The catfish that you saw is a White catfish. They are in the Hudson year round. They are up near the Tappan Zee, and come down the river to spawn in the spring. I don't know about on the NY side of the river, but over here on the Jersey side, they are very popular to fish for every spring. They are caught all the way down to the GW bridge, so I'm sure that seeing (and catching) them down near NY bay isn't that uncommon. I wouldn't eat them, or anything else out of the river for that matter, but I know of alot of people that eat them, and they aren't Radioactive (YET!!!)...lol.
The funny thing is, most people wouldn't eat any fish out of the river, but yet they would eat a striper that they catch in the bay or even down further. They don't figure that same fish might be one that lives in the Hudson during the winter. As far as the white perch, I've only seen them caught on the bottom, mostly on sandworms. You might be able to catch them on flies though. Who knows?
~tight lines~
~spinfreak~