The phenomenon is called Dolphin Assisted Fishing, and has been taking place in Brazil since 1847. The only other region in the world where dolphins are frequently helping fishermen catch fish, is along the coast of West Africa! So, how exactly are the dolphins helping the fishermen?
A group of dolphins will chase a school of fish towards the fishermen's boats. When the dolphins are ready, they roll onto their side, which the fishermen take as a signal to release their nets down to catch all the fish the dolphins have generously brought over to the boat. The dolphins get to have a feast on all the fish that are panicking and have successfully evaded the fishermen's nets!
What's truly amazing is that the dolphins haven't actually been TRAINED to do this by the fishermen, it's just something that the dolphins started doing! It's fascinating how intelligent dolphins are!