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One way is to use purse seine nets , it's a real good and effective way. Another way is to use small fish, live or dead on a hook and line in deep water over reefs. Dynamite is good also, especially by a coral reef.
You could use a big beetle, but the best kind of bait would be a small fish, such as a sardine, because tuna are big fish that eat smaller fish when in the wild.
you can catch them on the wharf in karmja if you are a non member or in catherby if you are a member. it is better in catherby as it is close to a bank