RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Raw Tuna
The Old School RuneScape Wiki also has an article on Raw Tuna.

Raw tuna are a type of fish which are caught from fishing with a harpoon at level 35 fishing, granting 80 fishing experience. With level 30 cooking they can be cooked on a fire or range to make tuna, yielding 100 cooking experience, and healing 10 hits when eaten.

Tuna can be caught at the following locations: