Sorry about the wait, Laotian shad it is, at least this one took you more than one guess, did that come off the top of your head or did you have to look it up? You ever keep one? Your turn Oddball.
I took a stb at looking at shad from that part of the world. I dove amid thousands of these fish when my ship went overseas (retired Navy).
This fish takes us away from the east. I caught one 200 miles up river when I was a kid. I kept it alive for 3 years in hard alkaline freshwater. It grows to about 18".
Although there are several from the gulf the southern seems the closest match on left eyed flounders, the dorsal fin bothers me a bit but the one in your photo might have the front portion of it't dorsal laid back, yours also lacks the obvious spotting.