SLAUGHTER BEACH, Del.  Sixteen nautical miles from the Indian River Inlet and about 80 feet underwater, a building boom is under way at the Red Bird Reef.

One by one, a machine operator has been shoving hundreds of retired New York City subway cars off a barge, continuing the transformation of a barren stretch of ocean floor into a bountiful oasis, carpeted in sea grasses, walled thick with blue mussels and sponges, and teeming with black sea bass and tautog.

“They’re basically luxury condominiums for fish,” Jeff Tinsman, artificial reef program manager for the Delaware Department of Natural Resources and Environmental Control, said as one of 48 of the 19-ton retirees from New York City sank toward the 666 already on the ocean floor.

But now, Delaware is struggling with the misfortune of its own success.

Having planted a thriving community in what was once an underwater desert, state marine officials are faced with the sort of overcrowding, crime and traffic problems more common to terrestrial cities.