A reliable source of essential proteins for feeding a Mars colony can be provided by farming fish in deep water units. Pelleted greenhouse offcuts can be fed to the fish, and their organic wastes in turn recycled to water the plants with rich nutrients. Set below the land surface level as protection from lateral radiation, the fish are further protected by leaded glass overhead, and the water in which they live. Raising of fertilised eggs is carried out in a concrete roofed sub-basement, the resulting fry being introduced to the main tanks only when they are large enough to quit the nursery. http://www.eosmarsprogram.org/Page43-06-Food.html