Tunas are quite hard fishes to catch as the fishing rod will turn red pretty quickly and several times while you're trying to catch them. They also have a ton of strength and will swiftly pull the line away while it's red.

So how do you get them?

There's a simple trick that the game never tells you, while the fishing rod is red you can't hold the button, but can still tap it rapidly to keep the fish close while you wait for it to turn yellow again. Simply do this and tap as fast as you can while it's red to avoid the tuna/ strong fish getting too far away and without risking the line breaking.

And voila, all the tuna and rare fishes you need! Hope this helps.