The kidneys are one of the body organs involved in excretion and regulation of the water balance within the fish.
The kidneys are paired organs located in the body cavity either side of the backbone. Seawater contains more dissolved salts than the body a fish (and freshwater contains less). This means that marine and freshwater fishes have different problems with regard to regulating the concentration of salts within their bodies. Their kidneys differ considerably in structure.
Mark McGrouther , Collection Manager, Ichthyology