Migratory fishes: Osmotic independence can be maintained only if the salinity of the external medium remains constant. If the salinity of this external medium fluctuates unduly, then the animal’s ability to osmoregulate breaks down. However, a notable exception to this are the migratory fishes which can continue to osmoregulate even when the external medium fluctuates drastically.
Osmoregulation in migratory fishes such as salmon is quite remarkable. This extraordinary fish spends the first three years of its life in fresh water after which it moves down to the sea. When it is full-grown, which takes about two years later, it makes its way upstream to spawn, after which it usually returns to the sea again. Three times in the course of its life, the salmon undertakes a dramatic journey from one extreme of osmotic environment to another. The way and manner the salmon navigates through this is still under investigation but it is presumed that it involves changes in the filtration rate of the kidney, and a reversal of the direction in which the chloride secretory cells transfer salt. From moving salts inwards in fresh water they must change to moving them outwards in sea water.
Similar events occur in eels. Here spawning takes place in the sea. After about three years, the young eels, or elvers, swim upstream to fresh water where they complete their growth. When mature, the adults migrate downstream to the sea again. As with the salmon, these migrations necessitate osmoregulatory devices which can adjust to changing conditions.