I know that there small mouth, and pike... not sure about catfish but i wouldn't be surprised if they were in there. And actually i just ran into an article about Merrimack river sturgeon,
http://www.onthewater.com/catching-a-merrimack-river-sturgeon/
And you mentioned catching trout on the Merrimack... On what part of the Merrimack to trout run? I'm guessing up in NH like Nashua throughout Manchester? I also herd in the spring that Salmon run by the dam in Lowell. is this true?
There are also Striped Bass as far down as Haverhill.