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Getting to the Bottom of Shark Mysteries:
March 19, 2000
@Sea correspondent/
photographer,
Tim Calver |
Atol das Rocas, Brazil -- Atol das Rocas provides plenty of water for it's local sharks, but where's a baby lemon shark to swim? The atoll's sand flats and shallows provide safety for small sharks, but most of these areas are completely dry at low tide. Deep water areas are the domain of large predators, including other lemon sharks. |
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| Brad Wetherbee checks the setup of a bottom monitor. During low tide, when Lama Lagoon is completely dry, it is thought that this location becomes a refuge for young lemon sharks. |
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As tides shift, the baby sharks of Rocas must swim to avoid danger -- they don't want to get eaten, they don't want to get left high and dry. Or this is what we think, at least. We'd like to understand a lot more about where the baby lemon sharks go, and why. And when you've got questions like those, you call in a shark-tracking expert. |
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Dr. Stan Spielman ties the finishing knot that secures a transmitter under the dorsal fin of a baby lemon shark. Designed to fall off in a matter of weeks, the transmitter will help us track the young shark in the mean time.
Working during the low tide hours, Dr. Brad Wetherbee and Brazilian student Andrey Castro re-install a bottom monitor in the main lagoon of Atol das Rocas. Using two cement blocks, Brad and Andrey ensure that the monitor will stay put until they return the next day to collect the recorded data.
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Dr. Brad Wetherbee is on loan to our expedition from the National Marine Fisheries Service in Rhode Island, where he tracks sand bar sharks in nearby Delaware Bay. He comes to our project with an impressive background in tracking everything from reef fish to tiger sharks, a large tupperware container full of electronic equipment, and some great flowered surf shorts.
The swift currents and shallow, rocky conditions of Atol das Rocas ruled out traditional tracking techniques using boats and hydrophones to physically follow sharks. Brad came equipped with stationary receivers that simply log a 'hit' whenever a tagged shark swims within range. "This is a good setting for this equipment." Brad told me, while pointing to locations on a chart where he had deployed his 'bottom monitors.' These foot-long tubes of electronics, enclosed in waterproof cases, are tied, strapped or bolted to the lagoon bottom so as not to be swept away in the currents. We have tagged seven sharks at Rocas, all between sixty and seventy centimeters long, and Brad has deployed seven bottom monitors to track their movements.
Brad produced a graph of the data recorded by the bottom monitor situated at the entrance of Lama Lagoon (our primary location for catching baby lemon sharks. See our dispatch from March 9-10). This instrument has been the most active of all that Brad has deployed -- five of the seven tagged sharks pass by it regularly. This indicates that Lama lagoon is an important part of the Rocas lemon shark nursery.
The graph shows the Rocas tides rising and falling like a roller coaster. At the very lowest tides, there is no water in Lama Lagoon, and of course no sharks. When the tide starts to rise, water rushes in at an incredible rate -- the lagoon's depth increases by more than one foot per hour. And around mid-tide, the sharks rush in. "The tagged sharks seem to aggregate at the mouth" says Brad, "...they're fighting the tide before streaming into the lagoon." Brad believes that baby sharks are drawn to the lagoon because they see it as a refuge. |
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