Whilst playing Sekiro: Shadows Die Twice you might have picked up some stray Treasure Carp Scales or even noticed that you can target the aquatic life in the game. Some of the aquatic life will kill you but some can be killed to obtain their scales. They only take a couple of hits but if they spot you they will try and swim away, they are not dissimilar to the scarabs from Dark Souls in this respect.
They can be handed into a guy who lives in a barrel next to the bridge in the Hirata Estates not too far from the Estate Path Idol. He will trade Treasure Carp scales for some interesting gear including new fighting techniques.
For the exact location of the vendor and where to get your first two Treasure Carp Scales check out the video below: