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What is osmoregulation?

Osmoregulation is the control of the levels of water and mineral salts in the blood. All fish which migrate from fresh water to salt water during their life cycle must go through this process.

The salmon is an excellent osmoregulator. However, like virtually all osmoregulators, the salmon is never in true equilibrium with its surroundings. In the ocean, the salmon is bathed in a fluid that is roughly three times as concentrated as its body fluids, meaning that it will tend to lose water to its surroundings all of the time. And, because the composition of its body fluids is so different from the ocean water, the salmon will be faced with all manner of gradients that are driving exchanges that will continuously tend to drive its body fluids’ concentration and composition beyond homeostatic limits. In particular, the very high concentration of NaCl (sodium chloride) in the ocean water relative to its concentration in the salmon’s body fluids will result in a constant diffusion of NaCl into the salmon’s body. Unless dealt with effectively, this NaCl influx could kill the salmon in a short time. In sum, a salmon in the ocean is faced with the simultaneous problems of dehydration (much like a terrestrial animal) and salt loading.
However, in fresh water, the problem is basically reversed. Here, the salmon is bathed in a medium that is nearly devoid of ions, especially NaCl, and much more dilute than its body fluids. Therefore, the problems a salmon must deal with in fresh water environments are salt loss and water loading.