They use clicks to rebound off of objests and that tells them if they have something in front of them like boats,fish, and other things. Yes, there are 5 differents kinds of dolphins that live in 5 different fresh water areas. The rest of them live in the salt waters. Schools of tuna usually stay underneath of the pods of dolphins. Tuna fisheries put out their nets for the tuna and sometimes dolphins get tangled up in them and can die.
Man is the dolphins greatest predator.
They are usually killed by man from the nets of tuna fisheries of for gastronomy. After man comes the shark, and third is other dolphins. Dolphins are carnivores. They usually eat some fish and squid. The differnt foods they eat are according to where they live and their ecosystems.