One place they might survive is Lake Saltonstall, a natural lake down just east of New Haven. It is very deep and carved out by the last ice age.
A few years back when Bob Orciari was the Inland Fisheries Western District Manager, we were discussing a series of sonar markings that registered on a state boat that indicated huge fish just off the bottom at the deepest point in the lake down along the north east shore line. We met that Spring at the lake, rented a boat, and I installed my Lowrance Paper Graph so we would have a permanent recording. We also brought some dead alewife herring and some heavy freshwater gear for fishing.
We found the markings which showed as boom-a-rang marks about 15' off the bottom. There was no indication that huge carp will go that deep and no other species was even close to that big. Huge Brown Trout would be more towards the upper end of the thermocline, etc.
Lake trout could be that deep though and as the ice age increased over time to its maximum, native lake trout from other water systems could be flushed down to the south. The state record lake trout is something like 29lbs, caught years ago, in Wono (lakeville Lake). How did it get there?
During the peak of the ice age, maybe 15,000 years ago, the oceans were about 400' shallower than today and the Long Island Sound side of the Race, from the RI land-mass to the Orient Point land-mass on the north shore of the Long Island area, was a land barrier that formed a huge fresh water lake. The East River and the Hudson River had a couple of hundred miles of down hill run to reach saltwater. So Saltonstall and the "Long Island Sound" area could have held all kinds of northern species as their last refuge.
Anyway, deep jigging with dead herring, jigging spoons, etc. just off the bottom and continuing up about half way produced zero hits or hookups. We worked it for a couple of hours before we quit and went walleye fishing up at the other end of the lake. So we don't know what these "fish marks" were. Maybe not fish at all but old timber but we never got hung up either and the marks seemed to move around though a paper graph has no GPS function and does not have the detail of today's electronics.
So the mystery continues, if you fish Saltonstall head over to that area and run your electronics and see what you can print.