Narrator] Marshall can finally get to the other part of his job.

I usually just pick my head up, look, see what he's doing once in a while, and then I just shell a bunch.

[Narrator] Cleanly shelling a scallop is a knife skill you learn on the job.

[Lebel] I've shelled a lot.

Add it out, I bet I'm close to 10-, 12,000 pounds somewhere in that range, I've shelled my whole life.

Take your knife, pinch just like that.

I got another bag, sweet.

[Narrator] These are called "diver" or "dayboat scallops".

Shelling them immediately keeps them from freezing on cold winter days, and when you're on a good spot, it's all about speed because the scallops keep coming.

All right, I need to get that bag.

[♪♪♪♪♪] [Lebel] Is that a wrap?

[laughing] [Tarr] Yeah.

[Lebel] Figured.

That's an orange scallop.

To me, they're the sweeter ones to eat.

If I'm gonna eat one raw, that's probably the best one to have is the orange ones.

That's good.

That's really good.

If I'm going to have one from my dinner, I prefer one about that size and not the big one.

But the best way to eat 'em is just like that.

That's perfect.

[Narrator] After six dives, they've harvested and shelled about 700 scallops, less than half of their total allowable catch.

[Lebel] Washing the scallops out, so they can be ready to be sold on the market.

[Tarr] How'd we make out?

[Lebel chuckling] Okay, I guess.

[Tarr] Around five gallons.

Quite a difference in size there.

This stuff's worth a little more money.

That stuff is still plenty good to eat, delicious.

I'd say $800.

You take the expenses out of that.

Marshall gets a share of it, and I'll still have some money in my pocket.

A little dinner, a little bit of scallops for my birthday dinner tonight.

So I know where they came from.

Pretty sure they're fresh.

It is an honest day's work, and a hard day's work, but it's enjoyable.

[Narrator] For David, it's a decent haul after a slow start, but just 40 years ago, annual scallop landings in Maine could be up to three times what they are today.

Back then, unregulated fishing brought in so much from the Gulf that it seemed to feed the entire world, and one of the main draws was Atlantic cod.

[Lamb] There are stories that you could lower a basket weighted down with some rocks, and pull it up wriggling with cod.

Now we've got to work quite a bit harder to catch them.

[chuckling] [Narrator] That's because Atlantic cod has been all but fished out over the centuries.

Long gone are the cod days, where fish were routinely coming up five feet long, or weighing more than 200 pounds.

Today, even places like Cashes sustain only a remnant population of the once-mighty cod.

Though far fewer in number, cod is still a top predator here, so knowing what it's eating tells us what the food chain below it looks like.

[Lamb] Part of the science that we're doing is looking at their gut contents, trying to figure out what they're eating out here.

[Narrator] That will help them better understand the health of the entire system.

But they're not catching any cod, only pollock.

[Lamb] Another nice, beautiful Atlantic pollock.

That's the biggest one we've caught so far.

[Narrator] Though cod numbers have crashed, pollock still seem abundant.

Sampling more predator species provides a more complete picture.

[Lamb] I feel like we could be pulling these up all day and never get a cod.

[Rasher] Yeah, well, I think we should stop, but we have an N) of 3 now of large pollock, which I think is good.

[Narrator] Since the cod aren't coming to them, they have to go to the cod.

[♪♪♪♪♪] [Lamb] We have hand spears, which we use underwater.

That way, you can see the fish that you're trying to catch.

The problem is the cod are a bit flighty.

They don't let us get that close.

So it's a real challenge, really hunting underwater.

[Narrator] With cod finally in the bag, their scientific investigation continues on deck.

[Lamb] So this is what we came out here to find.

This is cod, Gadus Morhua.

This is the fish that was the mainstay of New England's economy for centuries, and in the late '80s and '90s, it was completely overfished to the point of collapse of the fishery.

It's got a pretty healthy population out here at Cashes Ledge, and we think that it has the potential to even repopulate other areas in the Gulf of Maine if we allow them that space to grow and reproduce.

[Narrator] It's hard to believe that just 60 years ago, fisherman were pulling in cod weighing 50 to 60 pounds on the regular.

Just a little more than the one- to two-pounders Robbie and Doug caught.

This is what 500 years of relentless fishing can lead to.

[Lamb] So fork length is 30.5.

Total length is 31.

Weight is 760 grams.

[Narrator] After weighing and measuring their samples, the team takes an even closer look.

[Rasher] And now we're going to be doing stomach contents.

Did you get a bag ready?

[Lamb] So this is the stomach of the cod.

We're going to find out what's inside of it.

Look at that.

Crab.

-No way.

-Yeah.

Oh, my gosh.

So that's either a Jonah or a rock crab -about halfway digested.

-Wow.

We don't really see many crabs underwater, and the ones that we do see are hiding in dead mussel shells.

So it's clear that they're under extremely high predation pressure because of these abundant cod out here at Cashes Ledge.

That's one of the really obvious differences between the coast and out here.

On the coast, we see cancer crabs, and lobsters, and urchins roaming around freely on the bottom through the kelp forest, and they're quite large, and they're quite abundant, with the exception of urchins, and that to me suggests that there isn't a whole lot of fear of predation in the coastal system, whereas here we see, you know, quite the opposite.

Very few individuals walking around, and the crabs that we do find are quite small, so very different world out here.

[Narrator] Healthy ecosystems have all of their pieces.

At Cashes, it's all about predators.

Cod might be more familiar to us as fish and chips, but to baby crab, lobster, and smaller fish, they are a ferocious predator.

Without them, the system unravels.

Imagine the teeming savannah of Africa without lions, or Yellowstone without wolves.

When top predators like cod disappear, prey species lower on the food chain, like lobsters and crab, explode in number, changing the seascape and potentially allowing other species to move in.

[Rasher] So this piece of white muscle tissue from the Atlantic cod will tell us a story about what the cod has been feeding on over about the last six months, and so this allows us to have a deeper view into understanding the food web, and the predator-prey interactions that are happening out here over longer timescales than we can observe.

[Narrator] Even the limited presence of cod may be helping maintain balance in the system.

Amazingly, there's new science that lets us look well beyond the tissue sample.

You just take off your gloves, take out the tube, and then you just fill that... [Narrator] Shane Farrell's a marine chemical ecologist.

He studies DNA found in the water, and can find traces of every animal that moves through here.

[Shane Farrell] It may not look like much, but in one small litre of water, we can tell what animals and other organisms are there, from bacteria all the way to whales and sharks.

The power of environmental DNA.

[Narrator] One of the things Shane can see without a microscope disturbs him.

He suspects this is an invasive species of red turf algae.

It's already wreaking havoc on coastal kelp, and Shane has brought an invention to help him investigate how it causes trouble.

He calls it "BOMS", short for "benthic organic matter suckers."

These suckers allow him to sample the water right next to the red algae.

Shane thinks it might be exuding something that undermines kelp.

There's been this proliferation of this red algae, and it's taken over the bottom, and it's created these very lush, thick carpets.

Very, very detrimental.

We can only visually identify it really under a microscope, and what I'm looking for is a proliferation of cells around the main axis, but then at the tips, it becomes uniseriate, where there's only one cell, so it's very distinct.

This is, in fact, Dasysiphonia japonica.

Wow.

Even out here.

-So... -Yeah.

-Nowhere is safe.

-Nowhere is safe.

[Narrator] The reason this is so worrying is a survival tactic that this species brings.

[Farrell] I'm studying how these algae might have nasty internal chemicals that they exude into the water or onto their surfaces.

[Narrator] The red algae, in effect, are waging chemical warfare on the largest kelp forest on the East Coast of the United States.

When it comes to this newcomer, the kelp is just not equipped for this battle, and neither is the rest of the system.

[Farrell] We don't know much about its chemical makeup.

Other species that have entered the Gulf of Maine from the Pacific have shown that they are very chemically potent, and have compounds that make it not great to eat.

[Narrator] It's not part of the food chain, so nothing naturally removes it.

If Shane's hunch is right, it could be a one-two punch, but it'll be months before he gets back his data.

Meanwhile, in spite of the presence of an invasive like red turf algae, Cashes may still offer Brian the opportunity to showcase some of the Gulf's bright spots.

We're getting ready to go diving.

[Skerry] I've been working on this Gulf of Maine project for about four years now, and during that time, it's been very frustrating photographically, because so many of the subjects that I sort of naturally expected to be there just were not there, even simple things like invertebrates, and I was desperately hoping to find sea stars, you know, these beautiful, big, somewhat colorful sea stars, but over these last four years, I just wasn't seeing it.

[Narrator] After four long years, Brian's luck has finally turned.

[Skerry] You know, if I were to try to explain to somebody what the Gulf of Maine is, what it what it looks like, what it feels like, it would be this, it would be a single frame, a single photo that shows an explosion of life.

All of these animals, you know, fish and invertebrates, and kelp in the background, all these things in one place.

It's a Garden of Eden, and that, hopefully, will translate visually so that people understand.

[Narrator] To the naked eye, this scene is dreamy, but not for the mussels attached to the rock.

What you can't see is the sea star prying open the mussel's shell with its suctioned feet, and that's not the worst part.

The sea star then sends its stomach full of digestive enzymes out its own mouth to dissolve and eat its meal.

When you speed things up, it's a feeding frenzy.

Most of the seats at this diner are taken, and the sea stars on the move hunt for another spot.

[Skerry] I've been literally looking for a scene like this for three or four years since I've been working on this project, and haven't found anything close to this.

We did see this invasive red algae, which has done so much damage along the coastal kelp forest, and that gives me great concern for Cashes, but, you know, by the same token, I see that all the pieces are otherwise in place.

Cashes still remains largely healthy.

There are fish, there are invertebrates, there is healthy kelp.

I suppose anything can happen, but I'm certainly in the... in the corner of resilience.

[Narrator] That's supported by Shane's environmental DNA research.

It shows that Cashes does still have all its pieces, despite warming waters and harmful invasive species.

The water collected revealed the presence of 20 different fish species, from Atlantic cod to wolffish, to pollock to cunner.

Passing through are large schools of fish, such as Atlantic herring, and mammals like humpback whales.

[♪♪♪♪♪] Compared to an expedition in 1987, the kelp has retained its density, weight and length.

Cashes Ledge is still intact, its kelp forest resilient and relatively healthy.

For Jon Witman, who first started researching here some 40 years ago, the expedition has instilled a lot of hope.

[Witman] This is what I dreamed of.

The kelp forest looks pretty good.

We don't see these sorts of densities along the coast that indicate higher resilience.

It may actually serve as a climate refuge for intense warming effects in the coast.

If there is a place that should be protected in the Gulf of Maine, this is it.

[Narrator] Cashes' variety, its biodiversity, is the promising marker of its health, and the hope, not just here, but for the global oceans.

[Skerry] If you look at Earth from space, we very much live on a water world, and every other breath that we take comes from the sea.

This oasis that is still largely intact, it will pay dividends, it will pay you interest.

Those fish and those animals will thrive, and that will spill over so that the entire Gulf will benefit from it, and as a result we will, too.

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