We don't kill humans because we're humans. Dolphins are not humans. Your arguments about their levels of sentience are still basically opinions based on research, no one can prove a dolphin is sentient. The whole sex thing I won't even bother with, because it's unproveable. There's plenty of research that will claim lots of things.. need I quote the research showing pythons taking over the US?

Most research studies are biased according to what the researcher wants to prove. No, it's not the way research is supposed to go, just like interpeting data goes according to what result you want to get in many cases. Elephants are said to be sentient.. then great apes.. or whichever super special animal you're fond of. Just because they are more intelligent than most animals.. or show some signs of self-awareness.. still doesn't mean they are equivalent to a human being.

Dolphins seem to have a language.. seem to be very intelligent.. seem to be a lot. But the proof isn't there. Extra brain activity was supposed to be their echolocation being used and interpted in the last study on dolphin brain waves that *I* read.

Until they are proved to be sentient people.. they're animals. No matter how cool.

Oh.. and I would not use bison as an example of sustainable harvesting. They were taken to the brink of extinction and saved through more luck than anything else. People did not think about their numbers, reproductive rates etc.. then say "We should kill xx bison a year". They wholesale salughtered until there weren't any.