In my experience observing cormorants in the bays of Long Island and on Long Island Sound, they seem like solitary critters. In this story they sound communal. Can they exist in both manners?
When they find bait fish they eat in large groups.
In some parts of the world, there are colonies where after many years, the cormorant poop is so thick that it is actually harvested as a nitrogen source for fertilizers. Hence the nickname "guano bird".
Incidentally, as an undergrad at UMass-Boston, I worked with the guy who wrote the Double-Crested Cormorant's entry for the "Birds of North America" (a set of reports that is intended to provide complete, definitive accounts of the biology of every bird species in North America).
I don't know where you were looking, but there are colonies of these garbage pails all over the Island.
I see them roost in the tall willows here on Irondequoit Bay (next to Rochester, NY) by the thousands. Their droppings completely cover one of the sand banks.