Maintaining the proper level of salt within the body of a fish is critical to the health of any fish. Saltwater fish have a particular problem to deal with because sea water contains a much higher concentration of salt than the salt that is inside the body of a fish. This causes a tendency for salt to leak inside the fish and upset the internal body process. To combat this ‘leakage’ fish that live in salt water have very efficient kidneys and ‘ion pumps’ in their gills to collect the extra salt that has leaked in and excrete it back into the sea.

Some fish like the majestic Atlantic Salmon and the slippery European Eel have ‘multi-purpose’ kidneys and ‘ion pumps’ and are able to live comfortably in either freshwater or saltwater. Most fish do not have this ability and are only able to live in either freshwater or saltwater.

Saltwater fish are found in the open seas and inshore areas such as rock pools and in river estuaries where currents of freshwater mix with currents of saltwater to form a murky muddy mix. These estuary fish also have some ‘limited’ ability to cope with variable degrees of saltiness.