I don't have my fish bio books handy, but I wouldn't consider it a preference more than what their body is designed to do. If a fish can regulate salt content like anadromous fish then their organs would function like they should. Now if the SW fish in question have a range that is tighter because of life history then the organs should function best in that range I believe. I mean if you drop the salinity too much the fish would die and probably too high as well.What about osmoregulation wouldn't they have to consume less water and filter less salt through their kidneys if the salinity is lower? Not disagreeing I'm just confused as to why a fish would prefer a higher salinity.