I thought that the Atlantic salmon fishery on the Merrimack was to stocked surplus broodstock similar to a fishery near me ?
I doubt that there's a season for wild fish.
This is what I was thinking of.... In NH not Mass
I don't think that you're going to find Haddock or Pollack in the river either
People fish for them in the winter, but off shore and deep.
There used to be a good in-shore fishery for tommy-cod but I think that that's even dried up.
There might be hold over stripers however. There's more of those than people might think.
As for flounder, there's 2 kinds. The season for winter flounder is short, just a few weeks at the end of the winter and there's not the numbers that there were 20-30 years ago. You could fish for them with a fly rod I suppose, but you'd be the only one.
Summer flounder, also known as fluke, are more common and are often caught while striper fishing, usually at slack tide. In the late summer they'll follow bait schools right into the surf zone. I catch a couple every year.