“Large epipelagic predator fish, like this Atlantic blackfin tuna, have a deeply forked tail and a smooth body shaped like a spindle tapered at both ends and countershaded with silvery colours.
Epipelagic fish tend to be found in coastal water above the continental shelves, where land runoff can provide nutrients, or in those parts of the ocean where up-welling moves nutrients into the area.
Predator fish are usually fusiform with large mouths, smooth bodies, and deeply forked tails.
Ocean waters that are exceptionally clear contain little food. Areas of high productivity tend to be somewhat turbid from plankton blooms. These attract the filter feeding plankton eaters, which in turn attract the higher predators. Tuna fishing tends to be optimum when water turbidity, measured by the maximum depth a secchi disc can be seen during a sunny day, is 15 to 35 metres.” Wikipedia