This is one of the stories, or mythologies as we call them, that lend life to the yoga poses we now know. When you heard this story, can you think of a detail that could be true, and even proven? Or do you dismiss the whole story without finding the nugget of truth or wisdom?
One thing is true-- and has been proven-- that is fish can hear. In mid 1930s, an Austrian biologist named Karl von Frisch conducted a study with a blind catfish named Xaverl. He would put food at a certain spot in Xaverl's clay shelter, and because Xaverl can smell it, he would soon swim over and eat his food. After a few days of establishing this routine, the biologist began to whistle before putting the food down. On the sixth day of this new routine, the biologist whistled, and even before putting the food down, Xaverl was already at the designated spot, proving that fish can hear sounds.
There is truth everywhere, whether it is that fish can hear, or animals can feel, or dairy is bad for our health, etc. But we must be sharp enough to pick up the truth, open enough to listen so we do not allow our prejudices to dismiss what could be important information, and humble enough to change our hearts and minds so that like Matsya, our listening could also lead us to enlightenment.