The U.S. Army Corps of Engineers, Baltimore District, places shell to restore oyster reefs in the Chesapeake Bay tributary of Harris Creek, April 1, 2015. The shell comes from processing plants in the mid-Atlantic region and is permitted to be imported and placed in the river. There is not sufficient natural shell available to restore oyster habitat in the Bay. Therefore, artificial reefs, known as substrate or alternative substrate, have been constructed at restoration locations throughout the Bay. Substrate may be any combination of oyster shell, clam shell, or alternative substrate such as crushed concrete, rock or granite.The shell is 2 to 3 inches in diameter. (U.S. Army Photo by Sean Fritzges)