- You can't swim with a shark without just being fascinated by how this animal has evolved over the years of their tremendous success story and to see how they've adapted and in so many ways, There's so many species of sharks that it's just to me a subject that there's no end of questions to ask.

- [Narrator] And to answer some of those shark questions I'm in the ocean tank at the North Carolina Aquarium on Roanoke island.

Yep.

That's me.

And that's Sean Harper, the aquariums dive safety officer and those are sand tiger, nurse, and sandbar sharks.

Harper says the shark's behavior in the tank mimic makes their behavior on the wrecks off the North Carolina coast.

- [Harper] Very indicative of what you would see here, the sand tigers, other sharks species such as the sandbar sharks are swimming around the wrecks.

- [Narrator] And that's why I'm in the ocean tank more on that in a moment.

But first let's go off the North Carolina coast where more than 3000 shipwrecks, dating back to the first English settlements are found on the ocean floor.

Treacherous weather, ocean currents, shifting sandbars, and even world wars gave the North Carolina coast the nickname, the graveyard of the Atlantic.

But those wrecks have become the havens of ocean life, especially for sharks.

- The history of the wrecks is one very cool aspect of it.

The Marine life that are associated with the wrecks, I'm a biologist, so that really attracts me.

and the fact that you can see these natural aggregations of large numbers of large sharks and be swimming, shoulder to shoulder with them is just, there's not many places in the world where you get to do that.

- [Narrator] There are about 73 species of sharks that live or pass through North Carolina waters every year.

- [Harper] So it's not a scary aspect for me.

I'm always aware of their space and try to not encroach on them, to startle them or to make them aggravated in any way.

- [Narrator] But it's the sand tiger sharks that really intrigue scientists.

And it's not just because sand sharks are the predominant species on the wrecks.

- Where these sharks go.

When they come back, how long they hang out.

Do they move from wreck to wreck or stay?

We do know just from where sand tiger sharks tend to aggregate and even watching their behavior in our aquariums and stuff that they do like to be near structure.

They don't seem to like to be out in the wide open ocean.

- [Narrator] It turns out sand tiger sharks, not only like to hang out on the shipwrecks off north Carolina's coast, scientists say the wrecks appear to play an important role in a sand tiger sharks life cycle.

- We're catching almost all big females on these shipwrecks off of North Carolina.

This is interesting.

I'm actually a veterinarian and so one of the tools that we use a lot for aquatic animals is ultrasound.

'Cause ultrasound relies upon sound waves.

It works really well in water.

We ultrasound fish and sharks all the time in our care, in our aquariums.

So we were ultrasounding them and not only are most of these sharks that we are catching off the wrecks in North Carolina big females, but most of them we're pregnant.

So now we have a whole other big question, okay?

Why are they all pregnant?

And why are we not catching females that aren't pregnant?

Why aren't we catching males?

- Some of these females that are gestating are potentially hanging out on these wrecks for longer periods.

Whereas the males might be passing through seasonally and then other females that are taking their break from pupping might be also migrating through.

- [Narrator] Female sand tiger sharks give birth after a 12 month pregnancy.

The eggs hatch inside the mother.

There are normally two offspring.

- There's two big questions.

We don't really know where they mate.

And possibly even more important from a species standpoint is we don't know where they pup.

It appears that these wrecks are what you might call a gestation ground.

So for whatever reason this is where these pregnant females, one of the things we have learned we do know about Sand tiger sharks is that they have a biennial or triennial reproductive cycle.

So they get pregnant one year.

Then they take one or two years off to resume their energy stores.

So those non-pregnant females are living somewhere else in those off years.

- North Carolina aquarium researchers are joining with scientists at aquariums and universities along the east coast to better understand sand tiger shark behavior.

Part of that research involves catching the sharks and inserting a tiny tracking device into the sharks underbelly.

- This is the transmitter that is inside the sand tiger shark.

These would last for up to 10 years.

So once a shark is tagged, we can follow that animal for quite a long period of time.

- [Narrator] Receivers to record the sharks movements are attached to shipwrecks.

There are 39 receivers on coastal shipwrecks off the North Carolina coast.

- This is actually the acoustic receiver, essentially like a little hydrophone.

So it's listening for pings from the acoustic tags that are on the sand tiger sharks, as well as other species of fish that researchers have tagged.

So it's listening and then recording.

Up here is the hydrophone at the very top.

So that's where the signals are being received.

Inside here is basically the electronics that are recording and storing all that data and the batteries that keep it running.

- [Narrator] Some early findings indicate sand tiger sharks are migratory with a range from Georgia to New York and they swim from structure to structure.

- They also tend to follow temperatures a lot.

So we don't know if something has to do with the thermoclines in North Carolina.

We're very close to the gulf stream here in this area where these wrecks are.

We don't know exactly, but that's part of the things that we're trying to tease out from the information of timing and movements.

And when they come and stay on the wrecks and when they leave.

- The Sand tiger sharks have been a staple of public aquariums for as long as I can remember.

They do well in aquariums but for as long as we worked with them there's still gaps in our knowledge about these animals.

And so I feel like it's kind of a way that aquariums, public aquariums, can return the favor.

- [Narrator] Which brings us back to the aquarium's ocean tank.

I'll admit I was a little nervous, even with Sean diving with me, but I was mostly in awe watching a creature so graceful, so powerful, so beautiful despite the teeth, and so mysterious.

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