That assumes that people would recognize them as sapient. It wasn't until extremely recently that people acknowledged that nonhuman animals can even feel pain. The prevailing argument for the vast majority of human history was "they don't really feel pain, they only look like they do but really they're just faking it."
Then look forward at how people react to things like AI in speculative fiction. Modern audiences frequently get hung up on the idea that there might possibly be an artificial intelligence at some point in the future that is on the level of a human. No matter how intelligent or self aware they are depicted, you always get a sizable chunk of the audience who says "they're just machines, they're not really people, they're just good at pretending to be intelligent beings."
If we found a species of mermaids, there would almost certainly be plenty of people willing and even eager to kill and eat them. Some people might think that's awful, sure, but there would never be a shortage of people who proclaim that any appearance of intelligence or self awareness by those mermaids is really just a ruse.