The diet of the juvenile individuals of the whale sharks is unknown. However, it is assumed that they feed on small teleost fishes and zooplankton. It is estimated that a young individual can feed up to 46 pounds of food each day. The large individuals prey on schooling teleosts (of all sizes) and crab larvae. They also prey on squids, anchovies, krill, copepods, jellyfish, mackerels, fish eggs, sardines, small tunas and other small nektonic organisms. Also they may feed on macroalgae and phytoplankton.
It is known as a filter-feeder because it uses a special technique called filter-feeding method. In this method, the shark separates suspended food particles from water as the water flows over its spongy filter plates. Megamouth and Basking sharks are the only other two species (of sharks) that use this technique. Amusingly, its teeth are not busy during feeding but remain at rest. This is because it uses either of the following two techniques in feeding i.e. ram filtration or active suction feeding.
In the former technique, a shark goes forward but stays at constant speed and keep its mouth open while sucking food-bearing water into its mouth. This method is also known as passive feeding.
While in the latter case, the shark opens its mouth as well as shuts it down by gulping down huge volumes of water, which is then released by means of its gills. As more and more food particles build up on filter pads, they block these pads. For that, whale sharks are often seen to do ‘coughing’ in order to cast off these particles.