mm 



APRIL 2006 



The Biggest Fish 



Unraveling the mysteries of the whale shark 



By Steven G. Wilson 



One hot, windless May morning, five of my 

 colleagues and I boarded our small research 

 boat and motored out into the waters of 

 Western Australia's Ningaloo Reef. We were search- 

 ing for whale sharks — the worlds largest fish — hop- 

 ing to attach electronic tags to several animals to study 

 their migration patterns and diving behavior. Waiting 

 for our spotter plane to locate a shark, we passed the 

 morning in casual conversation. 



Finally the radio crackled to life. "I've got two 

 sharks, about a mile off Tantabiddi Passage!" The 

 vessel suddenly transformed as everyone scrambled 

 to gather and don masks and fins. As we skimmed 

 over the water, I struggled to attach a dart and tag 

 to my Hawaiian-sling polespear — a sport-fishing 

 spear powered by a thick rubber band. Five minutes 

 later, two large, dark shadows were looming beneath 

 the ocean's surf ice, about a hundred feet oft the bow. 



I plunged in, but once in the water I had to ask 

 the boat crew for directions to the unseen giants. I 

 swam toward where they signaled. A cobia came in- 

 to view, a game fish that often accompanies whale 

 sharks, and I knew I was close. Then, slowly, the 

 outline of a gaping oval mouth and, behind it, an 

 upright tail fin resolved from the featureless blue 

 background. As I swam closer, the shark's tapered 

 body and distinctive checkerboard markings also 

 came into focus. 



My attention, though, was drawn to the first of its 

 two dorsal fins. The base of the fin was the target for 

 the dart and tag. My colleagues in the water measured 

 the animal (fifteen feet long) and determined its sex 

 (female), while I positioned myself along its side. 

 When they were finished, I cocked the polespear and 

 released it. The dart penetrated the whale shark's tough 

 hide, but not as deeply as I had hoped. Unless adjust- 



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NATURAL HISTORY April 2006 



