Among the unique adaptations of salmon are an ability to move between fresh and salt water, the preference to migrate while water is turbid to avoid predators, and the development of unique, local populations, with variations apparently favoring genetic diversity. Despite appearances, salmon are among the most highly evolved fish.
Salmon adapted to move from fresh water to salt water as young fry, then back into fresh water as spawning adults. This migration process is accomplished both by physiological and behavioral adaptations.
One of their most interesting adaptations is in the deceptively simple act of drinking water. As young fry leaving fresh water for the first time, salmon spend several weeks to months in brackish waters, gradually adapting to the greater salinity levels of the open ocean. As the seawater is much higher in sodium than their blood, they begin drinking copious amounts of water, yet produce far less urine, as their urine becomes highly concentrated to remove the salt. Conversely, when returning to their home river to spawn, they reverse the process, again spending several weeks in brackish water to re-adapt to fresh water. Even though they stop drinking water entirely, they urinate far more to prevent water loading.
A study reported in 2012 in the science journal Nature noted that Pacific pink salmon adapted to warmer water temperatures by migrating early. Scientists believe these actions may have a genetic as well as a behavioral basis.