For what it's worth, I personally believe that dolphins, along with elephants, great apes and a few other animals, should receive protection similar to that of humans - that is, killing them should be considered a far greater offence than killing most other animals, and essentially defined as a murder. Or a manslaughter, etc.
The problem, of course, is where we would draw the line and how we'd be able to determine the extent of a given species' sapience. Without the ability to actually, definitely state how sapient these animals are - or even to know if a cross-species scale of sapience can be successfully drawn up - it's just a lot of guesswork and speculation. I freely admit that my personal ethical standards in this might seem arbitrary. Where's the cut-off point? I consider killing a dolphin or an elephant an act of great immorality, but what about, say, pigs? Pigs are often said to be very intelligent, and I quite happily support the slaughter of pigs so that I may feast upon their succulent flesh. And what about "outlier" intelligences like the octopus or the corvids?
It's an interesting dilemma.