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What mechanism allows anadromous fish (like salmon) to switch from salt to fresh water and back? How does the fish "know" to switch its physiology and do other fish possess the biochemical machinery for this? How difficult would it be to genetically engineer salt tolerance into a freshwater fish?

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Osmoregulation is a homeostasis regulation mechanism followed by salmons to adapt on both salt and fresh water.

Osmoregulation is the active regulation of the osmotic pressure of an organism's fluids to maintain the homeostasis of the organism's water content; that is, it keeps the organism's fluids from becoming too diluted or too concentrated. Osmotic pressure is a measure of the tendency of water to move into one solution from another by osmosis. The higher the osmotic pressure of a solution, the more water tends to move into it. Pressure must be exerted on the hypertonic side of a selectively permeable membrane to prevent diffusion of water by osmosis from the side containing pure water.

In order to reduce the dehydrating effects of salt water, salmons drink several liters of water per day, but in fresh water salmons doesn't drinks at all. The only water it consumes is that which goes down when it feeds. When an ocean-dwelling salmon drinks, it takes in a lot of salt(NaCl), which exacerbates the salt-loading problem. In fresh water, the salmon's kidneys produce large volumes of dilute urine, while in the ocean , the kidneys' urine production rates drop dramatically and produce a very concentrated urine.

Salmon use to deal with the NaCl fluxes driven by the gradients between the salmon and its surroundings. In their gill epithelial cells, salmon have a special enzyme that hydrolyzes ATP and uses the released energy to actively transport both Na+ and Cl- against their concentration gradients. In the ocean, these Na+-Cl- ATPase molecules 'pump' Na+ and Cl- out of the salmon's blood into the salt water flowing over the gills, thereby causing NaCl to be lost to the water and offsetting the continuous influx of NaCl. In fresh water, these same Na+-Cl- ATPase molecules 'pump' Na+ and Cl- out of the water flowing over the gills and into the salmon's blood, thereby offsetting the continuous diffusion-driven loss of NaCl that the salmon is subject to in fresh water habitats with their vanishingly low NaCl concentrations.

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