I would doubt that the Merrimac supports spawning based on what I think I know, but I think that the published science on stripers may be pretty spotty. Striped bass eggs are semi-buoyant and supposedly need to be able to float for 48-72 hours in brackish water after they've been fertilized. I think the distance to the first dam on the Merrimac is too short to support breeding.
In the Hudson they have something over 200 miles to the first dam at Troy, and the various Chessie estuaries are pretty far upstream as well. The Delaware never gets publicized as a breeding ground, but in trips to PA this year I heard multiple independent reports of 4" fish from Easton to New Hope, and the Delaware is the largest undammed river on this coast.
That said, there is definitely a spawning stock up in New Brunswick. I found a French language web page on "le stock du bar raye dans la Miramichi" which documented spawning in that famous salmon river.
All of which brings me back to the 6" microbass I caught in the wash at Chatham this spring. Where the hell did they come from?
The last reason that I have doubts about local spawning is the description of the spawn in the Chesapeake system in John Cole's "Striper". It's apparently a pretty formidable occurrance with huge bass rolling on the surface and lots of eggs and milt clouding miles of water. I would have expected to have heard reports from some of the usual suspects if that had been observed in any Cape or local system.