Near the coast of California in the United States you can see a very big kind of fishing boat, which is more than sixty metres long. It sends a helicopter out to look for dolphins. When it finds them, the helicopter goes down near to the water. The dolphins, and the tuna fish that are with them, are afraid of the helicopter and they swim away from it- but in front of them is boat. When they are near the boat, the fishermen pull a big net around the dolphins and tuna, and close it like a big bag. The net is 1.6 kilometres long, and more than 100 metres deep. They pull the net onto the boat, drop the dolphins back into the water, and the tuna stay on the boat. But this often hurts or kills the dolphins. These big boats have killed at least seven million dolphins since the 1960s.