Perhaps this is the best way to legally consider dolphins. They have approximately the same rights as human children, until any one of them decides to ask for more.
Rules of thumb regarding treatment of immature or disabled humans is not at all appropriate.
Now, will dolphins ever communicate their desire to achieve greater legal consideration? Not unless we put some more effort into researching communication with dolphins (and even then, I strongly suspect not). Not really a problem though I think.
You miss my point. It's not a requirement for any individual, but it IS a requirement for members of a species in general. Most humans are functioning members of society. The ones that aren't, whether it's temporary or permanent are protected implicitly and explicitly through social contract and the laws we've created.
No dolphin will ever be a functioning member of our society, thus dolphins are not part of society, thus dolphins do not take on the responsibilities of being in a relationship with humans, thus dolphins are not entitled to parallel status.
Is it? Considering we have never before in recorded history extended such consideration to another species, it seems unlikely that there are existing standards we can look to.
There are many things you can observe "most" humans doing, but that does not mean those things are all prerequisites for special legal and ethical consideration.
How can you possibly make that sort of generalization when there is only one species that is commonly accepted as having rights?
We can do whatever we want. We can give citizenship rights to gummy bears because they look like real bears and bears have two arms and two legs just like people. It would be illogical and counterproductive to any real advancement for society, though.
Exhibiting some rudimentary social behaviors doesn't qualify as accepting the responsibilities along with the rights accorded with status.
What does that even mean?
Participates in one.
We are proposing a re-definition.
That's not in the least what I'm suggesting.
Dolphins cannot participate in our society by agreement. They don't understand what participating is. They're captured or born in captivity and then trained to perform a few tricks. They are beautiful and fascinating, but they have no contributions to make to the fiber of our society that can't be made by other animals, machines, or even simple objects.
They are able to do nothing further. Giving them a new status is just semantics. They are fundamentally not functioning members of our society. Not individually. Not as a species.
Poor people don't contribute to our fiber of society in a way that can't be replaced by machines, said Mitt Romney.
It has nothing to do with how capable she is of accepting responsibilities in her current state.
Non-humans have no such implicit acceptance of responsibilities and inclusion within the framework of human society by humans.
"I'd like to share a revelation that I've had during my time here. It came to me when I tried to classify your species and I realized that you're not actually mammals. Every mammal on this planet instinctively develops a natural equilibrium with the surrounding environment but you humans do not. You move to an area and you multiply and multiply until every natural resource is consumed and the only way you can survive is to spread to another area. There is another organism on this planet that follows the same pattern. Do you know what it is? A virus. Human beings are a disease, a cancer of this planet. You're a plague and we are the cure." 
There is no chance any animal/fish is going to survive the brunt of us 9 billion in next fifty years.
I won't be so sure considering that unlike a Virus we are aware of the damage we are causing and are already trying to mitigate it.
In this dynamic, we as a virus race will choose the easiest way out even at the cost of all the species of this planet. 9 billion of us will, mark my words, only kill, eat or ridicule whatever that is left of the ecosystem today.
I am actually afraid about cannibalism picking up too in distant hungry countries which are already off balance today. It's pretty grim.
Dolphins will continue to be enslaved, slaughtered or even ridiculed for any "equal treatment" justified exactly the way this thread shows. Ground reality will of course be worse.
To which I do agree with your statement that "we are aware" and that there is an exit for us even though some will definitely try to mitigate, protect and stay behind.
I'll take that bet.