They are one of the organs involved in excretion and regulation of the water balance within the fish. Freshwater and marine fishes are faced with different problems with regard to regulating the concentration of salts within the body. Their kidneys differ considerably in structure.
Freshwater fishes have larger kidneys than marine fishes.
They have a higher concentration of salts in the body tissues than the surrounding water. Conversely marine fishes have a lower concentration of salts in the body tissues than the surrounding water. The kidneys of freshwater fishes remove water and re-absorb salts and sugars. They produce large amount of very dilute urine. The kidneys of marine fishes however conserve water.