This wasn't the first merperson you'd have to deal with. It certainly wouldn't be the last.

You were wandering along the beach, savoring what had been a rather calm, uneventful day, when a rattling hiss startled you out of your thoughts. It took a while to spot the source of the sound, but you eventually found it-- a large, black and white merman, beached on the shoreline. Its fins were raised and eyes narrowed, sharp teeth bared as it prepared to snarl again. "Leave," it barked out.