The large number of Cownose rays happens every year in the same spot, but does not last long. They gather together and once the group hits a certain size, they migrate. A group of rays is called a fever.
LONDON - APRIL 07: A Scuba diver trys to touch a Cownose stingray at the Sea-Life London Aquarium on April 7, 2009 in London, England. The newly refurbished aquarium costing 5 million GBP houses thousands of creatures representing 500 species including the world's largest collection of Cownose rays and over 10 species of shark. Housed in 2 million litres of water with 65 displays the Aquarium is also a leading centre of marine conservation in the UK. (Photo by Peter Macdiarmid/Getty Images)