Level up by catching fish. Be careful though: A boss fish lives in every level.
Dodge incoming attacks or you have to start all over again. As you level up the boss will change its color. Once this big boy turns green you are able to harvest him as well.
Use WASD to navigate around the world.
Press J to reach out your chalice. Every fish within radius will be harvested. Additionally, your character will become invincible for a brief moment. Use it to dodge incoming attacks.