To set up, each player has three little fish in the first line of an empty sea tile, with the board being comprised of four boards pushed together to form a square. To become bigger, fish must eat plankton. When you move to a space where there is a fish of the same size or smaller than you, you capture it. Small fish are fast but vulnerable; they can capture small fish but can be captured by any fish, and they are the only ones that can go through shipwrecks. Medium fish can capture small and medium fish, and they can't go through shipwrecks. Big fish can capture fish of any size, and they can't go through shipwrecks.
On a turn, you move one of your fish two spaces or two of your fish one space each. Fish can move orthogonally, and you must resolve the action of a space before taking your second move. Move into a space with plankton, and your fish grows in size; land on a birth space, and you add a little fish from your reserve to the board. The shipwreck serves as a tunnel for small fish, and the "?" space makes you draw a tile, take its action, then discard it. You might rotate part of the board, put a fisherman to work, or have an unexpected birth. Race to eat five fish first!