NARRATOR: In a remote corner of Australia…
FISHERMAN: Oh no!
NARRATOR: …a group of bull sharks is targeting sports fishermen
with unprecedented ferocity.
FISHERMAN: Holy ****!
NARRATOR: Showing evidence of a sophisticated new hunting behavior.
-You need to hook 10-15 fish… -FISHERMAN: Oh, look out!
…just to get one to the boat.
NARRATOR: Now, a research team is launching an ambitious investigation
using ground-breaking technology…
COLIN: Oh, he's got it! How good was that?
NARRATOR: …to find out if one of the world's most dangerous sharks…
FISHERMAN: Oh, there it is! Ah! Look at the size of it!
NARRATOR: … just got smarter.
Weipa, a remote mining town located on the edge of northeastern Australia.
Each year,
hundreds of sports fishermen are drawn to its shores by seasonal fish migrations.
But more and more,
their catch is being stolen in audacious underwater raids.
Some fishermen even claim the sharks have learned
how to recognize and follow individual boats.
Shark biologists Dr. Johan Gustafson
and Dr. Mariel Familiar Lopez are here to investigate.
What's actually happening here is a behavior called depredation
and that's when a fisherman's caught a fish and they're winding it in
and then a shark will come and take that fish.
NARRATOR: Locals claim they are losing as much as 80% of their catch to the sharks.
The level of depredation described by the fishermen
hasn't been seen anywhere else in Australia or even the world.
(dramatic music)
NARRATOR: Johan and Mariel have timed their arrival
with the height of the tuna fishing season.
But with a weather front moving in,
they have just 10 days until strong winds whip up a powerful swell
and force their research vessel back to port.
We have a variety of equipment we're going to utilize.
We've got acoustic tags, cameras,
and for the first time ever, a new cage design.
If what the fisherman are saying is true,
I want to know why.
(ominous music)
NARRATOR: Johan and Mariel team up with local fisherman, Wade Alleyn.
Together, they're heading to one of his favorite fishing spots.
JOHAN: What's your experience with the sharks versus the anglers?
Definitely have an issue with the bull sharks.
I believe they've definitely tuned in on the boats. They relate the boats to food.
NARRATOR: Bull sharks are often depicted as mindless, opportunistic killers.
But they're armed with an array of high-performance senses
they could be using to target the fishermen.
Smell many times more powerful than humans.
Vision that pierces through the murkiest waters.
And hearing that can pick up sound from over a mile away.
Do you think they're mainly bull sharks, the ones that are taking the fish?
I mean, it's hard for us to identify all the time
because they'll eat our fish out of sight.
But there are definitely a lot of bull sharks down here.
NARRATOR: To find out for sure,
and see if the situation in Weipa is as bad as the fishermen claim,
Johan and Mariel go fishing for themselves.
MARIEL: We're using a fishing line camera to be able to ID the shark.
Is it a bull shark?
Also, we're going to be able to see a bit more information
about the behavior that's occurring. When does the shark come in?
There we go. Let's get it in the water.
Right, we'll send this all the way to the bottom.
(tense music)
How's my technique, Wade?
WADE: Spot on, mate. I couldn't do it better myself.
(dramatic music) (fishing line whirrs)
JOHAN: Yep, yeah. (grunts) Oh! Oh, it's a fish! I got a fish!
(dramatic music)
It's taking it down.
(grunts) Alright, I'll try and bring it up.
(grunts) Oh, it's gone!
-MARIEL: You got bitten off! -JOHAN: Yep. I think we got sharked.
NARRATOR: Within moments of snaring a fish,
Johan's catch is stolen by an unmistakable underwater raider,
a bull shark.
JOHAN: Wow. That's a big shark. Over three, three and a half meters.
The fishing line camera did a great job.
Alright? We were able to confirm the species
and the first fish we caught, it was taken by a shark in 20 seconds.
That gives more weight to what the fishermen are claiming up here.
This is the first time I've been able to actually capture a depredation event
so close-up, underwater. This is amazing.
MARIEL: Yeah. It is really amazing footage.
JOHAN: Now we've seen one shark under the water from the view of a camera.
We want to see more. We want to see are there other sharks around?
So we need to get in the water with them.
(dramatic music)
NARRATOR: In a place as remote as Weipa, that's a significant challenge.
You need to remember, we're diving in an area where sharks have never seen a cage.
NARRATOR: Sharks have an acute sense of hearing.
They use it to locate their prey, but also keep themselves out of danger.
Loud, unfamiliar noises can put them on edge.
MARIEL: Sound is an important thing in an aquatic environment
because it travels four times faster than in air.
JOHAN: The thing with sharks is loud, sudden, sharp noises tend to spook them.
NARRATOR: The team has come prepared with a radical prototype
designed and built by underwater cinematographer Colin Thrupp.
A noise canceling cage made entirely of plastic.
So how do we reckon this is going to go when we get in the water?
Do you reckon these joins are going to hold?
I mean, a cage made out of polyethylene pipe
sounds a bit nuts when you first talk about it,
but when you actually get these things together, it's pretty strong.
NARRATOR: A world-first, the cage has been forged
using cutting edge electro-fusion welding,
a technique that bonds plastic by super-heating it
to over 200 degrees Fahrenheit.
I've been told that an earth mover can hit this and then bounce off.
MARIEL: Okay.
But I haven't heard what happens if a bull shark hits it.
-MARIEL: Well we'll test it. (laughs) -That's what we're here to do.
We're here to test it. We'll see.
(energetic music)
(indistinct speech)
JOHAN: What I'm hoping is that these sharks
become more accustomed to this plastic cage,
which is quieter and a little bit more stealthy.
It's going to minimize the noise and hopefully get us closer to the sharks.
MARIEL: If the plastic cage works and sharks do come closer,
this could be a game-changer of how we study shark behavior,
not just up here at Weipa but around the world.
JOHAN (over radio): Topside, topside. Please begin lowering the cage.
NARRATOR: Monitoring the descent of the experimental cage
is divemaster, Che McGuiness.
So, can you give us an indication of the cage?
JOHAN (over radio): The cage is sitting fine, over.
NARRATOR: On the surface, Wade casts out.
The water visibility is unseasonably poor
but his on-board sonar reveals there are big animals lurking below.
COLIN (over radio): I have a big bull shark, 10 meters out.
They're staying right on the edge of our periphery.
We can only just see them.
(tense music)
NARRATOR: The sharks move in closer,
and there's more than expected.
COLIN (over radio): There's at least six, maybe seven bull sharks
directly beneath us right on the edge of the murk.
(tense music)
JOHAN (over radio): They're not sure what we are. They're keeping a safe distance.
NARRATOR: Encountering this big pack of bull sharks
is the first clue something different is happening in the waters around Weipa.
A lot of the times I've seen them just by themselves.
But here I'm seeing a group of bull sharks together.
NARRATOR: Bull sharks are normally solitary and fiercely territorial.
They're even known to be cannibalistic.
Do these sharks hang out here all the time?
Do they hang out together all the time? What's the pecking order in this group?
We have a lot of work ahead of us to answer these questions.
(dramatic music)
COLIN (over radio): This is great!
JOHAN (over radio): All we need now is for Wade to get a fish on his line.
CHE: Divers, divers. Just letting you know.
The swell and wind has increased swell and wind has increased.
If it becomes unsafe or unworkable, let us know. Over.
NARRATOR: Another challenge the team must deal with has reared its head.
JOHAN (over radio): Copy that. Let's just see how we go.
NARRATOR: This year, the whole of Australia is suffering from La Niña,
an atmospheric phenomenon that plays havoc with the weather.
Recent rainwater has been pouring out of nearby rivers, reducing ocean visibility.
Whilst unpredictable winds on the surface
are churning up the coastal currents below.
(dramatic music)
CHE: Johan. Johan. Does the cage feel steady?
JOHAN (over radio): The cage is bouncing a lot.
MARIEL: We are concerned because this is a plastic cage.
It hasn't been really used a lot of times, and we haven't really tested it that much.
NARRATOR: The lightweight plastic cage
is being pushed and pulled by the turbulent water.
If the ropes connecting it to the boat snap,
Colin and Johan could be swimming unprotected from the sharks.
CHE: Johan, Johan, copy?
(radio static)
CHE: Johan, Johan, Johan, do you copy? Over.
(dramatic music)
JOHAN (over radio): Topside, topside can you hear me?
CHE: I can hear you now. I can hear you now.
NARRATOR: The current's getting stronger.
Tipping the experimental cage on its side.
CHE: Has the cage stabilized, over?
COLIN (over radio): The cage is buckling under the pressure.
You need to get us up now, over.
CHE: Coming up now. Coming up now. Over.
COLIN (over radio): Copy that.
(tense music)
MARIEL: We got you at the surface. Just wait there for a second.
NARRATOR: Safely alongside the mothership the damage becomes clear.
COLIN: We had raging current by the end of that,
you could just see this thick plume of just muck
just come through and sweep us and really tip the cage.
The plastic cage didn't like it at all it was really bowing under that pressure.
JOHAN: We were getting a little bit banged up underwater,
but the plastic cage did do what it was supposed to do.
We did get close to those sharks, and they did seem to not be spooked by us.
I think we had around six to ten sharks at any one time
swimming around us up nice and close. We could almost touch them.
(indistinct chatter)
NARRATOR: Johan and Mariel still need to observe sharks raiding the fishermen
from underwater.
But it will take several days for the cage to be repaired and strengthened.
COLIN: Yep! Grounded.
NARRATOR: It gives them time to investigate a different question.
JOHAN: According to the fishermen,
it's the noise of their vessels or the shape of their vessels
that's attracting sharks to these fishing grounds.
NARRATOR: Some locals claim an even more radical sea-change,
that the bull sharks have learned
how to distinguish and follow individual fishing boats.
(ominous music)
MARIEL: Johan and I have devised a little experiment
to see if we can prove or disprove what the fishermen are telling us.
Are the sharks really following their boats?
(dramatic music)
NARRATOR: To find out how shrewd the sharks are,
they need to be tagged with acoustic transmitters.
These will monitor their movements.
But to fit them, the sharks must be brought alongside the boat.
JOHAN: In my experience, rod and reel has always been the best option just for us,
but also for the welfare of the shark.
We're there the whole time with it. We can bring it in quickly.
We can handle it faster and we can release it faster.
NARRATOR: Johan straps in.
Landing a shark means going head-to-head against its power and strength.
(tense music)
(fishing line whirrs)
(dramatic music)
JOHAN: Oh, were on! Were on! (grunts) Yep, we got one! (grunts)
It's a fair bit of weight. That's a decent shark.
You can feel a strong head movement.
Oh! We are gaining ground, boy!
MARIEL: Once we get the shark up close to the boat,
we're going to be putting in a head rope and then a tail rope
and then we're going to flip it upside down.
So, belly up, to try to put him in tonic immobility.
NARRATOR: To roll the shark over, the team need to bring it closer to the boat
and maneuver it with a rope attached to its tail.
(dramatic music)
-JOHAN: It's definitely a bull. -MARIEL: Oh, it's a female.
(groans)
-WADE: She's feisty. -COLIN: She's feisty.
-JOHAN: Tail rope is on. -COLIN: Good work!
MARIEL: It's upside down, and she's getting calmer.
Tonic immobility is when a shark is put belly up.
JOHAN: Basically, they go into a transient sleep state.
They're not completely asleep or unaware.
They can snap out of this, so we have to be quite cautious still.
NARRATOR: The quicker the team works,
the better it will be for both them and the shark.
First up, the acoustic tag.
JOHAN: It'll allow us to map the movement patterns
and behaviors of bull sharks in this area,
and it's extremely pivotal to this experiment.
MARIEL: Okay. Tag is on.
NARRATOR: Measuring in at over eight feet,
this bull shark is a prize catch.
The decision is taken to fit an additional fin camera.
MARIEL: It can tell us so much about that shark's behavior.
How fast is a shark swimming?
Are they swimming really close to the surface or are they going really deep?
NARRATOR: This treasure trove of data will pop off the shark automatically
in a few days.
It's time to release the hook.
JOHAN: Alright, let's get rid of this thing. Yep, off.
-WADE: Let her free? -MARIEL: Let it go.
-JOHAN: Let it go. -MARIEL: Woo-woo!
Perfect. Good release, guys.
(dramatic music)
NARRATOR: Before they run their experiment,
Johan and Mariel need to fit as many acoustic tags as possible.
Working like a well-oiled machine, and with the sharks' welfare in mind,
they catch and swiftly release four more bull sharks.
JOHAN: Done!
All these tags that we're putting
is getting us closer to an answer of what's going on here.
JOHAN: We're going to give them 24 hours to relax
and go back to a more natural behavior before we start our experiments.
(music drowns out)
(action music)
JOHAN: The tagging experiment is designed to prove or disprove
the fishermen's claims that sharks are following their fishing boats.
NARRATOR: Johan and Mariel split up into two boats
and both deploy hydrophone receivers.
JOHAN: We can detect the pings that are coming off those tags.
(sonar static)
NARRATOR: Two of the sharks they tagged are quickly located,
still lurking together in the same fishing spot.
MARIEL: Yes Johan. I've got two of our tagged sharks here.
How about you?
JOHAN (over radio): Uh, hold on. We're going to come to you. Over.
NARRATOR: The experiment can begin.
Johan is in Wade's fishing boat. He'll cast out to get the sharks attention.
Once they're stealing fish off his line,
the fishing boat will move several miles down the coast.
Mariel will track the tagged sharks from the second boat to see if they follow.
MARIEL: Johan is going to start fishing from the other boat
and he's going to wait for a depredation event.
(tense music)
Yeah. We're hooked up.
(sonar static)
Wow. That's a good-size one.
(machine beeps)
(eerie music)
Oh. No, no, I'm getting shut. No, that's changed.
I think that's a shark. That's a shark. Oh! (groans)
(Johan groans)
(tense music)
(groans)
Oh! Oh, no. It's off.
(over radio) Mariel, Mariel? Got bitten off by a shark. So, it is happening here.
I still got our two tagged sharks here.
JOHAN (over radio): Okay. Let's pack up everything here.
We're going to move down to the other site and we're going to see they follow. Okay?
MARIEL (over radio): Roger that.
NARRATOR: The big question is:
have the sharks followed?
MARIEL (over radio): Johan, Johan, do you copy?
Yes, I read you.
I don't pick up any tagged sharks over here. Over.
JOHAN (over radio): Well look, we've been here for 30 minutes now.
I think that if they were following us from there,
we would have picked them up on the hydrophone by now.
That's a result for us. It means they haven't followed us from that site.
NARRATOR: The tagged bull sharks are nowhere to be seen.
But at this new site. even without fishing,
the on-board sonar reveals a school of different sharks lining up below.
WADE: That's a shark there.
JOHAN: That is interesting. What does this mean?
NARRATOR: The experiment has delivered an unexpected result.
MARIEL: We don't think that sharks are following the boats.
What's more likely is this depredation behavior is much more widespread
in the population of bull sharks up here at Weipa.
(dramatic music)
NARRATOR: Rather than being followed by specific sharks,
all the bull sharks in the region may have learned how to carry out these raids
and could be ambushing the fishermen wherever they go.
(dramatic music)
We need to remember that this was just one small experiment
with only a few tagged sharks.
But what it does indicate is that the scale of these depredation behaviors
could be occurring far more widespread and intense than what the fishermen realize.
NARRATOR: It could explain why such a huge proportion of the fishermen's catch
is being taken.
But why do so many bull sharks in these waters
appear to be conducting these raids?
(dramatic music)
Forces beyond Weipa's shores are likely playing a role.
(boat horn)
Australian fish stocks have decreased by 31% over the past decade.
Scientists believe this pressure could be forcing sharks
to alter their natural hunting behavior.
But it still leaves many questions unanswered.
JOHAN: We want to know, why is it so concentrated here in Weipa?
(ominous music)
NARRATOR: Now halfway through their time in northern Australia,
the team has just five days until strong winds will force them back to port.
Colin has fixed and strengthened the plastic cage.
It's ready for him and Johan to go back in the water.
So far we've only seen the depredation behavior from the fishing-line camera.
But what we want to do is see it from the plastic cage
and that's the intention of this dive.
We know that there is multiple sharks down there.
Wade's up top. He's going to be fishing, and hopefully we'll be able to see,
is there a hierarchy between different sized sharks?
What's the etiquette at this dinner table?
NARRATOR: On this dive, a 360-degree drop camera will help fight back
against the unpredictable water visibility.
We're really hoping to get a lot of information from it.
NARRATOR: The cage is lowered into position.
(tense music)
On the surface, Wade casts out. Shadows emerge deep in the water.
JOHAN (over radio): I've got two bull sharks.
Two bull sharks. Approximately five meters below us. Over.
COLIN (over radio): The sharks are coming in closer.
JOHAN: They're getting more confident with us.
COLIN: We've got three bull sharks around us now.
NARRATOR: Topside, Wade has a fish on.
(tense music)
JOHAN (over radio): They're just circling.
COLIN (over radio): Not coming in too close to take the fish.
NARRATOR: The sharks are biding their time.
But as the fish on Wade's line tires, they strike.
JOHAN (over radio): Big bull coming up, big bull.
Here he comes. He's going for the fish.
(intense music)
COLIN (over radio): Oh! He's got it! How good was that!
MARIEL: We were expecting to see the shark come really fast
and just snap it from the line. But that's not what we saw.
JOHAN: It came in slow. No sense of urgency at all
and just basically plucked the fish off the hook.
I've never seen that before. It was incredible.
Another thing I noticed was the biting tactic.
So, the sharks took out the propeller of the fish.
It bit off the tail section first.
And then they came swooping around again, slow and calm, and they took out the rest.
The general perception about bull sharks
is that they are just mindless, aggressive hunters.
And that is not what we're seeing here.
What we are seeing is a calm, more intelligent approach.
NARRATOR: It's strikingly similar to a tactic used by a legendarily smart hunter.
Killer whales also patiently stalk fishing boats
striking with intelligent, surgical precision
once trawlers start to bring in their catch.
Scientists believe Orcas do this to deliberately save energy.
Are sharks now rivaling this clever thinking?
JOHAN: Think of it from a shark's perspective.
When they hunt, they expend a lot of energy.
So, a hooked fish presents an opportunity
for a more efficient way of getting a feed.
NARRATOR: Back beneath the boat,
underwater cinematographer Colin catches a glimpse of something unusual.
COLIN (over radio): I think I can see a smaller bull shark.
(tense music)
Yeah, definitely a young bull shark. That's really weird.
JOHAN: Col reckons that he saw a juvenile bull shark.
To me, that's unusual.
Why would there be a small juvenile bull shark
out in the open ocean in adult territory?
Bull sharks have a really interesting reproductive cycle.
The females come from the ocean up rivers
to lay their pups in freshwater environments.
NARRATOR: Rivers offer protection and security for bull shark pups.
Only once they reach six to seven feet or around eight years old,
will they head to the ocean.
If Colin did actually see a juvenile bull shark this far out offshore,
that's quite a significant find.
NARRATOR: It means there could be a bull shark nursery somewhere nearby.
JOHAN: If that's true, that means juvenile bull sharks are leaving their nursery
and they're mingling in with these adults on these fishing grounds.
I think that's something that needs further investigation.
(tense music)
(action music)
JOHAN: To prove that there is a nursery,
we need to go and catch juvenile bull sharks.
And in order for us to do that, we need to go upriver.
NARRATOR: Located just 30 miles from the offshore fishing site,
the Mission River runs through Weipa
and is one of the largest nearby sources of freshwater.
Searching for baby sharks in an open river is notoriously difficult.
The trick is to follow the food.
We've got a bunch of birds just hanging off under the bridge.
(birds calling)
JOHAN: It's good. A lot of birds are a good sign.
NARRATOR: The birds are taking advantage of a crowded ball of fish
near the surface of the water.
It's a sign that normally means there are predators lurking below.
COLIN: Where we get little fish big fish follow, right?
JOHAN: That's right.
MARIEL: Maybe we should start to get our gear together.
What do you think we should put down first?
-Let's go drum lines first. -Good idea.
NARRATOR: In these silty, low-visibility waters diving is not an option.
The team needs to bring whatever's hunting the fish up to them.
JOHAN: This is really actually quite exciting
because we think that we found a location where there are juvenile bull sharks.
MARIEL: We're going to set up some drum lines.
Just think about a really long fishing line.
JOHAN: Okay?
-MARIEL: Got it? -COLIN: Line.
-Perfect. -Here's your way.
COLIN: Bring us a little bull shark, please.
JOHAN: Once those yellow floats are no longer visible,
or if the entire big float bobs underwater,
we're "shark on."
NARRATOR: They back off the boat,
and wait for something to bite.
(dramatic music)
MARIEL: Guys. I think the floats are down.
JOHAN: All right, cool. Yep, the floats are down,
we can't see the yellows. That's great.
COLIN: He's definitely on. (grunts) He's still on. He's still on.
There he is. Here he is. Here he is. Alright.
JOHAN: Oh, yes. That's a nice little belly.
It looks like-- oh, that could be a bull. That looks like-- that looks like a bull.
-COLIN: Bull shark! -JOHAN: This is great. I think we're on.
COLIN: This is exactly what we came here for.
NARRATOR: Genetic analyses of this juvenile
could provide vital details about Weipa's bull shark population,
but only after it's brought alongside the boat.
COLIN: Guys, get that tail rope on.
-Thank you. -COLIN: She's strong.
Hang on, hang on she's come back around. Swing back round.
NARRATOR: Baby bulls are born with a mouth full of razor-sharp teeth.
And they're even more agile than adults.
(all exclaim)
JOHAN: He's going-- he's going too far forward.
-Just keep this in gear, in back here. -COLIN: You got it.
JOHAN: They might be small, but they still can bite.
Fighter, yeah. I lost it.
(dramatic music)
-COLIN: Whoa! -JOHAN: He's a fighter.
That's great.
(tense music)
-JOHAN: Towing, off. Ready? -COLIN: Yes!
JOHAN: Grab that rope.
MARIEL AND COLIN: Oh, it's a female.
MARIEL: Hello, cutey.
-NARRATOR: The shark is under control. -JOHAN: It's one meter 40.
I would say she's been here, for about five years.
NARRATOR: Measuring under five feet,
it's highly unlikely this shark traveled here by sea.
She's definitely from this river system.
So, I'm pretty confident there's even smaller ones around.
Capturing this juvenile bull shark
indicates that there is a nursery in this river.
This means that there is an easy connection
between this nursery and the offshore population.
NARRATOR: An easy connection means more opportunities
for juveniles to mingle with adult sharks out at sea.
Bull sharks don't have parental care, and they're usually solitary animals.
So, seeing them interact together, we get a lot of questions.
Previous research has shown that a lot of species, including sharks,
have the ability to learn new behaviors in a social setting.
Is that how bull sharks in Weipa
are actually picking up this depredation behavior?
(dramatic music)
NARRATOR: Catching a juvenile bull shark
also allows Johan and Mariel to test another theory.
Alright, were going to need a biopsy sample.
Bull sharks tend to migrate with the seasons
because the water temperature changes.
So as it cools, bull sharks will move into the more constant warmer waters.
NARRATOR: But that rule might not apply in this tropical part of northern Australia.
JOHAN: Up here in The Top End,
the water temperature doesn't really fluctuate as much.
NARRATOR: The sharks around Weipa
could be resisting their migratory instincts and putting down roots.
JOHAN: If that's true and they are staying in the one area,
that means social learning behaviors
could be offloaded to younger sharks more often.
NARRATOR: Proving this theory will take years of field work and tagging.
But cutting-edge genetic analysis could offer a sneak-peek at the results.
DNA can tell us a lot of interesting stuff.
We can test it against other populations around Australia.
It can tell us if there's any genetic differences.
It can also tell us how related sharks are to each other.
JOHAN: I think we're good here.
I think we can get ready to release, so we're going to take off the tail rope.
MARIEL: Okay, tail's out.
COLIN: Off she goes.
COLIN: Whew! How good?!
-Oh, nice. That was great. -Yes! What fun!
NARRATOR: To complete their investigation
and find out if the bull sharks around Weipa are putting down roots,
Johan and Mariel need to get DNA samples from adult sharks back out at sea.
On the way, there's a temporary diversion.
(beeping)
NARRATOR: A ping from the fin cam
they attached to a big bull shark several days ago.
(beeping, continuous)
We're trying to find it by tracking its signal
so we can download and get the data.
MARIEL: I think it's coming near the shore.
JOHAN: There it is.
MARIEL: Oh yeah, I can see it.
JOHAN: The data from the fin cam showed us some amazing information.
Generally, bull sharks that I've tagged
spend more of their time at the ocean floor during the day.
NARRATOR: Normally they use the cover of night to patrol at the surface.
That's not what we're seeing here in Weipa.
We're seeing the bull shark spending time at the water's surface
and then going all the way back down to the sea floor during the day.
NARRATOR: The fin cam captures the shark repeatedly diving
and then returning to the surface.
JOHAN: This is what we call "yo-yo diving."
The yo-yo behavior is generally associated with a type of patrolling behavior.
Basically, the shark's looking for food.
But why is this bull shark doing it during the day when they're generally nocturnal?
That is a significant question.
NARRATOR: It's yet another anomaly amongst the local bull sharks.
Is it somehow related to the recreational fishing?
Are they looking for recreational fishing catches?
This is a good piece of the puzzle,
but we have to remember that it is only one animal,
one incident in one video shot so far, but it does start to raise questions.
To conclude something, we really need to get more data first.
(ominous music)
NARRATOR: It's the team's final day in Weipa.
Strong winds will soon force the mothership back to port.
JOHAN: This is our last dive here. And we need to get more DNA samples.
Basically, the higher the number of biopsy samples we have
is the greater our strength in our conclusions that we can come up with.
NARRATOR: To collect the DNA, Johan is armed with a modified spear gun.
This will maximize his chances of getting multiple samples from one dive.
Wade fishes topside once again to draw the sharks in.
COLIN (over radio): Yes, we've got two sharks below us. Two sharks. Over.
MARIEL: Although we're seeing the bull sharks,
we still need to get them a little bit closer
so that we can get a biopsy sample from them.
NARRATOR: The cage is positioned closer to Wade
to increase Johan's chances of a clean shot.
COLIN (over radio): Bull shark at 12 o'clock.
six or seven meters out.
(suspenseful music)
NARRATOR: It's high-noon for Johan,
but before he's had a chance to pull the trigger, disaster.
COLIN (over radio): There's this murk coming through.
NARRATOR: A cloud of silt envelopes the cage.
JOHAN (over radio): We're right on the edge of visibility.
NARRATOR: Giant bull sharks just meters away from the divers
have become invisible.
COLIN (over radio): The visibility is getting worse and worse.
NARRATOR: With fish in the water,
an unseen shark could easily collide with the cage.
(intense music)
CHE: Surface to divers, we have a 20-minute bottom time
due to conditions have just started to creep up, up here.
Okay divers, you've got 20 minutes, 20 minutes, 20 minutes.
NARRATOR: Time is running out to get the DNA samples.
(tense music)
COLIN (over radio): Johan. Bull shark to your left.
Coming in closer. Take the shot! Take the shot!
Got him! Yes!
NARRATOR: It's the perfect shot.
The shark is none-the-wiser and disappears as if nothing's happened.
CHE (over radio): Divers. Divers. Five more minutes. Five more minutes. Over.
NARRATOR: There's still a few more minutes on the clock.
(dramatic music)
COLIN (over radio): We've got another one coming in.
Yes! A second biopsy in the bag. Nice shot.
NARRATOR: It's another crucial sample.
COLIN (over radio): Divers to surface. You can start bringing us up.
Mission success.
CHE: Copy, copy. We'll bring you close to the surface.
COLIN (laughs): That was a better dive, wasn't it?
JOHAN (laughs): That was awesome. That was great. It actually worked really well.
I don't know about you, but I count that as a win.
JOHAN: I think we've done well.
NARRATOR: Despite the unpredictable weather, the team have what they need.
We got several biopsy samples from a range of different sizes of bull sharks
that we've sent to the laboratory to analyze.
(dramatic music)
NARRATOR: The DNA results are in.
It's a small sample size, but they've delivered truly intriguing data.
JOHAN: What the DNA has shown us, is that the shark we sampled upriver
is half related to the large female
that we sampled offshore in that last cage dive.
NARRATOR: Further analysis indicates that the third sample
is also related to both these sharks.
JOHAN: Due to the small sample size that we have, I'm completely surprised,
and I did not expect any of these sharks to be in any way related to each other.
MARIEL: We don't know if it's a mother or a father
but we do know that they're sharing an ancestor in-between them.
NARRATOR: It's a strong family link connecting the bull sharks in the nursery
to the adults raiding the fishermen at sea.
JOHAN: It may also implicate that compared to other areas
these sharks may not be moving in large numbers.
NARRATOR: The closer related the sharks, the smaller the gene pool,
and the less they could be moving away.
JOHAN: If they're not migrating the large distances
and they're spending more time in this one area,
that could explain why the depredation events
are so much higher and concentrate here in Weipa.
(dramatic music)
MARIEL: If the populations of bull sharks up here at Weipa are localized,
they would be exposed constantly to fishing activities,
increasing their likelihood of depredation behaviors.
NARRATOR: Together with the presence of the nearby bull shark nursery,
it could put Weipa at the epicenter of a perfect storm.
MARIEL: It's definitely possible that juvenile bull sharks
could be learning the behaviors from adults.
JOHAN: We've seen smaller sharks leaving the river earlier…
JOHAN (over radio): Definitely a young bull shark.
JOHAN: …and potentially starting the social interactions
and learning procedures at a younger age.
MARIEL: Seeing this bull shark behavior has been amazing.
However, it's also been surprising.
NARRATOR: The raids on fishermen have been exposed
as the work of a methodical, calculated hunter.
They're highly intelligent.
They're analyzing their environment before they do anything.
NARRATOR: These smart bull sharks might be at the vanguard
of an adapting shark behavior that could soon spread elsewhere.
JOHAN: Recreational fishing is increasing worldwide.
We're starting to see a lot more anger towards sharks,
and that could lead to a downward spiral towards shark conservation.
That's why it's important when we get opportunities like up here in Weipa
that we grab them with both hands.
There's a lot more to uncover.
And I am looking forward to coming back and spending a lot more time here.
(dramatic music)