Growing fish in Cages system
By: Uri Ben Israel an Aquaculture expert
About cage farming tilapia
Raising fish in cages is a successful system used in many parts of the world. It has replaced the traditional fish-growing method of earth ponds. Its advantages are well recognized and it is widely spread all over the globe. The first and most important advantage is the unlimited amount of water surrounding the cages. This unlimited water supply provides vast amounts of oxygen and running water, which is necessary for productive fish farming. In addition to that, the financial investment in such a system is much lower. This system of cages is simpler in daily operation in comparison to any other commonly used method of growing fish.
Site Selection and Placement of Cages
Large bodies of water tend to be better suited for cage culture than small ponds, because the water quality is generally more stable and less affected by fish waste. Exceptions are entropic waters rich in nutrients and organic matter. Small (1 to 5 acres) ponds can be used for cage culture, but provisions for water exchange or emergency aeration may be required. Cages should be placed where water currents are greatest, usually to the windward side. Calm, stagnant areas should be avoided. However, areas with rough water and strong currents also present problems.
Cages may be moored individually or linked in groups to piers, rafts, or lines of heavy rope suspended across the water surface. At least 5 meters should separate each cage to optimize water quality. The cage floor should be a minimum of 4 meters above the bottom substrate, where waste accumulates and oxygen levels may be depressed. However, greater depths promote rapid growth and reduce the possibility of parasitism and disease.
The optimum fingerling size for stocking in final grow out cages is determined by the length of the growing season and the desired market size. The shorter the growing season, the larger the fingerlings must be at stocking. The use of male populations, which grow at 30-40% the rate of female populations, will result in larger fish, greater production and a reduction during the grow-out period.
Recommended stocking rate of tilapia fingerlings depends on cage volume, desired harvest size and production level, and the length of the culture period.
Water exchange is less frequent in large cages, and therefore the stocking rate must be reduced accordingly to fish size.
In tropical or subtropical regions with a year-round growing season, a staggered production system could be used to facilitate marketing by ensuring regular harvests, e.g., weekly, biweekly, or monthly. The exact strategy will depend on the number of cages available and the total production potential of the body of water.
The cage system is serviced by a number of floating elements. The work between the cages is performed using motorboats. The use of motorboats serves several purposes: stocking fish into the cages, harvesting fish from the cages, extracting fish to the processing plant, transferring fish between the cages, feeding the fish and a vast number of other activities requiring movement between cages and shore. In addition, the motorboat is used to move the whole cage to the shore for harvesting, restocking or net preparation.
Stocking and harvesting fish can also be done by using special fish tanks on the service boats that contain water with oxygen, which comes from a special bottle of oxygen. The extraction of the fish is done part by hand and part by mean of a special device for the extraction of fish called the Archimedes Screw (Fish Elevator).
At a central point in the lake a wharf will be located to serve the cages by the boats. The wharf will be the main service point which will concentrate all cage related matters such as loading and unloading of food sacks, fuel station, repair and maintenance of the cages, exit point for the divers and a place to tie up the boats at night, and to leave extra floating technical elements.
From time to time, when the cages are emptied of fish, the nets will be transported to shore and washed in a specially designated facility. The fish farm will be serviced by a number of divers whose role will be to watch for damage to the cages, repair the nets in case of tears caused by predators or by any other causes.
Elevator for fish harvesting
Total production in cages increases as the stocking rate is increased. However, there is a density at which tilapia become too crowded and water quality within the cage deteriorates to a point that causes a decline in growth rates. In cages, production should be limited to 30 -50 kg per cubic meter. Tilapia continues to grow above these levels at gradually decreasing rates, but they convert feed poorly, and the risk of loss due to oxygen depletion or disease is greater. For maximum turnover of mar
ketable fish, it is best to limit production to levels that do not depress growth. The total number of cages that can be deployed in a lake and therefore total fish production, is primarily a function of maximum allowable feeding rate for all cages in that body of water. The total feed input is related to number and size of fish in the cages (the biomass) and is limited by surface area of the pond.
Cages advantages and disadvantages
Some advantages are:
Flexibility of management
Ease and low cost of harvesting
Close observation of fish feeding response and health
Ease and economical treatment of parasites and diseases
Relatively low capital investment compared to ponds and raceways
Some disadvantages are:
Risk of loss from poaching or damage to cages from predators or storms
Less tolerance of fish to poor water quality
Dependence on nutritionally-complete diets
Leveling turned over – This may occur when anaerobic material flows up from the bottom of the lake, increasing toxic chemicals and lost of oxygen in the water.
The cage has three main parts. The outer part which floats on the water surface is made from 2-3 HDPE tubes. The diameter of the tubes is 250mm and they compose the upper floating base (as seen in the pictures) to which the fish growing net is tied.
The second component is the net, where the fish are kept and grown. The net is completely submerged under water in a depth as determined by the farm’s requirements.
The third component is the mooring system located at the bottom of the lake which anchors the cage system to the designated location above water surface. The mooring system is anchored to the bottom with heavy anchors (as illustrated below). Out of this anchoring system are a number of cables protrude which connect to the cages and thus anchor the cage so that it cannot move. This system is designed to withstand strong winds and large waves. A number of mooring systems will be placed in several locations in the lake. Each of the systems will carry 24 cages. Each system will be named so it will be characterized and identified by the farm’s computer.