Big fish was removed from Old School RuneScape after an update.
The Big fish is a random event. While fishing, a large fish would sometimes appear in a fishing spot and eat the player's fishing equipment, then spit it somewhere nearby, usually 3 to 10 squares away. When this happened, the player had to find their equipment quickly before someone else saw it and took it or it would disappear. When the player had found the equipment, then they could go back to fishing. When this event happened, the fishing spot would usually move to a new location as well.