There aren't many Atka Mackerels in the Eltro Sea, but that may be a good thing since they feed on smaller fish. Although being rare, most Atka Mackerels are caught in Ocean Water or Reefs no deeper than 100-200m and are tasty no matter how you cook them.
Atka Mackerel can be caught from various fishing locations throughout the game.
- Sea of Olvia: Atka Mackerel can be caught off the coast due East of the Olvia Coast Node, in the Sea of Olvia.
- Luivano Island: Atka Mackerel can also be found in the Luivano Island's, near the Node Manager.
NOTE: These are just the locations that I have personally found the Atka Mackerel, feel free to edit it to provide more Locations and Information.