by
Richard A. Collins1
Professor of Biology
University of Central Arkansas
Conway, Arkansas, U.S.A.
Introduction
The technique of culturing fish in cages has become known as “cage culture” in the past few years. This short and convenient terminology will be used in the present report. Cage culture of fish refers to that method in which fish stocked at a high density are fed in a cage-like enclosure suspended at the surface of a body of water.
Cage culture of fish began in Asia in the early part of the present century and has been studied and applied in other parts of the world only during the past decade or so. The earlier application of this method in Asia was very limited in productivity because of the unavailability of properly balanced, prepared feeds. The common carp was the principal species cultured in Asia during the first many years, and the feed consisted mainly of wastes from human and animal food. This old method is still used to some extent in parts of Asia. A scientist for the Food and Agriculture Organization of the United Nations told me only a few days ago of seeing this primitive method still being utilized in the Mekong River.
Modern methods of aquaculture involving well-balanced feeds, disease and parasite control, instrumentation for the measurement of water quality, etc., have been applied recently to cage culture of fish. The method was researched in the United States in the 1960's by myself, Dr. William M. Lewis, Dr. H.R. Schmittou, and later by many others. Scientists in Japan and Russsia also conducted studies on this subject during this period. As a result of the use of these modern methods, the culture of fish in cages has become a practical and successful method and will now take its place alongside the other known methods such as open ponds, raceways, pens, recycle systems and combinations of these.
Cage culture should not be thought of as a method to replace other methods. More correctly, it should be considered as a method to be utilized in situations where other methods are not applicable. For example, large reservoirs, irrigation canals, ox-bow lakes, deep ponds resulting from excavations, estuaries, and other bodies of water where the use of other aquaculture methods are not feasible. Since cage culture can be conducted in the same body of water where sportsfishing and other recreation takes place, it allows for multiple uses of places such as reservoirs.
Cage culture is also particularly suitable for certain kinds of research. Genetic studies provide an example. If the F1 progeny of several crosses are to be tested for some character, such as rate of growth, then cages could be quite useful. All progeny could be placed in the same body of water, thereby eliminating the variable of different water quality in different ponds. Also cages can be utilized more inexpensively than can separate ponds. Other examples could be: /1/ comparison of quality of feeds, /2/ disease and parasite studies, /3/ nutrition studies, /4/ behavior, /5/ isolation of brood stock.
Cage Construction
Cages may be built in a variety of sizes from 1 m3 to as large a size as is convenient to handle with the equipment available. For research purposes, cages of 1 m3 to 2 m3 are more efficient. Cages of 3 m3 can be handled by hand, but larger cages require mechanical equipment.
The materials used in constructing a cage can be varied depending upon what is available. In more undeveloped areas, materials such as bamboo have been used successfully. The best materials to use are wood or metal for the frame, styrofoam for flotation, and plastic coated wire or plastic mesh for the cage. However, the flotation may be accomplished quite well with plastic jugs or barrels. Of course, glass containers should not be used because they may break. The wire used for constructing the cages does not have to be plastic coated, but corrosion may completely destroy a cage in three months if there is no protective coating. The rate of corrosion varies directly with the concentration of electrolytes in the water. An opaque cover is necessary for most species of fish, especially if the water is highly transparent. Most fish react adversely to a shadow or moving object above them, and the opaque cover shields them from normal activity necessary for feeding and tending to the fish.
When selecting material for construction of the cage, one should take care that the mesh size is 12 mm or greater. Sizes smaller than 12 mm will reduce the circulation of water through the cage. Also, algae growing upon the cage will completely close smaller openings within one month, depending upon the fertility of the water. If smaller mesh sizes are necessary for very small fish, then the wire should be kept free of algae, a lower density of fish stocked, and the fish should be moved to cages with a larger mesh as soon as the fish are large enough to be retained by the larger mesh.
Nylon net or similar synthetic materials should be used with caution. If there are fish-eating mammals, turtles, or other animals that will attempt to get the fish from a cage, these animals may chew holes in the netting and allow the fish to escape.
Stocking
Several species of fishes have been cultured successfully in cages, some of which are native to Europe. The common carp, American catfish, rainbow trout, salmon, and yellowtail are some species that have been cultured in cages extensively. Those species that are omnivorous adapt to pelleted feed more readily.
It is important to handle fish carefully when they are being harvested and moved for stocking into cages. Fish that are injured or stressed excessively while being handled may appear to be healthy at that time, but they will die during the first week they are in a cage. Of course, proper handling of fish is not peculiar to cage culture, the same principle applies to any type of fish culture.
The number of fish to be stocked in a cage varies with each species, size of the fish at the time of stocking, and length of the growing season. The upper limits of density of fish in cages have not been clearly established, and that also will vary with the species. However, I have successfully cultured the catfish Ictalurus punctatus at a density of 217 kg per cubic meter of cage, and the rainbow trout at a density of 85 kg/m3. And still, I do not think these densities are the highest possible. So, whatever density one can culture per cubic meter must be taken into account when determining the number of fish to be stocked. For example, if channel catfish double their weight approximately each 30 days during a growing season of 150 days, and 200 kg are to be grown in 1 m3, then 300 fish of 21 g, 200 fish of 31 g, or 150 fish of 42 g should be stocked. An additional number of fish, perhaps 5%, should be stocked to allow for normal mortality.
Cage culture is a high density method of culturing fish. If the cages are not stocked to achieve a high density, then one of the principal advantages of the method is not realized.
Feeding
Fish in a floating cage get no food except what is given to them by the culturist. /There are a few exceptions to this in special situations. One example in Asia is caged carp placed in a sewer stream where they feed partially upon benthos flowing through the cage. /Therefore it is necessary that the feed be complete, i.e., contain all the necessary vitamins and a proper balance of carbohydrates, lipids and proteins. Fish fed on diets of a supplementary nature will generally do well for about one month and then cease feeding and die from lack of some ingredient in the feed. The fish should be fed all they will consume once each a day, either just before dark or soon after dawn.
A floating pellet is the best form of feed, but a sinking pellet may also be used. The advantage of using a floating pellet is that the correct amount to feed can be easily determined. Floating pellets can be easily observed, so providing the amount the fish will consume without wasting feed can be determined each day without disturbing the fish. As the fish grow the amount can be increased. However floating pellets are more expensive to prepare than sinking pellets. If sinking pellets are to be used, a tray can be placed into the cage and the pellets can be poured into the tray. Figure 1 shows a cage for use with floating pellets.
When comparing the dry weight of pelleted feed to the wet weight of fish, conversions of 1.1/1 to 2/1 are common. I have routinely obtained 1.3/1 with catfish and 1.54/1 with rainbow trout.
Routine feeding should be done with normal care. Unnecessary bumping of cages with the boat or other loud, sudden noises around the cages should be avoided. An electric motor is very convenient to use on a boat when feeding the fish.
Water
The best quality of water for cage culture is that which is infertile and does not have much fluctuation of dissolved oxygen during each 24 hour period or during an entire season. Highly fertile waters generally have very low dissolved oxygen at dawn, and caged fish in high density do not tolerate dissolved oxygen concentrations as low as do fish which are loose in a pond. The principal reason, of course, is that fish which are loose can “pipe” water from the surface and the caged fish cannot.
Cages do not need to be where there is a current of water. In fact, cages placed in currents may have a greater loss of feed. Fish in a cage move continually and cause an exchange of water to an extent that there is generally no difference in dissolved oxygen inside and outside a cage if the size of the mesh is sufficiently large.
A minimum depth of one meter underneath a cage is necessary to ensure adequate circulation. Also the feces of the fish can sink to the bottom and not influence the dissolved oxygen around the cage.
Caution should be taken not to overstock a body of water. A given volume of water will support only a limited amount of fish whether they are loose or caged. For example, if a pond is stocked to capacity, then cages of fish should not also be placed in the pond.
Effluent waters from electric generating plants can also be utilized for cage culture, especially during the winter. The warmed effluent waters can function to extend growing seasons, and in some situations provide for year-round growth of fish. If the warm effluent water flows directly into a reservoir, then cage culture would be a good method to use.
If the body of water used for cage culture is an excavation pit that is a closed lake, i.e., one in which there is no outlet for water, then caution should be taken in its use. All added fertility from fish feces and dissolved feed will accumulate in the closed lake. During the first two years or so there generally will be no difficulty, but after that time accumulated fertility may cause a depletion of dissolved oxygen during the summer months.
Disease
Disease and parasiticism occur in caged fish just as in pond-reared fish. Some common disease organisms are Chondrococcus columnaris and Aeromonas liquefaciens. Protozoans and monogenetic trematodes are also common. Prevalent disease organisms and parasites differ with different species of fish and experience with each species by the culturist is necessary.
Treatments for some diseases have been elaborated by specialists in this field of study. Antibiotics such as tetracycline have been effective against some bacteria. Most commonly the antibiotic is sprayed onto the pelleted feed, allowed to dry, and then fed to the fish. Prepared, medicated feed from the feed mill could also be used if large quantities are involved.
In the treatment of external parasites, the cage of fish can be placed into an enclosure slightly larger than the cage, and then the water in the enclosure can be treated with chemicals such as Dylox, formalin, or malachite green. While the cage of fish is in the enclosure, care must be taken to agitate the water adequately to prevent a depletion of dissolved oxygen. Short-term treatments of a few minutes would be appropriate if the enclosure method is used.
Mortality of fish in cages is highly variable. If the fish are healthy, handled carefully at the time of stocking, and there is no disease, then mortality is usually less than one percent. However, either of the above factors can markedly increase the mortality. Disease can usually be detected early in a cage because the fish can be seen at the time of feeding and the amount of feed consumed reduces at the onset of disease. In fact, this early detection of disease is one of the advantages of the cage culture method. Most mortality of fish in cage culture is due to the culturist, i.e., human error, and not to the method of culture.
Sampling of caged fish periodically is desirable in some types of research. However, it should be noted that sampling disturbs some species to an extent that they do not feed well for several days after the sampling. Catfish are sensitive to sampling, but rainbow trout do not react adversely to any significant degree. Feeding records will reflect whether a species is sensitive to the disturbance required in sampling.
In summary, it can be said that cage culture of fish is a tried and proven method, and it will always be a part of modern-day aquaculture. The method is not new, but new techniques and information have improved the method to an extent that it is now economically practical. It is currently being used routinely in some places, such as Japan, and will be used more extensively in other parts of the world as knowledge and experience are gained by culturists.
Fig. 1 A CAGE FOR USE WITH FLOATING PELLETS
1. ÁBRA KISÉRLETI HALTENYÉSZTÖ KETREC ÚSZÓ HALTÁP ETETÉSÉRE
