I heard a fun question the other day which caused me to write it down for a Daily Doodle. "Do fish yawn?" I know, great question!
Fish do open their mouths sometimes, but that is typically to attract mates or deter aggressors. They do not breath in the same way we do (obviously), since they do not live in an environment of air. Water passes through gills, allowing the fish to absorb the oxygen. This means that they need to open their mouths and swim in order to increase their oxygen saturation. So no, Fish do not yawn, at least not in the way we do.
I tried to keep it short, but if you're an empathetic person like me, you've already yawned just reading about yawns... sorry.