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22 August 2003

The Sharks of Summer

by Forrest M. Mims, III
Geronimo Creek Observatory
Seguin, Texas

Way back when I was a student at Texas A&M, I swam out beyond the surf zone one night while visiting Padre Island with some friends. This seemed like fun, until I realized that the water had become quite deep. It was time to head back to shore, but it was so dark I didn't know which way to swim. There were few buildings on the island in those days, so there were no lights to mark the direction of the shore. Somehow I made it back okay. But if I had known then that sharks patrol those waters, I would have never taken that night time swim. Shark attacks in the waters off Galveston and Padre Island are very rare, but they occasionally happen. Florida, which has a much longer coast, has many more attacks. According to the International Shark Attack File, 24 of the shark attacks reported from around the world so far this year occurred in Florida waters. The only reported attack on the Texas coast this year occurred June 12, when a teenaged boy was bitten while swimming at Padre Island. Recently my family visited the Bob Hall Pier on Padre Island just north of the Padre Island National Seashore.

 

This small hammerhead shark, and some much bigger ones, can be seen at the Texas State Aquarium in Corpus Christi. Photograph by Forrest M. Mims III. Click image to enlarge.

While walking to the end of the pier, we noticed a small hammerhead someone had caught. The obvious question is are large hammerheads also present? Some of the fishermen at the end of the pier were glad to tell us about the sharks that cruise those waters. In fact one had just caught a 4-feet long black tipped shark.

We arrived too late to see the shark, which had been sliced up for bait. But we did see its jaws. They were easily large enough to bite a substantial chunk from an adult thigh.

If large sharks are cruising the waters off Padre Island, why is it that shark attacks there are very rare? One reason became evident during a night time visit to the Bob Hall Pier. Bright lamps along the pier cast a bright glow on the water below, which was swarming with hundreds of fish. With such an abundance of tasty fish, why would a shark waste its time on people?

The science of shark attacks is limited, but there's reason to believe that some attacks are cases of mistaken identity. Reduced visibility can be a factor in these kinds of attacks, and the waters in the surf zone off Padre Island are quite murky because of the fine particles of suspended sand. A few years ago I measured the clarity of the water off Padre Island with a special light detector connected to a very small computer anchored to the bottom with a weight.

The water was so opaque to light that I nearly lost the sensor package. When I eventually found it, the computer had recorded very little sunlight during the 30 minutes or so it recorded light intensity on the bottom a few feet below the waves.

So perhaps water clarity might play a role in shark attacks off Padre Island. But that doesn't explain attacks in clear water at other beaches. Some of those attacks are intentional, and they fall in two general categories. In some intentional attacks, the shark bites and flees. In others, the shark repeatedly attacks its victim.

Since these kinds of attacks are very rare, I hope you don't let the highly remote chance of a shark encounter stop you from enjoying the beach. You're more likely to be killed in an accident on the way or by a lightning bolt than by a shark. If you stay home and mow the lawn, killer bees might be lying in wait. If you decide to have a picnic, a rattlesnake or a rabid skunk might join the fun.

Then there are the yellow jackets nesting on your front porch, the black widow spiders hiding under the porch and the brown recluses inside your closet, all waiting to sting or bite you if the sharks don't get you first. If you decide to do nothing at all, a meteor might crash through your roof or a mosquito might sneak inside your house and give you a case of West Nile virus.

So go ahead and enjoy the beach during these remaining days of summer. Just keep an eye out for a triangular fin slicing through the water in your general direction.

If you survive your swim, be sure to stop by the Texas State Aquarium on the way home. They have some very nice captive sharks on display.

This feature was originally published in Forrest Mims's weekly science column in the Seguin Gazette-Enterprise, Seguin, Texas. The column is written for a general audience.