One waypoint on the road to perfection in Stardew Valley is completing the fish collection. If you find every last one, Willy will send you a stardrop in the mail the next day. On top of that, collecting every fish is an entry in Qi's Perfection Tracker, and it unlocks the Master Angler achievement.
Stardew Valley: How to Catch Every Fish
With a good lure and a few crab pots, anyone can fill in the fish list of Stardew Valley.