NARRATOR: Bull sharks bite with more force, pound for pound,
than any other species of large shark.
But in Texas waters, they don't frequently
turn their teeth on humans.
That is, until 2010--
three attacks in less than a year.
The safety and livelihood of local residents
is under threat.
Galveston is a tourism mecca, but having a bunch of shark
attacks in a short span makes people question
whether to go the beach.
NARRATOR: All three attacks take place in the waters off
of Galveston Island, so wildlife journalist Chester Moore
returns to the scene to search for clues that
may shed light on the mystery.
And near the water's edge, he spies something striking.
CHESTER MOORE: This is an incredible plant
called Sargassum.
NARRATOR: Sargassum is a floating seaweed.
Much of it originates in the Sargasso Sea,
a vast 3,000-mile long stretch of ocean in the North Atlantic.
But closer to home, Chester has noticed something sinister.
This seaweed seems to attract sharks.
When I'm out there fishing and I see a bunch of Sargassum,
I know I'm probably going to catch sharks.
NARRATOR: What is it about this tiny plant that
appeals to sharks, and how could it
explain the spike in attacks?
Biologist Larry McKinney has been
studying the Texas coastline for more than 40 years.
He says the answer may come down to what
lies hidden within Sargassum.
This is a mobile habitat.
NARRATOR: The seaweed provides food,
refuge, and breeding grounds for an array of critters.
The types of animals that are associated with Sargassum
are particularly small, starting with small crabs and shrimp.
NARRATOR: These animals, in turn,
attract a variety of larger species
like fish and sea turtles, the preferred prey of sharks.
LARRY MCKINNEY: Any place that would
concentrate a food source, sharks will always come.
NARRATOR: Normally, Sargassum floats to the Texas shore
in small bits and pieces.
LARRY MCKINNEY: Today, we have kind of a normal amount
of Sargassum.
NARRATOR: But around the time of the attacks, something changed.
What I'm looking at here are satellite
photographs of the coast of Texas during 2010, 2011.
If you look just off shore, now each one of these green dots,
these are big patches of Sargassum.
They were coming in in mats that were
the size of a football field.
You can actually see Sargassum from space.
[music playing]
NARRATOR: In Galveston, the seaweed
buries the beach in huge heaps.
Experts believe this sudden influx of Sargassum
may be the result of a variety of environmental factors
including strong winds and a steady rise
in ocean temperatures.
And the seaweed invasion isn't limited to Texas.
Beaches in Florida, the Caribbean,
West Africa, and South America are also inundated.
LARRY MCKINNEY: No matter what they tried
to do to clean it up, within a few days,
more were coming ashore.
NARRATOR: It's a worldwide Sargassum siege.
[music playing]
Could sharks be following this seaweed snack into the Texas
shallows, thereby increasing the risk of an accidental run-in
with humans?
According to McKinney, sharks do follow
the mats, but only so far.
Well, you see, most of the fish feeding on Sargassum
is out past these breakers.
Once it gets up into these sandbars,
it starts rolling over and over.
The small animals do drop out and they disperse.
NARRATOR: So once the seaweed reaches
the shallows where humans are, it loses its appeal.
So the idea that Sargassum attracted sharks into this area
is probably not very likely.