I have wondered about this often with cetaceans. I feel like personhood is such a
human concept and completely depends on who is defining it. It calls into question, "What is personhood to begin with?"
There have many documented cases of dolphins rescuing drowning human swimmers or protecting surfers from sharks. I feel like it takes a good amount of intelligence and empathy-like qualities for dolphins to be able to recognize the peril of a human creature--foreign to their natural environment--and decide to treat him as one of their own where there is no obvious benefit for the dolphin.
My opinion is that we should give them the benefit of the doubt when it comes to personhood.
EDIT: This video of killer whales is also perplexing. https://www.youtube.com/watch?v=DWsN63PRCW8
A group of killer whales beach themselves to grab some sea lion pups for lunch, then proceed to toss them around and eat them. There was one seal they decided not to eat so one of the killer whales beached himself again in order to release it safely on land. It really does feel like there's complex mind behind this behavior.