Freshwater Fish

• Catfish are a freshwater fish.Catfish image by Pavol Kmeto from Fotolia.com
  
  Freshwater fish have a high content of salt in their bodies. As they take in water, they retain all the salt they can and excrete excess water through their urine. This process is known as osmoregulation.
  
  

Saltwater Fish

• Tuna are saltwater fish.tuna image by Amjad Shihab from Fotolia.com
  
  Saltwater fish do not need a lot of salt in their bodies because of the environment that they live in. Their bodies consist mostly of water. As they take in water, they excrete the excess salt through their urine and retain the water. The regulation of salt and water in their bodies is also known as osmoregulation.
  
  

Environment

• If a fish is put into an environment that it does not belong in, the osmoregulation system breaks down. Freshwater fish are not made to excrete salt, so if they are put into saltwater, they will retain too much salt and die of salt poisoning. Saltwater fish are not made to excrete water, so if they are put into freshwater, they will become waterlogged and die.
  
  

Exceptions

• The eel can adapt to any environment, from a pond to an ocean. Some fish --- like the redfish and sea trout --- also live in estuaries, environments where freshwater meets saltwater and where the amount of salt fluctuates. Anadromous fish, including salmon and shad, live in saltwater but swim up freshwater streams to mate.