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.