The Native Americans in the San Francisco Bay constructed the Emeryville Shellmound and over 400 other shellmounds made up of inedible shellfish shells from millions of meals consumed at or near the shellmound sites. The size of these mounds indicate that they had to have accumulated over hundreds if not thousands of years and indicate a well used and stable food resource. The inedible shells they threw aside from their catches of shellfish eventually covered some hundreds of thousands of square feet, sometimes tens of feet thick. Most of the shells are from oysters (Ostrea lurida) which occurred in large oyster reefs throughout the San Francisco Bay area. How they are harvested is unknown but may have been by hand or by using oyster rakes. Tule (Schoenoplectus acutus) canoes were often used for fishing, moving between shell beds, hunting and fishing sites in the Bay area and the extensive Sacramento–San Joaquin River Delta.