Final answer:
Fresh-water fish are considered osmoregulators as they actively manage their internal salt concentrations and water balance, regardless of the lower solute concentration in their surrounding freshwater environment.
Step-by-step explanation:
Fresh-water fish are considered osmoregulators because their tissue concentration of solutes exceeds those of the surrounding water. Osmoregulation is a physiological process by which organisms maintain water balance and the concentration of solutes within their bodies, irrespective of the surrounding environment. Unlike osmoconformers, which match the osmotic pressure of their environment, osmoregulators actively control the concentration of salts and other solutes in their tissues.
In a hypotonic freshwater environment, freshwater fish conserve solutes and excrete excess water due to the lower solute concentration in the water relative to their tissues. They achieve this by actively taking in salts through their gills and excreting dilute urine. Conversely, in a hypertonic marine environment, saltwater fish face the opposite challenge and must excrete excess salts through their gills while conserving water.
Organisms like salmon are special cases; they are euryhaline, meaning they can tolerate a wide range of salinity and are able to osmoregulate in both fresh and saltwater environments. To adapt to the changing environments, salmon undergo hormonal changes that trigger the necessary osmoregulatory responses.