To catch fish in Castaway, you have to click on the bobber in the square once the fish has nibbled on the bait long enough start the fishing mini game. You want to click on the bobber in the square while the left-right slider is in the green. For the tougher catches, you will need to give the left-right slider lead. If you have any further questions on fishing in Castaways, please watch a Let's Play of the game where they are doing fishing. That should clear up any issues you have. Enjoy your games!

Thank you for your kind comment on my first workshop upload! :) Means alot!
I also did my first map aswell, based on my own IRL city, take a look and toss in a comment or give it a thimbs up! :D