It's wierd that you found it that far down the Hudson. Traditionally they are a freshwater fish, and the Hudson is brackish at times up to Poughkeepsie. However, if it has been very rainy, the runoff can make the river fresh to a more southern point. I also believe that at different depths (up to 200 feet) at some points there is a difference in Salt / Freshwater, with one sitting on top of the other due to runoff. Maybe this played a role in the fish getting so far south. Also, it looks like it has been dead for a while, glazed eyes, and slimy, and the current could being it that far rather quickly (within a day). Looks like a small one too, those things get huge!! When I lived upstate on the Delaware River, we used to snag them with treble hooks, then hold on!!! Also used to chum with corn in Cayuga lake, and drop a hook filled with corn on the bottom, they are a good fight! I know the amish people used to spear them on the finger lakes for food, but we never ate them, just a fun catch.