Construction Of Aquaculture Cages

Cages for fish culture have been constructed from a variety of materials and in practically every shape and size imaginable. Basic cage construction requires that cage materials be strong,  durable, and non-toxic.

The cage must retain the fish yet allow maximum circulation of water through the cage. Adequate water circulation is critical to the health of the fish, in bringing oxygen into the cage, and removing wastes from the cage.

Location of the cage in the pond is critical to proper circulation through the cage. Mechanical circulation and aeration through the cage may be necessary if stocking densities are high (Figure 1), cages are large, or water quality deteriorates during production (see SRAC Publication No. 161, Cage Culture – Site Selection and Water Quality).

Cage materials:

Cage components consist of a frame, mesh or netting, feeding ring, lid, and flotation. Cage shape may be round, square, or rectangular. Shape does not appear to affect production with most freshwater species.

Cage size depends on the size of the pond, the availability of aeration, and the method of harvest. Most fish farming supply companies sell manufactured cages, cage kits, or materials for  constructing cages. The most common cage sizes utilized in ponds are: cylindrical— 4×4 (diameter x depth) feet; square—4x4x4 feet and 8x8x4 feet (length x width x depth); and rectangular—8x4x4 and 12x6x4 feet (see figures).

The frame of the cage can be constructed from wood (preferably redwood or cypress), iron, steel, aluminum, fiberglass, or PVC. Frames of wood, iron, and steel (unless galvanized) should be coated with a water-resistant substance like epoxy, or an asphaltbased or swimming pool paint.

Bolts or other fasteners used to construct the cage should be of rust-resistant materials. Mesh or netting materials that can be used include plastic coated welded wire, solid plastic mesh, and nylon netting (knotted or knotless).

Mesh size should be no smaller than 1/2 inch to assure good water circulation through the cage while holding relatively small fingerlings (4 to 5 inch) at the start of the production cycle. A larger mesh size can be used if large fingerlings are stocked. The feeding ring or collar can be made of 1/8- or 3/16-inch mesh and should be 12 to 15 inches in width. The feeding ring keeps the floating feed from washing through the cage sides.

All cages should have lids to assure that fish do not escape and that predators (including people) do not have access to the cage. Lids may be made from the same mesh material as the rest of the cage or can be made of other materials, such as plywood, masonite, or light gauge aluminum or steel. Plywood, masonite, and steel will need to be painted with exterior or epoxy paint.

Some cage enthusiasts believe that opaque lids (particularly in clear ponds) reduce stress on fish by limiting their visual contact with outside disturbances.

No scientific research has been performed to support or dispute this observation. It should be remembered, however, that the manager of the cage needs to be able to observe feeding behavior and have easy access to the cage to remove uneaten feed and any dead fish.

Flotation of the cage can be provided by styrofoam, waterproofed foam rubber, sealed PVC pipe, or plastic bottles. Cages can also be suspended from docks.

Plastic bottles should be made of sturdy plastic (e.g., antifreeze or bleach bottles) and should have their caps waterproofed with silicon sealer. Floats should be placed around the cage so that it floats evenly with the lid about 6 inches out of the water.

Cage construction:

The simplest cage design to construct is a 4×4 feet cylindrical cage fashioned from 1/2-inch plastic mesh (Figure 2). The mesh comes in a roll 4 feet wide, and a total of 21 feet of plastic mesh is used per cage.

Thirteen feet of mesh is used for the cylinder with two 4- foot panels for the bottom and lid. The cylinder is formed around two metal, PVC, polypropylene, or fiberglass hoops at the top and bottom of the cage. A third hoop is used to form the lid.

Cages can be laced together with 18-gauge bell wire (plastic coated solid copper wire), stainless steel wire, hog rings, or black plastic cable ties (white cable ties should not be used as they deteriorate in sunlight).

For detailed instructions on the design of a 4×4 round cage see SRAC Publication No. 340, 4-H Aquatic Science Project, Guide to Raising Catfish in a Cage and Publication No. 341, 4-H Aquatic Science – Catfish Cage Culture Record Keeping Project.

Figure 3 shows the design of a 12x6x4 cage which is popular in commercial cage culture of catfish. These cages are usually  made from coated (a commercially available plastic net coating) knotted nylon netting and floated with a 4-inch PVC frame. Lead weights along the bottom seams of the cage maintain the cage’s shape in the water. All cages need feeding rings to keep floating feed inside the cage.

Feeding rings should be about 15 inches in width and should be attached to the cage so as to extend 3 inches above the water level with 12 inches extending into the water. Feeding rings can be attached to the cage or suspended from the lid. Suspending the feeding ring from the lid reduces the amount of fouling around the sides of the cage. The feeding ring is prone to fouling because of its smaller mesh size.

Even with a feeding ring some feed will be washed out of the cage by aggressively feeding fish. Some cages have been constructed with solid or fine mesh bottoms so that sinking feed can be fed during cold weather.

Fouling has been observed as a problem with these cages. If sinking feed is going to be fed, it is probably better to construct a removable feeding tray with 3- inch or higher sides (to keep feed from being swept off the tray by the swimming action of the fish).

Feed can be dropped into the cage above the tray and the tray can be removed to check for feeding response and cleaned as needed. Cage lids should be attached so they are not readily dislodged  by wouldbe predators yet can be easily opened to remove dead fish.

Lids are commonly attached so that they hinge along one side of the cage and are wired or hooked along the other sides for easy access.

 

 Author:

Michael P. Masser