Tuna ocean sea underwater fish fishes 12. Learn more about life in the sea and the challenges facing our oceans. The southern bluefin tuna thunnus maccoyii is a tuna of the family scombridae found in open southern hemisphere waters of all the worlds oceans mainly between 300s and 500s to nearly 600sat up to 25 metres 82 ft and weighing up to 260 kilograms 570 lb it is among the larger bony fishes. Coastal fish can be contrasted with oceanic fish or offshore fish which inhabit the deep.