When I was young I read a SF short story about a man traveling into the future to (IIRC) see the effects of a nuclear test or something of the like. He encounters a race of "mermaids" that have evolved from the fallout from the nuclear war that resulted from the tests.
Edit 2: I distinctly know that the protagonist was a time traveler, because (spoiler) the "mermaids" sabotaged his time machine at the end of the story to prevent him from going back in time and warn his people of the effects of the nuclear war, which would cause them to never exist. I wasn't myself for several days after reading that ending :)
I have no idea what it is called or who wrote it, so any tips are much appreciated!
Edit 1: I read it in the 90s, don't remember when more accurately sadly. It was in a book of short stories translated to Swedish.