Certain fish have this internal ability, Salmon is a prime exaple. King salmon live in the pacific ocean, Yet you can catch all you want in lake Michigan. Its usually part of the life cycle to be in fresh water at some stage of life, but if land locked they can not get to the salty water and just eat and grow. They can not complete the life cylce and reproduce as they need the salt water for that. So they will be like HSB, put and take.....lots of coastal fish have this ability but seems to not have been explored very much. little differant than salmon, they hatch in salt water migrate to brackish and fresh water estuaries as juviniles and once mature move out to larger more salty water again. In the St johns river certain salt water fish (even adults) can be found all the way down to orlando area around the head water of the river.
see the salinity tolerance range on the southern flounder here. 0- 35 ‰
http://www.sms.si.edu/IRLSpec/Parali_lethos.htm
