Freshwater fish in salt water
What would actually happen if a fresh water fish had to live in salt water?
For most fish, they would die. But some, like eels and salmon, can move
freely between the two at certain stages of their lives. To do this they have special
mechanisms of excretion and absorption of salt and water.
If you put a freshwater fish into saltwater, most fish would lose weight
(from losing water from its body) and eventually die. Approximately 2% of all 21000
species of fish actually move from freshwater to saltwater or from salt to fresh at some
point in their lives, the move would kill any other fish. But even with these special
varieties of fish, the move must be gradual so their bodies can adjust, or they too,
will die from the change. If you want to learn more about why the freshwater fish will
lose water, (or why a saltwater fish in freshwater would gain water), look up the words
"diffusion" and "osmosis".
Tom F Ihde
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Update: June 2012