data show that fewer sharks are present in El Nino 

 years than during La Nina years. 



In La Nina years, ocean temperatures and sea lex - 

 els in the western tropical Pacific are relatively high; 

 during El Nino episodes, water temperatures and 

 sea levels are lower. Both patterns have long-range 



Signs of overfishing have begun to appear: 

 catches have declined, and fish have gotten sm 



effects on climate and currents from Australia to 

 South America, as well as in many other parts of the 

 world. I began to suspect that the El Nino phe- 

 nomenon somehow negatively affects the w hale 

 sharks' food supply at Ningaloo Reef. 



To confirm my suspicions, I tirst had to deter- 

 mine what the whale sharks eat along the reef They 

 were already known to teed on schools of a tropi- 

 cal species of krill, Pseudeuphausia latifrons, but no 

 one could say whether it is their primary food. To 

 answer that question, I examined fecal samples from 

 whale sharks, which divers had collected at the reef 

 over several years. All the samples included crus- 

 tacean remains that resembled krill, and a genetic 

 analysis later confirmed that the species was indeed 

 P. latifrons. Surveys using sonar to look for krill while 

 whale sharks were congregating on the reef also 

 turned up plenty of krill, forming schools about the 

 size of a football field and some ninety feet deep. 

 Most tellingly, when my colleagues and I came 

 across schools of krill, we almost always found whale 

 sharks feeding on them. 



Do krill populations fluctuate with El Nino, 

 too? As part of a study on fish larvae, biolo- 

 gists from the Australian Institute of Marine Science 

 in Townsville set traps each month for two con- 

 secutive summers. I was able to study the krill and 

 other zooplankton they caught. As luck would have 

 it, the first year had strong El Nino conditions and 

 the following year strong La Nina conditions. In line 

 with my hypothesis, krill abundance proved to be 

 much higher during the La Nina year. Two years of 

 data is not proof, but it does offer a good lead. 



How does El Nino influence the production of 

 krill? The Leeuwin Current that dominates the reet 

 is stronger in La Nina years than it is in El Nino 

 years. Paradoxically, however, the stronger current 

 suppresses nutrient upwelling, and that leads to low- 

 er chlorophyll concentrations and a diminished sup- 

 ply of most kinds of zooplankton in La Nina years. 

 So what accounts for the high krill abundance we 



discovered in a La Nina year? For now, at least, that 

 remains a mystery. 



Although I finished my doctoral studies in 2001, 

 I still return to Ningaloo Reef each whale shark sea- 

 son. In 2002 I collected tissue samples for a genetics 

 study by a graduate student at the University of South 

 Florida m Tampa, comparing DNA from 

 whale sharks in the Atlantic, Indian, and 

 Pacific oceans. Once finished, the study 

 will show how much genetic mixing takes 

 ler. place between whale sharks in the three 



ocean basins. That should shed some light 

 on how much impact regional fisheries 

 may have on the global abundance of whale sharks, 

 and thus guide efforts to manage and conserve them. 



My surprise encounter with the dorsal fin of a 

 whale shark resulted from an effort to answer an- 

 other basic question: Where do the Ningaloo Reef 

 whale sharks go in the winter, spring, and summer? 

 In 2003 and 2004, I joined three fellow marine biol- 

 ogists — Brent S. Stewart of Hubbs-Sea World Re- 

 search Institute in San Diego, Jeff J. Polovina of the 

 U.S. National Marine Fisheries Service in Honolulu, 

 and Mark G. Meekan of the Australian Institute of 

 Marine Science in Darwin — in attaching pop-up 

 archival tags to nineteen sharks. The tags record data 

 about the light level, depth, and temperature of the 

 tagged fish's environment until a preprogrammed 

 date. Then the tags detach, float to the surface, and 

 transmit their archived information to satellites. 

 From those data, the sharks' movements can be re- 

 constructed to within about a hundred miles. 



We recovered several months' worth of data from 

 each of six tags. All six sharks had moved northeast 

 after leaving Ningaloo Reef, and several individuals 

 had approached the Indonesian coast, where, we 

 feared, they risked becoming fishermen's quarry. 

 Our depth and temperature data also showed that 

 whale sharks inhabit a more extensive niche than 

 anyone had suspected. The animals spent most of 

 their time in surface waters, but they also dove oc- 

 casionally to depths of more than 3,200 feet, where 

 temperatures drop as low as forty degrees Fahren- 

 heit — a big change from the balmy eighty-four- 

 degree waters at the surface. Why do they dive? Per- 

 haps the sharks need to cool off, or perhaps they are 

 feeding on some unknown, deepwater prey. 



Since they spend so much time near the surface, 

 though, we realized we might track them much 

 more precisely with a different kind of tag: a satel- 

 lite-linked radio transmitter. With such a transmit- 

 ter, an animal's position can be determined to with- 

 in a mile anytime the transmitter's antenna is above 

 the sea surface. In 2005, with the help of John D. 



NATURAL HISTORY April 2006 



