Different levels of sub-surface salinity are available, including the surface layer. The salt concentration in the ocean is linked to the dissolved oxygen concentration. The dissolved oxygen is a limiting parameter of pelagic species presence.
Fishes have to make a biological effort to be in equilibrium with their surrounding conditions. Usually, every pelagic fish have very high metabolisms which mean that their oxygen consumption is high too. Dissolved oxygen is inversely proportional to salinity values.