Thousands of Sharks Gathering | Blue Planet | BBC Earth

A tiny island lost in the midst of the Pacific. It's the tip of a huge mountain that rises precipitously from the sea floor thousands of metres below. The nearest land is 300 miles away. Isolated sea mounts like this one create oases where life can flourish in the comparatively empty expanses of the open ocean. But all the creatures that swim beside it would not be here were it not for one key factor. The deep ocean currents. Far below the surface, they collide with the island's flanks and are deflected upwards, bringing with them from the depths a rich soup of nutrients. Such upwellings attract great concentrations of life. Most of the fish here are permanent residents. Feeding on the plankton, the tiny floating plants and animals that are nourished by the richness brought up from the depths. And they, in turn, attract visitors from the open ocean. All this action attracts even larger predators. Sharks. Hundreds of sharks. These silky sharks are normally ocean-going species, but the sea mounts in the eastern Pacific, like Cocos, Malpelo and the Galapagos, attract silkies in huge groups up to 500 strong. Silkies seem to specialise in taking injured fish and constantly circle sea mounts on the lookout for the chance to do so. But silkies are not the only visitors. Hammerheads gather in some of the largest shark shoals to be found anywhere in the ocean. Sometimes thousands will circle over a single sea mount. But these sharks are not here for food. They have come for another reason. Some of the locals provide a cleaning service. Following the last El Niño year, when a rise in water temperatures caused many sharks to suffer from fungal infections, the number of hammerheads visiting the sea mounts reached record levels.