When an animal the size of a very large double-decker bus makes a sudden 90[degrees] turn, it has to be for a good reason. As the satellite tracks started to come in from whale sharks that we had tagged off the Galapagos, they clearly showed that as the sharks were swimming away from the islands, they were all reaching a certain point and then making a very abrupt change in direction.
However, it wasn't until we overlaid the tracks onto a map of the sea floor that we could see that these movements were apparently in response to geological features deep in the ocean that the sharks couldn't possibly see. The more we analysed their movements, the more obvious it became that they were using seamounts, faults, fissures and even plate boundaries for navigation. But how?
RIGHT TO ROAM
Since the whale shark was first described in 1828, we've learnt little about its natural history. The largest fish in the ocean--growing to lengths of up to 18 metres and a weight of more than 30 tonnes--it's predominantly a filter feeder. Found in all the world's seas, apart from the Mediterranean, it's reputed to travel long distances, and while we know that it's ovoviviparous (giving birth to live young that hatch from eggs within the female), we don't know where it breeds or gives birth.
One of the more intriguing aspects of whale shark behaviour is their propensity to gather in large aggregations on a seasonal basis. Perhaps the most famous takes place around Ningaloo Reef off Australia's west coast. During the late 1970s, whale sharks were observed there in large numbers, feeding on fish larvae and the eggs from spawning corals.
Since then, about a dozen aggregation sites have been identified around the world. As well as around the Galapagos, these include areas off the coasts of India, Madagascar, Mozambique, the Philippines, Indonesia, Belize, Baja California and the Yucatan Peninsula in Mexico, where more than 400 individuals were spotted during one aerial survey.
The Galapagos Whale Shark Project (GWSP) was established to study the population of sharks that visits the islands each year between May and December. This period coincides with the cool season, when the rich Humboldt and Cromwell currents are at their strongest.
In more than two decades diving these waters, I've observed that the majority of sharks that congregate here are adult females ranging in length from about eight to 16 metres, and most appear to be in an advanced stage of pregnancy. There are very few sightings of males or juveniles, a factor that differentiates this from practically all the other known whale shark aggregations worldwide.
This is surprising in itself, but even more so is the fact that around 99 per cent of these sightings are at one geographical location in the far north of the archipelago--close to a tiny island called Darwin, with its renowned dive site, the Arch.
One of the GWSP's primary aims is to find out more about whale shark movements on a local, regional and, hopefully, global scale, as well as establishing their reasons for homing in on Darwin. With this in mind, this year, a group of scientists from the Galapagos National Park, the Charles Darwin Foundation, Conservation International, Ecuador, and the University of California, Davis--with funding from the Rapier Family Trust--undertook one of the most ambitious whale shark tagging programmes to date. The fieldwork, carried out in three 15-day sorties, involved attaching satellite tags to each individual we encountered, photo identification, laser telemetry and tissue sampling.
In order to identify individual sharks, we used a modified version of software initially developed by NASA for the mapping of constellations and deep-space objects. The characteristic white spots of the whale shark resemble the human fingerprint in that each pattern is unique to the individual. …