Ethics question: To kill a dolphin

See if you can help me figure this one out.

Question 1: Is it morally acceptable to kill a dolphin for food?

If you answer no:
Should a person then be able to be convicted of homicide for killing a dolphin?
Is it morally acceptable to kill a whale? If you said no again, how about a monkey? a wolf? a cow? Is there an IQ cut-off for what is acceptable to kill and what is not? Is there any other rubric we can use? Skip down to the discussion.

If you answered yes:
Consider the next question....


Question 2: What if it were determined that a dolphin is every bit as intelligent and sentient as a human? Would it be morally OK to kill a dolphin for food then?

If you answered yes:
Is it really OK to kill another sentient animal? If we were visited by an elite race of aliens, wouldn't we want them to consider us as equals and not kill us? Wouldn't we protest if they killed us for food?

If you answered no:
You seem to have decided that the cut-off for ethical killing is sentience: thou shalt not kill another sentient being. But what if we discovered that dolphins were about as smart as a mentally retarded adult? Is it OK then? What if they had the mental capacity of a 12-year old? A 5 year-old? A 2 year-old? Is there a cut-off IQ score below which it is OK to kill something? Is it then acceptable to kill a human being below that mark?


Discussion
My point is that there is a continuum of intelligence in the world. A mouse is somewhat more intelligent than a grasshopper, which is somewhat smarter than an tick, etc. As I said above, were we to encounter another sentient race we would certainly want them to consider killing one of us to be homicide, a crime of equal severity as killing one of their own. Therefore, if a dolphin can be proved by some measure to be sentient, shouldn't we want to extend that same moral maxim to the killing of dolphins?

All of this begs the question: where exactly is the line of sentience? Dolphins probably have a language that is as sophisticated as our own: there are 23 known dialects of dolphin-speak, and a dolphin can convey and understand much more information than a person (Aside: a much bigger portion of a dolphin's brain is devoted to speech, and they can therefore absorb much more information. A short series of clicks and whistles can likely convey as much information to a dolphin as a novel to a human).

Is there some rubric by which we can determine sentience? Can we establish a cut-off point below which killing is ethically acceptable? Should people spend a life in jail for killing a dolphin?

Please share any insight you might have to this question.