From the back of the book:
Welcome to Anglers from Planet X, the free fishing minigame supplement for the amazingly weird roleplaying game of anime comedy action: Teenagers from Outer Space.
Inside the pages of this carefully crafted document you will find everything you need so that your Teenagers can go fishing in freshwater, saltwater, alienwater, or the school fish tank!
After all, Teenagers from Outer Space is (sorta kinda) a Japanese (inspired) roleplaying game! And what sort of JRPG doesn’t have a fishing minigame?