A fish cage having a frame structure defining an enclosure with a top and a bottom and an intersecting side. The frame is adapted to rest on the bottom of an outdoor body of water and is covered with a first netting on the top and side but not the bottom. The first netting has mesh openings sized to permit ambient water and suspended particles therein to circulate through the cage but prevent predators and competitors for food of the fish under study from entering the cage. A second netting covers the bottom of the enclosure. The second netting has second mesh openings larger than the first mesh openings and sized to prevent the fish being studied from leaving the enclosure as well as permit plant and animal life at the bottom of the body of water to enter the enclosure, thereby providing a more natural environment for the fish under study. The enclosure includes a door to permit access to the interior of the enclosure.