Speartuna, also known as spearfishing tuna, can be found in various oceanic regions where tuna species are prevalent, such as the warm waters of the Atlantic, Pacific, and Indian Oceans. It is typically harvested by spearfishing, a method that involves diving underwater and using a spear gun to catch the fish. Local fishing communities or specialized seafood markets often sell speartuna, but it's essential to ensure that the fish is sourced sustainably. Additionally, some regions may have specific regulations regarding spearfishing to protect tuna populations.
First, you must be at least LV3 or higher to get speartuna. Go to the Gathering quest of Forest & Hills, then check the fishing spot right beside your camp spot (where you start when you initiate a Low Rank Quest) and there should be a giant fish there, biggest fish. If not, go to area 1 then come back. Keep doing this until you see the speartuna. For a higher chance of getting the speartuna, check high rank quests.
you can get some when you have ballista thingy upgrage on your fishing dock. you'll need a net though.