Coming up next on Jonathan Bird's Blue World, why do sharks hang out at shipwrecks?
The blue world keeps her secrets well, and today we're going to attempt to solve a mystery
about an amazing fish.
Every spring, Sand Tiger sharks converge at certain shipwrecks off the coast of North
Carolina. Nobody knows why they visit these wrecks or what they like about them.
They don't seem to be doing much of anything.
But scientists have a theory on what they are doing...and we're going to see if we can
confirm it.
Hi, I'm Jonathan Bird and Welcome to my world!
Our adventure begins in coastal North Carolina in the United States. We have loaded piles
of gear aboard a dive boat and we're heading way offshore...to an area of the ocean full
of world war 2 shipwrecks, known as the "Graveyard of the Atlantic."
The wrecks are located here, in a lonely part of the open ocean, more than 15 miles from
land. Eighty feet below me is the wreck of the Carib Sea, a ship that was sunk by a German
U-boat in World War two.
The Carib Sea has deteriorated over 60 years on the bottom of the ocean, but it still looks
like a ship.
My crew and I are suiting up to explore the wreck and the sharks that have come here.
We don our gear and jump into the water, descending down the anchor line into the blue.
What starts out as crystal clear water near the surface, becomes more murky at the bottom.
Out of the haze, a shark slowly materializes, swimming at a leisurely pace
Sand Tigers have rows of pointy teeth that look pretty scary.
These needle-like teeth are not designed for biting divers, but in fact they are good for
grabbing and holding on to slippery fish. They aren't good for cutting, so the Sand
Tiger usually doesn't take bites out of anything. They usually don't eat anything that they
can't swallow in one bite.
As I swim around looking at the sharks, they seem to be trying to act inconspicuous, as
if they are just waiting for me to leave so they can get back to what they are doing when
I'm not around. But what are they doing when I'm not around??
They could be coming to the wrecks to eat. Afterall, there are a lot of fish here. But
there are a lot of fish in lots of places. Why would all these sharks come to the wrecks
in North Carolina, all at the same time?
Biologists think this is no coincidence. They think the sharks come here to mate.
One clue are the sharks teeth that can be found on the wreck. As I look around, I find
a bunch of shark's teeth scattered on the wreck and in the sand nearby.
Sharks have a steady supply of new teeth being regenerated because they lose their teeth
every once in while when they bite something hard.
When sharks mate, the male bites onto the female to hold on. He often loses some teeth
in the process. Scientists think these teeth are a sign that the sharks are mating at the
wrecks!
They sure aren't mating while we're around, even if we use our super-quiet rebreathers.
So the next step is to place some remote cameras on the wreck and leave, to see if we can catch
the sharks doing anything without us around.
My crew and I place several remote cameras on the shipwreck. We will record for an hour
with nobody around so that the sharks won't be disturbed by our presence.
Later we look at the tapes to see if the cameras caught anything on tape.
See now what's surprising me about this is that I really expected to see something different
with the unmanned cameras. I mean when we're not down there, there's no divers, they should
be doing something now that we're not there. But they're doing the same thing—which is
nothing!
While we got some interesting shots of the sharks, we didn't see any mating. In fact,
they looked like they are doing the same exact thing they do when we are down there with
them
I would expect a lot more action.
Hmm, I think we are going to have to go back out and see what they do at night. I think
that's the only thing we can see because if they're not doing it during the daytime, maybe
it's a nighttime, maybe it's a nocturnal activity.
Could the sharks be mating at night? There's only one way to find out, and it's a pretty
spooky option.
We decide to take the boat out to the wreck to stay overnight. We are the first film crew
to attempt filming Sand Tigers on this wreck at night.
We head down into the darkness with video lights on our cameras, hoping to see something
incredible.
When we reach the wreck, we don't really see anything except pitch darkness. Until...
Out of nowhere, a dazed Sand Tiger shark suddenly comes right at me and scares me half to death!
During the day the Sand Tigers are far too shy to come this close.
They don't seem aggressive...but they do seem confused by the lights. They act like they
are sleep-swimming, almost like a deer in the headlights when the bright video lights
get too close.
After an hour looking for mating behavior and not seeing any, we give up and head back
to the boat. But we still have one last idea.
And you liked the camera positions earlier?
Those were good.
We will try the remote cameras again, this time at night.
Alright.
And the sharks do, they go through there, they go around the front of the bow, and then
when they circle around they come across that front...
But in order for the cameras to be able to see anything, we will need at least some light.
The following day, we set up a super bright movie light over the bow of the wreck by hanging
it from an air-filled lift bag. This 200 Watt HMI light is as bright as a 1,000 watt lightbulb.
If we hang it high enough over the wreck, it should throw just enough light to provide
some gentle illumination for the cameras, but not so much that will daze the sharks.
While Rick is hanging the light, Gator, Tim and I are setting up cameras.
Finally, everything is ready. Now we head back to the surface to wait for darkness.
Later, we dive back down to the wreck and turn on both the cameras and the light. It's
a quick dive, and then we return to the boat to wait.
Whoa! I don't know we might have just put down the world's largest plankton-attracter!
I don't know! But I can tell you one thing: the sharks are around the wreck at night.
Um hmm! Oh yeah, they're there!
In the morning we collect all the cameras again.
Reviewing the footage, our light above the wreck worked out well. In fact, it worked
so well that it attracted a huge school of fish below it.
I mean that big HMI light right over the bow of the boat really causes the fish to aggregate
underneath it, but it doesn't seem to attract the sharks, and the sharks don't seem to be
really attracted to the lights or the fish.
There are very few sharks in this footage so they're not together by any means.
Yeah, really it looks just like the day footage all over again.
We learned that the sharks are probably not feeding on these fish because if they were,
how could they not be attacking such an easy meal?
Every once in a while we see a shark or two pass through the school, but we don't see
any mating.
You might think that our entire experiment was for nothing because we didn't see the
sharks mating. But in fact it proved that the sharks don't seem to be very active at
night. And it showed us that they might not be mating at the wrecks at all, though we
can't prove it either way.
The ocean is filled with mysteries, many of which will never be solved. We're a little
closer to understanding the Sand Tiger shark, but we have a lot to learn. For now, we still
have no idea why the Sand Tigers come to the wrecks in North Carolina. This mystery will
continue to remain unsolved.