MarnaLou --
Yellow fin tuna swim in schools below the surface of the sea, and often have dolphins swimming above them. The commercial fishers aren't trying to catch dolphins, but when they're netting yellow fins inevitably they get some dolphins too. The dolphins die.
Albacore tuna schools behave in a different fashion (I don't remember the details) and when they are caught typically no dolphins are also in the nets.
-- David
