Expedition Journal

April 23, 1996
On the Trail of Bob and Martha

Bob and Martha are two Carribbean Reef sharks, named after JASON crew members. (I am not sure if Bob is named after Dr. Ballard or Dr. Hueter.) Bob was the first one caught, and Martha was caught on another day. These two are the only ones who have remained in the area. The other three Carribbean Reefs that have been caught have all fled the area. Yesterday Betsy Berg, a Teacher Argonaut, helped to catch a blacktip shark, but it was too small to put a transmitter and track.

Tracking is an adventure by itself. Most people will tag a shark, and then hope to catch it again someday. Here at JASON, we do things a little differently. We attach a transmitter to the bottom of their dorsal fin, and let the shark go. We then use a hydrophone system to keep track of it.

A hydrophone is a lot like a radio receiver. The transmitter attached to the shark lets out a frequency with a ping sound. When you put the hydrophone into the water and the earphones on your head, you can hear the pings, at regular intervals) if the shark is within range. The quicker and louder that you hear them, the closer the shark is to the boat; the fainter and slower, the farther away.

JASON also adds another unique part to this hydrophone system. Instead of just one hydrophone machine, JASON scientists use two, one on eiher side of the boat. By connecting them to the earphones, when you listen is is a lot like listening to a stereo system. If you hear the pings on the left side, it means that the sharks are to the left of the boat. You turn the boat to the left. If the sounds are on the right, the sharks are somewhere on the right side of the boat and you turn it that way. The object is to keep the sounds in the middle, with the sharks right in front of you.

You might want to ask how we know which shark is sending the signals, and which one is nearby. Well, each shark sends out a dfferent frequency sound. For example, Bob sends out a low pitched sound, while Martha's is quite high. On Saturday we knew that they were both out there because we heard both the low and the high pitched sounds coming to us via the hydrophone.

I had a lot of fun, even though we didn't catch any new sharks. It is real science, because we are now finding out that this particular species is territorial and likes to remain in the same place. Check back for the latest data as we follow Bob and Martha!!

Heidi Kuglin
(Week Two Argonaut)

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Gene Carl Feldman (gene@seawifs.gsfc.nasa.gov) (301) 286-9428
Todd Carlo Viola, JASON Foundation for Education (todd@jason.org)
Revised: 23 April 1996