I'll add a data point. I explored Scorton after one of the winter kills last year (I recall two significant events) and found the marsh littered with hundreds of carcasses all the way up to the culvert at Jones Ln. Interestingly, there were none on the other side of the culvert.
My experience bears out Larry's observations: the fish are not evenly distributed throughout the system, but where they are concentrated they number in the hundreds. The system must support thousands of fish over the winter.
I heard about, but did not personally investigate, cold-weather fish kills in Barnstable Harbor and Pleasant Bay as well. I also presume that every spot that had ick fish this spring had them because it had held them all winter.