Sardines eat zooplankton, which is also known as "animal plankton" and floats around within the sea. Zooplankton feeds on plankton, which is the plant organism that is at the bottom of the ocean food chain.
Sardines threaten the ocean ecosystem, however, due to their massive population growth reports the Daily Mail. The reason for the population growth is that overfishing has lessened the populations of predatory fish in the oceans including salmon, North Atlantic cod, tuna and swordfish in addition to sharks. These marine animals eat sardines, but with less of them swimming in the ocean the sardines are living longer and reproducing longer without any threats according to the Pacific Fishery Management Capitol.
Sardines are small fish that can live up to 13 years in the wild, though they tend to live approximately 5 years. Interestingly, sardines also perform a sardine run whenever the cool water current changes according to the National Geographic. During this run, the sardines are looking to travel hundreds of miles through the coast, and many predators come out of hiding to eat as much of the sardines as they can. It is a feeding celebration for predators because the sardines swim tightly and thickly packed in groups of thousands.