A Harpoon is a tool used in the Fishing skill to fish in fishing spots with the Harpoon Fishing spot option. Harpoons can be used at Cage/Harpoon fishing spots and Net/Harpoon fishing spots. At Cage/Harpoon fishing spots, players with level 35 or higher Fishing can catch tuna for 80 experience each while harpooning. When players reach level 50 Fishing, they can also catch swordfish for 100 experience each when harpooning at Cage/Harpoon fishing spots. Members who have reached level 76 Fishing can harpoon shark for 110 experience each at Net/Harpoon fishing spots.
When a player is able to catch two types of fish with a harpoon (for example able to catch both tuna and swordfish), players will catch more of the fish which requires a lower fishing level (in this case tuna) than the fish that requires a higher fishing level to catch (the swordfish).
You may aquire a harpoon from any fishing store around RuneScape.