The Fishing Event was an April Fools related event released in 2011. It consisted of fishing on the map called map 777. Instead of trying to get the cheese and take it back to the hole, players had the full two minutes to attempt to fish in the three separate fish bowls for either extra cheese or a prize.
The first bowl available to players is on the lower right hand side of the map, by fishing in here you may receive either Starfish Earrings or a Bony Fish (fish skeleton). In the second bowl to the left hand side of the map you may receive a Fish Hat or Cat Ears, and in the final bowl to the upper right hand side you may receive Fish Earrings or a Fish Bowl Hat.
Acquiring any of the items from the bowls isn't definite, instead of receive a prize players may just receive cheese.
In France April Fools is known as "Poisson d'Avril", or "April Fish", hence why this event's content is related to fishing and fish items.