text: Most vertebrates have their own internal milieu, that is, we maintain constant concentrations of ions and water that are different from those of the environment. This is achieved by means of the osmoregulatory system, which includes the kidneys but also, in the case of fishes, special excretory tissues in the gill area.\nSome fishes (cartilaginous ones) do indeed maintain a high internal concentration of salts as part of their osmoregulatory mechanism:  it's not sodium chloride, however, but urea, and therefore their meat is stronger-tasting and has a salty smell.