It seems many pond owners in the southern half of the US could use southern flounder as a bonus fish for their ponds. Flounder have proven to thrive in pure fresh water and have pretty good cold tolerance.
I personally have seen southern flounder thriving and growing nicely in fresh water ponds for several years running. I have researched online and found several university programs have concluded the same result.
It seems ponds that ice over all winter might be out of the running but if you have a pond that does not ice over all winter long your pond could be a candidate.
I found a research paper online testing the lower lethal temps of Southern flounder at various salinities. The one that matters here is the fresh water results. I will give the short version of the results. They started at 20C (68F) then took the water temps down to 4C (39.2F) and kept the fish at that temp for ten days before bring them back up to 20C (68F)and they had 10% mortality.
It seems that water temps of 40F is what your pond should not get below at least for more than a few days. I know many ponds in the lower half of the US might be in this category and this could be a new opportunity to expand your ponds potential.
The real issue is sourcing them at this point. If you are in coastal state you could consider a wild harvest and release. You will need to check you state laws regarding this. I checked here in Florida the with the fish and wildlife law enforcement. They said if a flounder was harvested legally there is not law against me releasing it in my 'Private" fresh water pond. (They also informed me it was a salt water fish and it will die)
I have found some hatchery Programs online, you could check and see if they have any to offer for sale.
The CCA/AEP Marine Development Center in Corpus Christi, Perry R. Bass Marine Fisheries Research Station in Palacios, and Sea Center Texas in Lake Jackson, in conjunction with the University of Texas Marine Science Institute (UTMSI),North Carolina State UNIV Aquaculture Dept. and UNC Wilmington's Wrightsville Beach facility.
So, It seems if your pond gets into the mid to low 40's during the winter and you can get a few flounder it is worth trying to establish a few bonus fish for your pond much like is done with HSB. They, as with the HSB should not be able to reproduce in your pond as the reproduction process does require higher salinity.
I am not a scientist nor a biologist, this is just an idea I tried with good success and wanted to share with others.
SOUTHERN FLOUNDER This is a picture of a flounder caught from a fresh water pond
