Look for tins of tuna with 'dalphin-friendly' on the label. Find out what that means. Can they show that they do not kill dolphins when they catch tuna? write to them or look at their website.
Organizations like Greenpeace, Defenders of Wildlife, and Friends of the Earth have lots of information about dolphins, and they have ideas about what you can do to help protect them, write to them or look at their websites.
Stop eating tuna! Or if you really like tuna, make sure that you only eat tins of dolphin-friendly tuna.