"Fish need food. The basis of the food chain in the ocean is phytoplankton, photosynthetic protista. These creatures require dissolved nutrients which are scarce in most areas of the oceans. The major limiting nutrients are soluble nitrogen and phosphorous compounds. "

The same could be said about the world's soil.
Without farming technology used on land, the land could not support 1/100th of current population. Without recent technology [last few decades] half of current human population could not be supported with older farming tecnology. The ocean is mostly a hunting and gathering type level of food production.