Shark sensing prey. Computer illustration of a sequence showing a shark sensing blood from a wounded fish and then catching that fish (lower right). Sharks are among the apex predators of the oceans, often the largest and most feared predators in the sea. The largest can reach a length of over 7 metres, and eat anything. Their main prey are large fish, seals and turtles. Many sharks are able to detect tiny traces of blood in the water and follow the blood to its source.