Osmotic balance is critical for fishes to maintain, particularly those that move between waters with differing concentration gradients of solutes, particularly salts. Anadromous fishes such as many of the members of the family Salmonidae migrate between marine and freshwater habitats for reproduction, and provide insight into many aspects of osmoregulation in fishes. In thinking about the evolution of the bony fishes, which evidence suggest first emerged in freshwater habitats, a key question relates to how the bony fishes were able at some point to invade marine habitats. Forms that migrate between marine & freshwater habitats (such as Pacific Salmon) may provide some clues to that evolution.
For this reflection, speculate on whether the ancestral line of the Salmonid fishes were marine in origin or freshwater in origin. That is, did anadromy in the Pacific Salmon evolve from 1) an ancestor that lived in the ocean but at some point gave rise to fishes that lived in the ocean but began moving to freshwaters to reproduce or 2) an ancestor that lived in freshwater but at some point gave rise to fishes that lived in freshwater streams but made excursions into the ocean for periods before returning to the streams for reproduction?