The distribution of tuna in relation to oceanographic conditions

[First paragraphs...]
Four species commonly occur-yellowfin tuna, Thunnus (Neothunnus) albacares; skipjack, Katsuwonus pelamis; albacore, Thunnus germo; and southern bluefin tuna, Thunnus thynnus maccoyii.
All these are pelagic oceanic species which occur, within their specific temperature ranges, mainly in the tropical and sub-tropical belts. Their presence is also dependent on suitable salinities and water clarity, and their greatest concentrations are generally near current convergences and upwellings where there is usually abundant food. Below the surface layer is the sharp temperature gradient of the oceanic thermocline which acts as a depth barrier to these fish. Water clarity is important, because the tuna largely depend on their keen sight when hunting for food. Thus the clear blue oceanic water is more favourable to them than opaque inshore water.