The purpose of harpooning the tuna is to catch the fish. The harpoon is a pointed weapon that is thrown or projected, especially with the purpose of wounding or killing an animal. In the context of fishing, a harpoon is a long spear-like instrument that is used to catch large fish, such as tuna. The harpoon is attached to a long line, and when the tuna bites onto the bait, the fisherman pulls on the line and retracts the harpoon, piercing the fish with the sharp point.