ABSTRACT
Two major groups of fish can be distinguished: the first is made up of rays and sharks and is of no concern to the fish farmer. Fish are found from surface waters down to the depths of the oceans, from coasts to the open sea and in all fresh waters. White muscles characterize sedentary fish while red muscles are found in fish which are permanently swimming. Fish mouths can open to a greater or lesser degree depending on the species and on the alimentary regime. Fish are immersed in a medium where the level of dissolved salts is most frequently different from that of their internal environment, particularly that of the blood; different mechanisms allow them to regulate their osmotic pressure. In continental Europe, where the culture of fish in fresh water and traditional mollusc culture are long-established activities, most of the available sites are already in use.
