NARRATOR: Historically, shark attacks on Reunion
have been rare.
Over the previous decade, the annual average
was just one incident.
But in 2011, the island is in crisis.
Mathieu is actually Reunion's fifth victim this year.
And it's only September.
Islanders are in shock.
It's vitally important that we find out
the reasons for these attacks.
There is a real problem here.
LORIS GASBARRE: We noticed that something was changing.
What?
We didn't know.
NARRATOR: Residents are desperate for answers.
Their safety and their livelihood is at stake.
LORIS GASBARRE: A small island lives with tourism.
If we don't have tourism tomorrow,
we'll have nothing left.
NARRATOR: So what's triggering this deadly spate of attacks?
To find out, the French government calls in leading
shark expert Geremy Cliff.
Clearly alarm bells were signaling.
Five shark attacks of which two were fatal.
That is a huge spike.
NARRATOR: Cliff believes the first step to solving
this mystery and preventing further attacks
is to identify the species responsible.
So he starts by analyzing Mathieu's incident.
And a chilling details stands out.
He was attacked by two sharks.
MALE: [speaking french]
NARRATOR: A double shark attack like this is extremely rare.
In fact, it's thought that such an event has only
been captured on film once.
In the year 2000 in South Africa,
a teenager is surfing, when he's suddenly knocked off
his board by one shark while a second shark
attempts to bite him.
Incredibly, the boy survives.
But this tag-team tactic may serve as a calling card
for the culprit in Reunion.
GEREMY CLIFF: Only certain shark species hunt together.
NARRATOR: Hammerheads search for food in large schools
that can consist of more than 100 sharks.
And lemons conduct coordinated assaults
by herding prey into one area.
Even blacktips hunt in groups in the shallows.
But Cliff thinks these species are all
too small to inflict the kind of damage
that caused Mathieu's death.
GEREMY CLIFF: One of the striking features
of this particular attack is the severe injuries involved.
NARRATOR: Such ferocity points to a much larger predator.
And only two species commonly found in the region
fit that bill.
GEREMY CLIFF: Firstly, we have the tiger shark.
Tiger sharks reach at least 16 feet.
They have the size, in terms of jaws and body size
to be able to tackle large prey.
And therefore, they won't be scared of attacking a human.
NARRATOR: Yet, Cliff doubts tigers are to blame.
They are totally solitary animals
and will hunt on their own.
NARRATOR: That leaves only one possible culprit.
To explain, Cliff turns to footage recently captured off
the coast of Reunion, as part of a study on the behavior
of the region's sharks.
It depicts one particular species that apparently
plays well with others.
GEREMY CLIFF: It shows clear evidence of two sharks swimming
together, one behind the other.
In fact, in nearly 20% of occasions captured on film,
they were seen foraging together.
The sharks seen here are bull sharks.
NARRATOR: Bull sharks weigh as much as 500 pounds
and are highly aggressive.
They typically attack on their own, often head-butting prey
before tearing it to shreds.
But as these images reveal, they also
use another hunting tactic.
GEREMY CLIFF: There's clearly an advantage
to work cooperatively and thereby
prove predation efficiency.
NARRATOR: Based on this evidence,
Cliff believes he's identified the type
of shark that killed Mathieu.
GEREMY CLIFF: So from that I would
conclude that a bull shark was responsible
for this particular attack.