Why can't most fresh water fish live in salt water and most salt water fish not in fresh water?
by 
wickedwillie
ANSWERS: 5
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Just like any other creature on earth, animals have evolved and adapted to their environment. The ocean is quite huge and therefore a very stable environment. Salinity, pH, and minerals change very little in such a large volume of water hence the fish have become very intolerant of changes. Bodies of fresh water are much smaller and more easily affected by the surrounding land and weather. Freshwater fish have learned to adapt to these changing conditions. Salt is a desiccant - it absorbs water. That's why we need salt to keep from dehydrating when we sweat a lot. If the water surrounding a fish contains too much salt, water transfers out of the fish. Freshwater fish have adapted to low salt conditions and will literally dehydrate in salty water. Putting too much salt in your saltwater aquarium will also dehydrate those fish.
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I don't have a quarrel with most of what the previous answer states. But, we don't need to consume salt to prevent dehydration. When we sweat we loose water and salt and allot of other nutrients. In most cases replacing the water is all that is needed. But in cases of extreme sweating it is beneficial to replace the salt and nutrients too. In answering the question about fresh and salt water fish, the term osmosis must be understood. Osmosis is a term used to describe the physical property exhibited when a difference in concentration exists on either side of a semipermeable membrane. This is stuff we learned in high school but forgot. Water will flow through the membrane in the direction that will try to equalize the differences in concentration. So, water will pass through the membrane, moving from the side of low concentration to the side with the higher concentration. This is the same mechanism that is working in your fish question. The salt water fish will die because too much water is entering it's body. The fresh water fish will loose water when placed in salt water.
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They are adapted to salt water conditions. There are many other minerals in salt water besides salt many species of fish need those other minerals in order to survive thus they die in fresh water. Salmon are an exception, they thrive both in sea and fresh water. There are many other exceptions as well, since a lot of fish live in coastal and delta and even river to sea waters.
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Their gills and bloodstream are not adapted to process saline water. The gills filter oxygen out of the water. A freshwater fish will die in saline water due to chemical imbalance in the bloodstream.
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Their gills and bloodstream are not adapted to process saline water. The gills filter oxygen out of the water. A freshwater fish will die in saline water due to chemical imbalance in the bloodstream.
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