If a fish is caught, it will be thrown back.
In most cases, the catch will not even be a fish, but instead a humorous item that will slightly disappoint the Sim, such as an old, worn-out boot. Fishing in The Sims is not tied to any skills or talent badges-whether the catch is 'good' or 'bad' seems to be random.
In order to fish, Sims need to be standing on a fishing pier, and there needs to be a rental shack, so they can rent fishing poles. Fishing is an object-based interaction in The Sims: Vacation, and differs significantly from the same activity in The Sims 2: Seasons and The Sims 3.