Is it possible to play solo? Without a GM.
Yes, with a few small adjustments.
During multiplayer games, you don't usually know what sort of fish is at the end of your line unless you take a specific power, but in solo there's no way to conceal that information from yourself, so you should play as if you have that power.
The game also uses a rock-paper-scissors mechanic, but you can roll randomly to determine which sign the fish throws.