Only 1 Skipjack and 1 Bluefish required, not the 2 x indicated under the Conditions.

Bluefish can be fished in open waters in the Seas of Olvia, so around Marka Island but anywhere where it indicates that sea on the mini-map.

Skipjack can by caught in Balenos Islands open waters outside of Velia bay south east of Luivano Island and on around there. Or harpooned more often in Balenos Islands, for instance to the N of Mariveno Island in deep water.

He wants 2 skipjacks and 2 bluefishes (he accepts them one by one, but you still need 2 of each).

I recommend fishing in deep waters near Luivano Island in Balenos Islands area, you can get both bluefishes and skpijacks there.