Re: advisability and consequences of introducing freshwater fish into saltwater

Hi Charles!

Fish, like all multicellular organisms, must maintain "salt" (really 
meaning solute) and water balance in their cells.  Fish that live in fresh 
water are evolutionarily designed to excrete lots of the water they ingest 
while retaining salt avidly.  For saltwater fish, the evolutionary 
pressure is in the opposite direction:  they excrete lots of salt/solute 
and retain water avidly.  Most organisms that are able to excrete lots of 
salt/solute are able to adjust their salt/solute excretion rate downward, 
but those that do not excrete lots of salt/solute are unable to increase 
their excretion rate by a large amount.  Because of this principle, I 
would think that you could probably acclimate saltwater fish to fresh 
water, but it might be difficult to acclimate fresh water fish to salt 
water.  The fresh water fish may just not have the biological equipment to 
excrete the excess salt it would ingest and might therefore dehydrate.

I hope that this answered your question.
If it did not or if you have any further questions, please e-mail me at 
idodge@student.med.harvard.edu.

Have a good day!
Ingrid Dodge