Some Fish Adjust To Varying Saline Levels

• Eels are among the fish that can live in either fresh or saltwater.eel image by Brett Bouwer from Fotolia.com
  
  No one is sure why some fish developed the ability to live in both freshwater and saltwater. What experts do know is that these mostly cold-blooded vertebrates live in environments that provide them with what they need to survive, whether that is food, stable temperatures or a way to escape predators. The biggest difference between freshwater and seawater environments is the amount salt present. But fish---like the other vertebrate animals, mammals, birds, reptiles and amphibians---have a 0.9 percent salt solution in their blood.
  
  

Scientists Have Theories On Why Oceans Are Salty

• Sea bass can also live in either freshwater or saltwater environments.sea bass with parsley image by Chef from Fotolia.com
  
  Scientists think that after the planet Earth was formed that it rained constantly. As water evaporated in the oceans, it came down onto the earth in the form of rain, dissolved minerals in the soils and then the water ran back into the seas, washing salts from the earth into the oceans, eventually making them saline, according to a paper by William A. Wurts, Ph.D., a state specialist for aquaculture at Kentucky State University Cooperative Extension Program.
  
  

Physiological Differences Allow Some Fish To Adapt

• Sturgeon are among the fish that have physiological mechanisms that allow them to live in either fresh or saltwater.hi sturgeon! image by alwayspp from Fotolia.com
  
  While fish that normally live in freshwater have physiological mechanisms to concentrate salts in their bodies when their environment does not offer them enough salt to maintain that 0.9 percent salt solution in their blood, fish that normally live in seawater have mechanisms that allow them excrete excess salts from their bodies when their environment is more saline than that.
  
  However, euryhaline fish species maintain both physiological mechanisms. That enables them to adapt to living in either freshwater or saltwater.
  
  

Freshwater Has 0.05% Salinity

• Bears hunt salmon in freshwater.i love salmon image by Chris Bibbo from Fotolia.com
  
  Freshwater rivers, streams, lakes and ponds generally have a saline level of about 0.05 percent. Therefore, living in that environment requires fish to have the physiological ability to concentrate salts within their bodies. Two bodily functions allow them to accomplish that task. Fish that can live in freshwater have gills that function to diffuse water while keeping bodily salts inside the fish. Also, living in freshwater requires fish to have well-developed kidneys because so much water must pass through them.
  
  

Seawater Averages 3.5% Salinity

• Seals hunt salmon in coastal waters.Sealion eating salmon image by Lori Pagel from Fotolia.com
  
  While freshwater has a salt level of about 0.05 percent, seawater has an average of 3.5 percent salinity. Brackish water, such as that found in coastal lagoons and estuaries where rivers and streams meet saltwater, has a salinity that is somewhere in between that of saltwater and freshwater, depending on conditions. That means fish living in salt water have to have some physiological mechanisms to get rid of excess salt in order to maintain blood levels of 0.9 percent salinity. They have to be able to drink saltwater and then excrete excess salt through their urine by having their kidneys produce small volumes of fluid that contains a high concentration of salt
  
  

Some Fish Migrate Between Marine and Freshwater

• Some fish that migrate between marine and freshwater environments spend most of their lives in seawater but go back to freshwater to reproduce, such as salmon or striped bass. But sea bass can also lay their eggs in seawater and the fish hunts for prey in both fresh and saltwater. Other migratory fish, such as eels, lay their eggs in saltwater but live in freshwater, while flounder live in the ocean or in the brackish water of coastal estuaries and lagoons. On the other hand, red drum fish live along coastal waters and lay eggs near the shoreline, juveniles generally grow up in bays or marshes. The sturgeon is similar to salmon as it usually spawns upstream in rivers. But sturgeon also spawns in brackish estuaries, and although they can tolerate a wide range of salinity, sturgeon usually live in brackish coastal waters.