The boatman typically sits, kneels or stands in the boat and either paddles it with a double bladed paddle or with his arms in a single person canoe when lying prone. If the boat was not woven tightly enough, then the boatman would find himself sitting, standing or kneeling in several inches of water. The tule canoes were often used for transportation to oyster mollusk and other shellfish beds that could be harvested at low tide. The Emeryville Shellmound or midden composed almost entirely of the inedible shells of different types of shellfish, presumably harvested utilizing tule boats, is an example of the over 400 shell mounds known in the San Francisco Bay area. These often massive shell mounds (the Emeryville Shellmound was originally reported as being high by long), were often built up over centuries of shell discards and showed a stable source of easily obtained shellfish utilized for many hundreds of years. It is believed that shellfish was a major if not the main source of food for many Native American people.