The sardine run is a unique and unexplained natural phenomenon of mass migration. The shoals of Sardinops sagax, visible on satellite photographs of earth, can stretch for up
…to 15km in length, with a width of 3,5km and a depth of 40 metres. Within the shoals the fish pack close together relying on the principle of safety in numbers for their survival. The spectacle of millions of tiny red-eyed fish (also known as pilchards) travelling in a tight shoals into the warm sub-tropical waters of the Indian Ocean causes a frenzy amongst all who come into contact with them. The mere presence of so many sardines attracts a frenetic following of predators - birds, dolphins, sharks, whales and game fish - all diving and darting into the shoals in their hundreds. .
This in formation is taken from: http://sharondavis.co.za/content/view/59/3/ ( Full Answer )