I'd be a lot more sympathetic to the cause of treating dolphins as if they had human rights if we first agreed to treat all humans as if they had human rights.
I am reminded of people who scoff at giving money to bums on the grounds that it is an inefficient form of charity. In most cases, here is what happens:
1. Bum asks X for spare change.
2. X thinks "That's not an efficient way to give. I would be better off donating the couple bucks in my pocket to a charity that I have researched carefully."
3. "Sorry I don't have anything on me."
4. X goes and spends the money on a cup of coffee and completely forgets about the incident.
Now, obviously that's not what always happens. But it's what usually happens, and it's ridiculous. I've had people tell me that instead of giving money to homeless people, I should go buy fast food and hand it out. Of course, when asked if they have ever done this themselves, the answer is invariably "no". It's so easy to do nothing instead of something if you can convince yourself that the something is not the absolute ideal.
Finally, affording dolphins human rights is not in conflict with affording humans human rights. We will not delay the process of preventing human rights abuses by also preventing the abuse of dolphins. To ask the question "How can we think about giving dolphins basic rights when so many humans don't have basic rights?" may feel righteous, but it certainly doesn't actually do anything to help either cause. All it does is to reassure yourself that you don't need to worry about the dolphin situation.