A Sharky Evacuation

Sometimes shark research yields unexpected results — especially when the barometric pressure drops.

As Tropical Storm Gabrielle bore down on Southwest Florida in the summer of 2001, scientists from Mote’s Center for Shark Research wanted to pull their remote acoustic listening stations in Terra Ceia Bay, below Tampa Bay, out of the water. There wasn’t enough time.

After the storm, the data from the listening stations was downloaded onto a computer that mapped shark movement. What the scientists saw was astounding: Thirteen tagged juvenile blacktip sharks fled their nursery habitat in the bay before the tropical storm battered the coast; the sharks returned five to 13 days later.

“I was just hoping my equipment would still be there when the storm was over,” said Dr. Michelle Heupel, staff scientist in Mote’s Center for Shark Research. “In the end we didn’t have enough time to get it out of the water, so I said leave it, and crossed my fingers, and it turned out to be a pretty fortuitous thing.”

Before the storm, Heupel was conducting a study to see how young blacktips used the nursery habitat. Twenty-five underwater acoustic listening stations in Terra Ceia Bay recorded the movements of blacktips tagged with transmitters. “The nursery area is considered to be their safety zone,” said Heupel. “We don’t normally see them leave the nursery for extended periods.

“Usually in late September and October you see them migrate and move in and out a lot more. They usually don’t leave until the water temperatures drop. We think the threshold is around 20 degrees Celsius (68 degrees Fahrenheit). It varies with when the cold fronts come through. The earliest I’ve ever seen them leave is late October. Usually it’s not until November. They all leave for the winter and go down to the Keys.”

Heupel looked at all the variables that might have led to the exodus.

Changes in salinity and noise associated with heavy rainfall and wind were ruled out. The blacktips left before any of these changes occurred. “I think that it was the change in pressure coupled with the fact that it came sort of out of synch with the tidal change,” said Heupel. “It was just a really odd pressure change that probably triggered them to leave.”

When the barometric pressure drops, the sharks’ perceptions of how deep they are is altered, she said, which might make them feel unsafe because they think they are in shallow water.

Here’s how it works:

Air presses down on the water of the bay. If the barometric pressure decreases, a shark in 3 feet of water feels as if it is in 1 1/2 feet of water. Scientists think that the blacktips, which are normally in about 10 feet of water, went into Tampa Bay to get to deeper water: 17 feet of water with the decreased pressure would feel like 10 feet.

“This is a really interesting look at these animals actually using their systems to detect a pressure change and respond to it in a natural environment,” said Heupel. “I just happened to be in the right place at the right time to record this behavior that no one has reported before. I think it certainly contributes to our knowledge of the biology and behavior of these animals.”

Published in the Summer 2004 issue of Mote Magazine