(waves crashing) (water bubbling) (gentle music) (narrator): As winter descends on the Great Lakes of North America, lake whitefish return to the rocky reefs where they were born, to spawn under cover of darkness.

It's one of the largest freshwater fish migrations on our planet.

But it's rarely been witnessed by humans.

This female looks to be carefully choosing where to lay her eggs, pursued by a male eager to fertilize them.

(gentle music continues) It's a dance as old as the lakes themselves.

But every year, fewer fish are taking part.

Lake whitefish were once a key part of an incredible freshwater ecosystem that's nourished the people of the Great Lakes for millennia.

But now, life in these vast waters is struggling to survive in an environment utterly transformed by a tiny invasive species, the quagga mussel.

(Elgin): There's quadrillions of quagga mussels in the Great Lakes.

In a week's time, every gallon of water in Lake Michigan could've passed through a mussel.

(Weidel): Those mussels created a system that is really now much different than what we had seen before.

It's just incredible the clarity of the lake.

(Sweetnam): Clear water, from an aesthetic perspective, people kind of love that.

But from an ecosystem's perspective it really means nothing's there, nothing's growing.

The offshore waters have been described as a biological desert.

(Smith): It's my opinion that invasive quagga mussels have been the biggest change in the Great Lakes ecosystem, really going back since the glaciers.

(suspenseful music) (narrator): The Great Lakes of North America form the largest interconnected body of freshwater on Earth.

While images of the oceans fill our screens and live in our imaginations, the vast and magnificent freshwater world has long been hidden from our view.

Only now, using state-of-the-art, underwater drone technology, are we finally able to explore the wonders hidden beneath the surface of the Great Lakes.

(gentle music) (ice creaking) (water bubbling) (narrator): All winter, the lake whitefish eggs have been maturing, safely hidden in the cracks between rocks.

(wind gusting) As the early spring sun thaws the ice, and starts to warm the water, the hatch begins.

(intriguing music) At first, the larvae get nourishment from their egg sacs.

But that won't last long, and they soon head out in search of tiny aquatic animals called zooplankton.

(♪♪) Life is hard for a baby whitefish.

If they don't find food quickly, they will starve.

They must also contend with predators trying to eat them, and sometimes, the nets o scientists trying to save them.

(water bubbling) (clattering) - All right.

So, we got some larval fish in here.

(boat creaking) This is historically, really one of the most significant spawning shoals for whitefish in Lake Huron.

And it's one of our study sites, where we've seen declines in whitefish populations.

And what we're out here looking at is whether they, um.... are getting enough food to eat.

I was interested in studying whitefish because of their importance in the commercial fishery and their importance to Indigenous communities.

I think they have a pretty unique role in the lake.

(water bubbling) (narrator): Lake whitefish, also known as Atikameg, or Atikmeg, in many Indigenous communities, is a cold-water fish related to salmon and trout.

They can be found throughout northern North America.

In the Great Lakes, they can grow to more than 6 kilograms, or 13 pounds, and live for over 30 years.

(whimsical music) Their mouths have evolved for digging into the lakebed, to feed on animals living in the soft sediment.

They often move in schools, grazing the lake bottom.

This feeding behaviour, akin to bison grazing grasslands, stirs up nutrients and leaves impressions on the lake floor.

Whitefish probably migrated into the Great Lakes shortly after the lakes formed from melting glaciers, about 12,000 years ago.

Their numbers likely once approached 100 million, playing a crucial role in shaping the lakes' ecosystems.

But now, their future hangs in the balance.

(Dunlop): Just to give you a sense of how much the population has declined, in southern Georgian Bay, um, we used to catch between about 10 and 15 lake whitefish in an individual monitoring net, whereas now we rarely catch a single individual.

It's a pretty dire situation for lake whitefish right now.

(water bubbling) (narrator): Whitefish began declining in the lower Great Lakes in the 1990s.

Recent years have seen them disappear throughout Georgian Bay, Lake Huron and Lake Michigan, with devastating consequences.

(intriguing music) (narrator): Indigenous communities have been hit hard by the whitefish decline.

This is especially true for the Sault Tribe, who have fished for centuries in Northern Lakes Huron and Michigan.

Each summer, Sault Tribe scientists undertake the grim task of estimating fish populations in their territory.

- We're just trying to get a sense of fish populations, all the species, all the age groups.

In this entire survey, we've happened upon maybe 20 or 30 whitefish this summer.

That's not really enough for the tribal commercial fishery to exist on.

No one is currently fishing throughout this whole area, and that's unheard of.

(Reining): It's kind of disheartening to see what's left of this fishery after being part of it for 30 years.

It's actually been devastated.

(seagull squawking) (narrator): When fish populations collapse, overfishing is often to blame.

That doesn't seem to be the case here.

- We do have a long history in North America of over-harvesting.

But the Great Lakes are actually managed really pretty closely.

(boat rattling) We're, in almost every case, not harvesting anywhere near the number of fish that our best math says we could harvest safely, and yet the decline continues.

That tells us very clearly that it's not a harvest problem.

It's a recruitment problem.

Getting from being a baby to an adult, that's where the problem is.

And that's what makes it so challenging.

(water rushing) (narrator): Like, Erin Dunlop, Sault Tribe scientists are also trying to figure out if baby whitefish are getting enough to eat.

(Smith): What we're looking for is large-bodied zooplankton, the food that would really make these young fish grow well.

(dramatic music) (narrator): The zooplankton are so small that Sault Tribe staff use a microscope to count their tiny catch.

(Smith): If we go back to 1970s, we'd see something like 700 large-bodied zooplankton per litre of water.

What we're seeing here typically is one to three zooplankton per litre of water.

We know we're orders of magnitude lower than places where, where whitefish are doing great, and, and we're pretty confident that's part of the problem.

(water splashing) (narrator): Erin's work digs even deeper into why baby whitefish aren't growing up.

(Dunlop): We've got larval fish.

We actually take out their stomachs and have a look at what's inside.

(contemplative music) (narrator): By closely examining their tiny bodies, Erin's team has discovered what the larvae are eating, and how fast they're growing.

- So, what we're finding is that the growth rates of larval fish are now just 50% of what they were, um, historically.

This is our indicator that perhaps these larval fish are not getting enough food to eat.

(Smith): If we were here a hundred years ago, every inch or two, they would have found a zooplankton to eat.

Now, they're really literally swimming thousands of body lengths in order to get a single meal.

We believe it's directly due to invasive quagga mussels.

(gentle music) (narrator): Historically, whitefish would have hatched into what's known as the spring algae bloom.

As the strengthening sun penetrates into the water, it encourages the growth of algae, turning the water green.

Zooplankton then feed on the plentiful algae and multiply The result is an exponential increase in food for baby whitefish, right when they need it to survive.

But now, the algae are being consumed by mussels.

In some years, the spring bloom has been diminished.

In others, it has disappeared almost completely.

(gentle music) (Smith): Sault Tribe's creation story is based on a relationship with Atikameg.

The creator led the people here when they were hungry and explained that they'd have this relationship if they took care of it.

And the Anishinaabek took care of this relationship for thousands of years.

And now, the number of fishers is at an all-time low.

The number of fish harvested is at an all-time low.

And that relationship between the people and the fish is really in jeopardy.

We really are at a point where we have to do something.

Time to just talk about it is long gone.

It is time to do.

(suspenseful music) (narrator): The Indigenou communities of Northern Michigan are taking extraordinary measures to protect baby whitefish.

These eggs were collected last fall from one of the few spawning aggregations left in Northern Lake Huron.

After maturing for several months, they're starting to hatch.

(intriguing music) (♪♪) (Aikens): The driving theory behind their decline in the Great Lakes is the lack of food.

There's just not enough food out there when they emerge from their eggs to get them off to a good start.

And so, if we kind of help them through that stage here in hatcheries, maybe we can help them out.

(narrator): Instead of the dry pellets usually fed to fish in hatcheries, Rusty is using live brine shrimp.

The goal is to better prepare the whitefish for their future in the wild.

(Aikens):So, hopefully the brine shrimp will, will mimic nature more, right?

Because it's basically a zooplankton.

We're hoping it helps build that prey drive, and kind of teaches them how to get their own food when they do get in the wild.

(gulp) (gentle music) The reason you find Sault Tribe in northern Michigan is because this is where the whitefish were.

There's members out there making a living off of fishing, and it's really been tough for them.

And also, people like having it on their tables.

So, the decline is a big hurt to our community.

We can do what we can, hopefully we can make some difference here in the future.

(contemplative music) (seagulls squawking) (narrator): To understand how things got so bad that the Sault Tribe is trying to raise whitefish in hatcheries, we must venture out into the vast offshore waters of the lakes.

(pully squeaking) (water rippling) It's here, far beneath the surface, where quagga mussels have truly taken over.

(pully squeaking) (eerie music) (rattling) (splashing) (Deck Hand): Surface!

(rattling) - We got 'em.

(narrator): Few have studied this invasion more deeply than Research Ecologist, Ashley Elgin.

(woman): Uh, 20... (Elgin): Today we are continuing our long-term monitoring of dreissenid mussels in Lake Michigan.

We are tracking dreissenid mussels because we have a survey that we've been conducting since before they even arrived.

(eerie music) (narrator): The quagga mussel is the slightly larger, and much tougher cousin of the infamous zebra mussel.

They're both native to Eastern Europe and are so closely related that scientists often refer to them together as dreissenid mussels.

(birds chirping) Both species hitched a ride to the Great Lakes in the ballast water of cargo vessels in the late 1980s.

(Deck Hand): Surface.

(clattering) (Elgin): The invasion was first dominated by zebra mussels being seen in the nearshore.

But starting in the 2000s, the quagga mussels were more prominent, and their numbers really exploded.

And they expanded into deeper waters and colonised all the lower Great Lakes almost completely.

(clatter) (narrator): A dive into the offshore waters of much of the Great Lakes is now a descent into a world dominated by quagga mussels.

Out here, most hard surfaces are encrusted in mussels.

(eerie music) (♪♪) But what's truly extraordinary about quagga mussels, is their ability to continue spreading from rock to the mud and sand that make up the majority of the lake bottom.

(Elgin): Quagga mussels, they do like to have a surface to attach to, but they are capable of colonizing soft sediments, and that's what's unique about them, as opposed to zebra mussels.

(eerie music continues) (narrator): Starting at somewhere around 50 metres or 160 feet, it's often difficult to see the lake bottom for the mussels.

(Elgin): What we see in those solid carpets, it's about 20,000 mussels you can get in a square metre.

In areas where they are attached and piled on top of each other, you can get 30,000 to 40,000 in a square metre.

Each mussel, as a conservative estimate, could filter one litre per day.

So, if you scale that up, it means that in Lakes Michigan and Huron, all the water could pass through and be filtered by a mussel in a week or two.

(narrator): Anything on the lake bottom out here, is a magnet for quagga mussels.

This is especially true of shipwrecks, like this one, which was discovered during the making of this documentary in Lake Huron.

Resting in over 80 metres, or 260 feet of water, it's believed to be the steamship Africa.

Built in 1873, it was lost in a storm in 1895, along with all 11 crew members.

(♪♪) But many of its secrets remain hidden under millions of mussels.

(drone squeaking) (Martin): There is just so many quagga mussels on this shipwreck.

Like there's hardly an inch of bare wood that I can even see.

All of these shipwrecks now are completely covered in quagga mussels.

So, I no longer see that bare deck, or the dishes on board, or the machinery.

Instead, I see a lump that I know, "Oh, hey, this is part of this machinery equipment," that I now recognize as a shape, not as the actual piece of equipment.

(muffled) Yeah, so, keep going... (narrator): Kayla Martin volunteers for an organization dedicated to the preservation of Ontario's shipwrecks.

(Martin): So, you're heading... (narrator): She's helping us document this newly discovered wreck, before it's destroyed by quagga mussels.

- Circle the wreck.

Quagga mussels have byssal threads, which burrow into the wood.

And that's how they hold on, and so they don't fly away, off into the current.

But these threads are slowly destroying the wood as they burrow into it.

Also, the weight of the quagga mussels, as they slowly build on top of each other, will cause fragile pieces of wood to collapse.

So, overall, quagga mussels are very damaging to shipwrecks.

Yeah, there you go, that's looking good.

These shipwrecks are a critical part of our heritage, which is why we're in a quest right now to document all these shipwrecks before quagga mussels destroy them.

(gentle music) (narrator): In recent years, quagga mussels have spread to even the deepest, darkest, coldest depths of the Great Lakes.

(Elgin): In our annual survey a few years ago, we added more deep sites, so that we can say, "Well, we'll, we'll, get these sites now, and then we can see when muscles reach these sites."

We get the before.

They were already at those sites, at the deepest sites.

So, confirmed they are at 200 metre depth.

(water whooshing) (narrator): The mussels now cover the bottom of most of the lower Great Lakes.

Only Lake Superior remains mostly untouched, likely because the lake lacks the minerals the mussels need to build their shells.

(Elgin): There's all this energy that was up in the water column that's now trapped in the mussels.

Then it starts with less food for zooplankton, and then the small fish that eat those zooplankton, and the larger fish that eat the smaller fish.

And it resonates throughout the food chain.

(Deck Hand): Surface.

(indistinct chatter) (Elgin): It's hard to understand how something that is this small, and even much smaller than that, rewired the food web.

- It's going by the bank?

- Yeah.

(narrator): Back on shore, the Sault Tribes effort to help baby whitefish survive the quagga mussel invasion, is well underway.

After a month in the hatchery, the larvae are moving to a new home.

(Aikens): The fish look good.

So, this morning we're gonna stock the ponds with our larval whitefish.

It's always exciting.

Uh, you know, it's another step in the, in the process, and to get to here is, is a minor victory in itself.

(man): Freakin' out.

(Aikens laughing) (water rippling) (narrator): The whitefish babies will spend several months here, hopefully growing strong enoug to survive in the Great Lakes.

(Smith): Our pond has lots of zooplankton, unlike the Great Lakes, and these fish grow great there all on their own.

We basically ignore them, we get to the late fall, and we can catch them and release them once they've gotten past that reliance on zooplankton.

(dramatic music) (narrator): After spending the summer in the pond, it's time to see if the whitefish are ready for the Great Lakes.

(man): Great.

- Big, beautiful fish, really nice.

- A nice size.

They look healthy.

They look good.

♪ (upbeat music) (Jason): Those fish hit the water and immediately started feeding, hiding.

These were fish that in my mind had a bona fide shot of going out and becoming adult fish and doing fishy stuff.

(narrator): With a little help, these 2,500 young whitefish now have a fighting chance of growing up.

But, for a species that once numbered about 100 million, this is a drop in the bucket, unless something changes in the Great Lakes.

- When I am talking about this story, I usually use whitefish because it's the easiest to show the link.

Quagga mussels reduce the food, decrease whitefish abundance.

It's super clear.

But if the invasion continues to increase, I'm not at all confident that there will be food for walleye, smallmouth bass, and many of the other fishes in the Great Lakes.

We're going to have to think far outside of the norm if we're going to save this relationship between the people and the fish.

(narrator): For a long time, doing anything to control the mussels was considered impossible.

But then, as often happens in science, somebody made an accidental discovery.

♪ (intriguing music) (SCUBA bubbles) In this case, that person was one of the Great Lakes' most respected underwater ecologists, Professor Harvey Bootsma.

The happy accident occurred at Harvey's underwater laboratory at Sleeping Bear Dunes National Lakeshore, in Northern Lake Michigan.

Harvey uses this laboratory to study what quagga mussels are doing to life on the lake bottom, which scientists call the Benthic Zone.

(Harvey): In 2016, we started a project here to remove mussels from a relatively small area, just 40 square metres.

And that was simply going down with paint scrapers and scraping mussels off of all the rock surfaces.

The goal at that time wasn't to see if we could remove mussels, it was just to see what happens to the benthic environment when we do remove mussels.

We thought within a year or so, they would start recolonizing the rocks.

And now it's six years later and those rocks still haven't been recolonized.

And that was kind of curious.

What we think is happening is that those rocks can't be colonised because round gobies, another invasive species, they like to eat on mussels, and they especially like to feed on small mussels because they can crush the shells of small mussels.

(narrator): Like zebra and quagga mussels, the round goby came to the Great Lakes from Eastern Europe in the ballast water of cargo ships.

But unlike fish native to the Great Lakes, gobies evolved to see these mussels as food.

(Harvey): In the early 2000s, we just saw this round goby population explode in the nearshore zone.

So, we think that they're preventing those rocks from being recolonized because they eat the small mussels before they get the chance to grow.

So, that got us and other people interested in the secondary question of, well, can we actually remove mussels and at what scale could we do it?

Could we do it at a scale that would have any impact at all on the ecosystem?

- Down to one!

- Down to one!

(narrator): Harvey's unexpected discovery jump started a movement to figure out if controlling invasive mussels might be possible after all.

- We might have two lines down there when we get on station.

When we find a spot, we'll throw out a marker float.

(narrator): Leading the way is Harvey himself.

Today, he's headed out from his home base, at the University of Wisconsin-Milwaukee, into central Lake Michigan.

Supported by his team of graduate students, Harvey is setting up the first mussel control experiment ever attempted in deeper, offshore waters, where the vast majority of mussels are found.

- What we'd like to do today is see if some of the experiments we've been doing in shallow water can be repeated in deeper water.

So, we're going to do a dive here in a depth of 20 to 25 metres, and we're going to try putting down a tarp, which we've done in shallower water, to see if we can kill mussels by putting a tarp over them for a couple of weeks.

(narrator): The tarp has the potential to kill mussels by depriving them of food and oxygen.

(Harvey): As far as I know, this is the deepest anybody's gone to try to get rid of mussels.

It's always a little more challenging when you're doing a deep dive.

It's a little more dangerous, so we've got to be cautious.

(narrator): As they descend, the water becomes unexpectedly silty, greatly reducing visibility.

It only gets worse on the bottom.

(Harvey): Sometimes it's a challenge working at this depth because as soon as we start putting stuff on the bottom, there's a silt and sediment layer and that gets kicked up.

So, it can be hard to see what you're doing.

(narrator): Despite the near-zero visibility, they manage to spread the tarp out and weigh it with chains, with just enough air left to get back to the surface.

- That was crazy.

It's usually pretty clear down there.

I think it's because it's so warm all the way down, it's getting all stirred up.

Couldn't see diddly squat today.

And gobies were having a great time down there, but we weren't.

(narrator): One month later, Harvey returns to check on the results of the experiment.

- Well, we hope to find dead mussels.

So, we'll lift the tarp.

I'm expecting we'll see dead mussels in the tarp.

So we'll see.

- Theyre both in.

(narrator): Once again, things don't go as planned for Harvey and his dive mate.

A strong current pushes them away from the experiment site, and they use up much of their air just trying to locate the tarp.

As soon as they start removing the tarp, the visibility is again reduced to almost nothing.

But as the silt clears a few minutes later, they're able to see the results.

Deprived of food and oxygen, the mussels did not survive.

(Harvey): It looks like all the mussels are completely dead under the tarp.

So, that's good, that's what we were hoping for.

Now we've got an area where we've wiped out all the mussels, and now we can track that over time to see whether the mussels come back.

- Got 'er?

- Yup.

(narrator): Despite this success, Harvey is not optimistic about using tarps for deepwater mussel removal.

- That was a mess.

- Yeah.

- As soon as we started moving stuff, couldn't see a thing.

- The planning went a lot smoother than the execution.

- Well we wasted half of our air looking for the thing.

Between the turbidity and running out of air.

- Not a good combination.

- Yeah.

When I was down there today, I thought we've got to come up with some other strategies because there's a lot more risks diving at this depth to do that kind of work than there are at 30 feet.

It's just a lot more challenging.

We can't stay down there very long.

So, we've got to come up with some other method.

(narrator): As Harvey considers a safer way to kill mussels in the heavily infested waters of Lake Michigan, his good friend Brenda is hard at work in Lake Superior.

She's trying to prevent mussels from invading the Apostle Islands, one of the lake whitefish' last refuges in the Great Lakes.

And she's doing it by hand.

(Brenda): We are using manual removal here in Lake Superior to try to get ahead of emerging infestations of these zebra mussels, because in Lake Superior it's a little bit of a slower burn.

We didn't see the explosion that we've seen in the other Great Lakes, but we're starting to see, almost little spot fires here and there around the lake.

And so we're trying to kind of snuff them out where we find them.

(narrator): The waters of Lake Superior were long believed to hold too little calcium for zebra and quagga mussels to build their shells.

But recent years have seen large numbers of zebra mussels colonize the Thunder Bay and Duluth harbours, possibly due to warming waters.

From Duluth, zebra mussel larvae may be drifting with the current for close to 100 kilometres, or 60 miles, before landing on the fish spawning shoals near the Apostle Islands.

The nutrient-rich waters around the Apostle Islands are one of the most important fish nursery areas in the Great Lakes, and a crucial refuge for lake whitefish, whose numbers have plummeted elsewhere.

A large-scale mussel invasion here would be devastating.

(Brenda): Hopefully we see densities that are no higher than we saw last year.

(indistinct chatter) - Wow.

- Add the two, which are in our bag.

(Brenda): Looks like we've got 14 live, and 15 dead.

- It's a big haul.

- This is more than everywhere else in the Apostles you dove last year combined, probably.

- Yeah, you're right.

(Toben) We're hoping that removal efforts will continue to keep these reefs as clear as possible.

But, if this is going to be the case, where we see year after year of establishment, then we might be fighting an uphill battle.

(narrator): In every other Great Lake, the coming of zebra mussels preceded a much more damaging invasion of quagga mussels.

Let's hope that's not the case for Lake Superior.

- So, we'll swivel it around.

(narrator): Back in Lake Michigan, Harvey is about to test a new mussel clearing device he's spent the last seven months creating.

(Harvey): So, this is our Benthic mussel masher, which we're hoping will mash mussels.

- You can see, we've made this mostly out of uh, scrap we had out in the backyard behind our building.

Had some old train axles back there, they're about 400 pounds apiece, and then the steel plate itself is about 200 pounds.

So, all together, we're about a thousand pounds here.

So, we'll put it down there, drag it across the bottom, and see how it works.

(tense music) (dramatic music) (♪♪) It definitely did something down there.

Yeah.

We may need to do some tweaking of it, but I think, once we've done that, we may have something that we can ultimately use to try to clear mussels from a very large area.

(seagulls crying) - It would be very gratifying if we could find ways to remove mussels from a large enough area that we did see some real positive responses in the fish community.

So, let's poke it and see what happens.

(Jason Smith): Until recently, most folks have kind of thought that mussel control was not doable.

I disagree with that.

I don't believe yet that folks in the Great Lakes region have the will to deal with them.

But if folks here had that willingness, I will always believe that it is a challenge we could deal with.

But, I'm not confident that's gonna happen in the next 20 years while I'm still working on these problems.

So in the short term, could we start to restore some rivers and get fish spawning in rivers again, like they did for 10,000 years.

(narrator): Long ago, there were two groups of lake whitefish in the Great Lakes.

Those that spawned in lakes, and those that spawned in rivers.

The river fish all but vanished more than a century ago after logging practices destroyed their spawning habitat.

(intriguing music) But the Sault Tribe and the Little Traverse Bay Band of Odawa are out to change that.

These Lake Huron whitefish were caught on their spawning grounds just hours ago.

They gently remove the eggs from the females and the sperm from the males.

The females don't survive the procedure, becoming food for the community, but the males are released to spawn again.

Once the eggs and sperm are combined, they have only 24 hours to put them in the river before they become too delicate to handle.

(soft music) 130,000 fertilised eggs are carefully placed into special incubators, and prepared for travel to their new home.

Their destination is the Carp River, which flows into Lake Huron not far from where the parents of the whitefish eggs were caught.

- We have our egg incubators loaded.

So we finally are going to put eggs back in the Carp River today.

(narrator): The Indigenous communities not alone in this bold experiment.

One of the world's largest conservation organizations, The Nature Conservancy, is a key partner.

Senior scientist, Matt Herbert, is the group's lead on the project.

(Matt Herbert): We've been talking about this for several years.

It also was kind of a dream, so to be - to be starting in on this, and be doing it, is very exciting.

So today is the very first attempt to try to help rebuild whitefish populations.

- And then you come down, yup.

- All right, here we go.

(narrator): With luck, the eggs will be safe until spring, and the larvae will hatch and escape through tiny holes in the incubators.

Free of quagga mussels and naturally high in nutrients, the river may be a far better place for larval whitefish to begin their lives than the open waters of Lake Huron.

- I'll be upstream of you Ron, but I won't fall on you.

(Jason Smith): It is really challenging in the Great Lakes themselves to make a living if you're a brand new, hatched fish.

There's almost no food.

But there's still food in these tributaries, and there's food right at the mouth of these tributaries in the lakes.

(narrator): The ultimate goal is for the baby fish to imprint on the river, an return here to spawn as adults, creating a new, self-sustaining population.

While this may seem like a long shot, Jason and Matt are actually following nature's lead, because whitefish are already returning to the tributaries of Green Bay, Wisconsin, all on their own.

(soft music) (Matt Herbert): About 20 years ago, whitefish were found to be running into one river, the Menominee River in Wisconsin.

And that was the first time that people had really known about whitefish running in rivers for around a hundred years.

And those populations have been increasing.

(narrator): No one knows precisely why lake whitefish returned to the Menominee River, but it's recently undergone a major environmental cleanup.

The return of the whitefish has benefited the whole river ecosystem, including these endangered lake sturgeon, who are vacuuming up eggs laid by spawning whitefish.

Lake whitefish have since spread to other nearby tributaries, helping make southern Green Bay the only part of the Great Lakes south of Lake Superior, where whitefish are truly thriving.

This is despite the area being heavily infested with quagga mussels.

(Matt Herbert): There's a huge drop-off right here.

We see the one place in the Great Lakes where whitefish are doing great, and we're just going to try to help jumpstart that exact same thing over here.

This one right here.

All right, hold 'er there for just a second.

This is something that is scale-up-able, to the size where we could really make a difference throughout the Great Lakes.

(bright music) (narrator): There is one final reason for hope for a Great Lakes ecosystem transformed by the quagga mussel.

Today, we're on a mission to use our underwater drone to help scientist studying the offshore ecosystem of the Great Lakes.

Our destination is the Mid-Lake Spawning Reefs, about 45 kilometres, or 28 miles, into the heart of Lake Michigan.

This is ground zero for the quagga mussel invasion, where the crystal-clear waters have been described as a biological desert.

(Zach Melnick): Yup.

Let's do it.

Okay good.

I'm gonna start motors.

(chiming) Okay.

I am descending now.

- That is some blue water.

- That is blue water.

(narrator): The area is so remote, it's rarely visited by humans.

So no one knows what we'll find.

- I would be amazed if there were whitefish out here.

- There is a fish there.

Yup,there's a few there.

Oh, this one's coming to say "hi".

You're a lake trout.

Oh, that's cool.

(narrator): Not only is the area teeming with big, happy lake trout, we encounter something no one was expecting.

- Oh, there are our friends.

Yup, a big school of whitefish.

- Oh yeah, yeah.

Nice.

Yeah.

- There's a whole bunch there.

I'm just going nice and slow, I don't want to upset them.

- Oh yeah, there's a ton of them.

Wow.

I mean those whitefish look pretty plump.

- That's good news, obviously.

And it's so crazy how they were nowhere else, but they really love this mound.

(narrator): Why they're here is a mystery, but it could be because the reefs, rising far off the lake bottom, cause currents that are concentrating nutrients, creating an oasis in the desert.

(Jason Smith): Fish are really resilient.

If you give them any opportunity to rebound, they will do so.

We just have to be the ones that help them start that rebound, and then get out of their way so that they can do it.

It won't be what it was ever before, but it could be magnificent.

(bright music) (narrator): With the incubators safely in the river, all that's left to do is wait.

- My 25-year vision of this is that we will stand right here in this spot, we will see whitefish, Atikameg, right here in this spot, and we will cook an amazing celebratory meal, right here, on this ground, watching those fish.

If we were standing maybe someplace just like this 200 years ago, what we would find is almost every species that lives in the Great Lakes would have had a life history that came up these tributaries and spawned.

So things like lake trout, lake whitefish, cisco - Otoonapii.

All of those species used these Great Lakes tributaries for spawning, for thousands of years, until things like logging stopped that.

I want these fish back in these rivers, for thousands of reasons.

I want our fishers to catch them, but really, I just want these fish back where they belong.

(soft music) (♪♪)