Is Sargassum Attracting Sharks to Galveston? | SharkFest

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 to 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 offshore, 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.

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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.

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NARRATOR: 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.