Tunas are in Seas of Olvia area (anywhere between Marka and Arita Islands).

Found some in hotspot (with seagulls) south of Invernen Island

Can catch in Marka Island

Sea of Nox - right to Lema Island (spot).

Can also catch in the Sea of Banto

For harpooning (Magoria Sailboat quest) I found tuna and swordfish south of Nada Island, in the Juur Sea.

Screenshot of Map Position
Magoria Fish Loot Table by color code
Map of color codes

Воды Оливии в косяке рыб(чайки)

Nesse HOTSPOT pesca atum de primeira!!!

Trouver un banc de poisson avec les mouette prés de l'ile de Lema.
Voir ma capture @+

Se puede encontrar en las proximidades De Isla Lisz hacia Isla Narvo y Isla Marka.
El triangulo de los Atún :-)

Got it from fishing hot spot north of Invernen island.

Fish Guide -> Oceanic Fish -> page 1

Voici un endroit ou vous pouvez trouver un hotspot pour le thon !

Словила на острове Рандис.

Titres :
Amateur de thon (31)
Prince des thons (160)
Gourou des thons (434)