Gates made either of rocks piled up or of wood are used to allow passage of water at high tide while not allowing the fish to escape. This is enough to maintain water quality. Natural fish ponds are configured in low-lying shallow bays. Across a channel a net is stretched and fastened along the bottom with mud, sand, and rocks. At high tide the net is raised and held up with branches or other support thereby enclosing the fingerlings within the resulting bay where they are fed until maturity.
Natural ponds are the most cost effective systems.
In Eritrea, a natural pond of about 25 acres was enclosed in three hours using only 150 square yards of net and 30 branches. Thousands of acres of calm, protected waters result from the barrier reef that parallels the Eritrean coast. Cages 15 by 9 by 3 feet framed of wood and covered with netting provide an ideal natural environment for fish.