No, because it said mag. If Sie was the subject it would say mögen instead, because the verb form changes to signify the subject in German. That's why all the words in a sentence can be in a very awkward or illogical order and a German speaker can still understand precisely what's going on.
How can I tell that "sie" in this instance is "you" and not "her" or "they"? I can see that "der Fisch" is the subject, so "mag" is conjugated to match "der Fisch". Just hearing "sie" I can't tell if it is capitalized or not. Is there another clue in the sentence that I am missing?
I had the German sentence given and had to translate it to English... It was actually capitalized, so it means "The fish does not like you (formal)". If it was not, it would mean "The fish does not like her". It could be "Der Fish mag dich nicht" instead as well, that would mean the same as the first one, but informal.