they're attracted not by one another but by easy food?
Basking sharks are the world's second largest fish, growing as long as 32 feet and weighing more than five tons. They are highly migratory, slow-moving animals often sighted close to the surface with their large mouths open to filter zooplankton from seawater. They are considered passive and no danger to humans other than that posed by their large size and rough skin. They and the larger whale shark, along with the megamouth shark, are the three shark species that eat plankton. -- ScienceBlog
