|Langston University Aquaculture|
Caged Fish Culture
Kenneth Williams and Glen Gebhart
THIS FACT SHEET summarizes fifteen years of experience with caged fish culture. Species have included channel catfish, rainbow trout bluegill, green sunfish, hybrid bluegill, hybrid striped bass, largemouth bass and tilapia. Channel catfish are the most common species cultured in cages, consequently this fact sheet will cover specifics of catfish culture. These principles also can be generally applied to other species. The Langston University Aquaculture Extension Program has produced several fact sheets that cover various specific aspects of caged fish culture. These include: "Cage Culture of Rainbow Trout", "Caged Catfish Culture Problems", and "Small-Scale Caged Fish Culture In Oklahoma Farm Ponds".
Our experience has demonstrated that caged fish culture can be successful if proper management techniques are carefully followed. However, many attempts at caged fish culture have failed when one or more recommendations were ignored. This fact sheet will compile successful caged fish culture techniques and highlight common mistakes.
Although open pond culture is usually the best method of raising fish, cage culture offers an opportunity to raise fish in existing ponds or lakes that were not originally meant for aquaculture. Most impoundments were built for watering stock, flood control, or recreational fishing. These bodies of water typically have irregular bottom contours, obstructions such as rocks or logs and deep areas that make seining difficult or impossible. Also many impoundments cannot be drained or refilled easily. Various fish trapping methods can be used under these circumstances, however, traps are not generally efficient methods of consistently harvesting commercial quantities of fish.
Impoundments considered for an aquaculture enterprise must meet certain basic requirements before fish can be successfully raised in cages. Larger ponds and lakes are the best candidates for caged fish culture. A pond must be about an acre in surface area, and contain at least 6 feet of water in at least one third of its surface area during the driest part of the year, (August and September).
Deep bodies of water, those that are greater than 12-15 feet, are of no advantage in fish culture and may at times contribute to oxygen depletion problems.
This happens because much of the water below about 6 feet has little or no oxygen. Lost feed and fish waste fall through the cage into oxygen depleted waters and settle on the pond bottom. When the water over turns naturally in the fall, the waste material is exposed to oxygen and begins to break down and decompose into constituent elements. Oxygen is depleted in this process and may cause a fish kill. A mixing device that destratifies water throughout the summer, may be necessary to avoid this situation.
Impoundments used for fish culture should be conveniently located so that fish can be fed and observed daily. Also, impoundments should not be located in areas easily accessible to the public, as caged fish are more vulnerable to theft or vandalism than fish reared in open bodies of water.
Aquatic vegetation such as weeds, algae or "moss" should cover no more than one-third of the pond's total area because these plants can contribute to oxygen deficiency problems later in the summer.
Consult our fact sheet entitled "Controlling Aquatic Vegetation With Grass Carp" for information concerning vegetation control.
Impoundments receiving runoff from livestock containment facilities should not be used for caged fish culture. Ideally, cattle should be fenced out of the fish culture impoundment. However, if it is necessary to water livestock; provide only an area large enough to allow cattle access for drinking. Do not let cattle stand in the pond. Waste products require oxygen to decompose; and if the quantity of waste is great enough, dissolved oxygen levels will decline, which may result in substantially reduced fish growth or even a complete fish kill.
Cages can be purchased from several sources or easily constructed by the fish farmer from materials available from companies serving the aquaculture industry. Cages can be made almost any size or shape but most round cages are 4 feet deep and a little less than 4 feet in diameter because rolls of plastic mesh or plastic coated welded wire are sold in 4 foot widths. Rectangular cages are usually 8 ft. x 4 ft. x 4 ft. or 8 ft. x 8 ft. x 4 ft deep. Optimum mesh size appears to be 2 x 1 inch. Plastic coated, welded wire is preferable for cage construction because of its durability. Muskrats and other animals cannot chew holes through this mesh. An anti-fouling paint can be applied to cage mesh if excessive algal growth reduces water flow through the cage. This is most commonly a problem in clear bodies of water. A feeding screen made from 1/8 inch plastic mesh, 1 ft. wide is attached to the upper inside perimeter of the cage to keep feed from floating out before it can be eaten. The cage is floated with blocks of foam or plastic jugs and anchored to a dock, cable or the pond bottom to prevent movement. The top of the cage is usually made of the same material as that used for the rest of the cage. However, in clear ponds an opaque cover that blocks the light may encourage light sensitive catfish to accept feed more readily. Cage construction materials cost from $2.00 to $2.60 per cubic foot of cage. Cages have a life span of 10 to 20 years with proper care.
Fingerlings and stocking rates
Stock healthy fingerlings. Diseased fingerlings or fingerlings in poor condition should not be purchased. These fish are at high risk of mortality when stocked in cages. Fingerlings for caged fish culture are stocked in the early spring from late March until mid-May. Stocking earlier, when water temperatures are below about 60 F., may result in cage worn fish. Stock graded 6 to 8 inch fingerlings to insure production of harvestable sized fish by the end of the growing season. Plan to harvest fish in one growing season to avoid over winter mortality.
Stocking rates are determined by the total surface acreage of the impoundment; and should not exceed 1000 pounds per acre in most situations unless aeration devices are used to prevent or remedy oxygen depletion problems.
Cages are stocked at a density of 9 to 12 fish per cubic foot. Higher densities are possible in some situations but cages should not be stocked at densities lower than 4 or 5 catfish per cubic foot. Slightly larger fish can be raised if fish are stocked at the lower recommended rates. At low cage densities catfish will fight and bite each other in an attempt to set up and defend territories. The wounds become infected with bacteria and the fish may die. Also, do not stock two year old fingerlings or wild caught fish into cages. These fish are accustomed to open water conditions and usually fight whatever the stocking density.
Feeds and feeding
Caged fish should be fed a 36% protein, floating pelleted ration with a complete vitamin pack. Catfish will convert this feed to fish flesh at a ratio of about 1.5 to 1.8 pounds of feed per pound of fish in cages.
Feed must float. Test a sample of each new lot of feed to determine that at least 95 percent floats. Fingerlings get a better start if a high protein, small pelleted trout ration is fed the first 4-6 weeks after stocking. Feed must be fresh, particularly during hot summer months because some vitamins break down rapidly with heat and so are not available to the fish. Vitamin deficiencies in caged fish cause a variety of problems. The most common being reduced growth and broken back syndrome from inadequate levels of vitamin C.
Fish feed molds easily, especially under the hot humid conditions of a typical Oklahoma summer. Many molds contain aflotoxins that can stress or kill fish. Moldy feeds should be discarded to avoid fish health problems caused by these toxins. To insure a mold free, high vitamin feed, buy no more than a 6 week supply at a time.
There are many formulae for determining the proper amount of food to give cage raised fish. Most methods are based on water temperature and the body weight of the fish or some combination of these factors. Perhaps the easiest method of gauging the amount of food given to a cage of fish is based on the amount of food eaten in a 5-10 minute period. Allow extra time when water temperatures are below 65 degrees F. This method automatically adjusts for fish weight and eliminates handling of fish or the guess work needed to determine body weight. Use a coffee can or some other kind of measurable scoop and count the number given to each cage.
After the specified time, estimate the amount of food remaining in the cage and reduce the ration given on the following day. If all the feed was eaten, increase the ration until a small amount is left in the cage. The amount fed can be adjusted on a daily basis but once every week is usually often enough. Caged fish should be fed daily. Fish can be fed at any convenient time, although it is best to feed at the same time each day. It is also best to keep the feeding routine constant. Keep disturbances around the cages to a minimum.
Cage harvest usually begins in September and continues though December. Cages are brought to the shore or to a dock where they are partially lifted from the water. The tops are opened and the fish are dipped out with a dipnet. Whether all fish are harvested at once or only a few at a time depends on the marketing methods employed by the producer. It is best to harvest the entire cage if possible. Frequent dipping into the cage will put fish off feed. They are also scraped and wounded when they bolt into the sides of the cage in their attempts to escape the dipnet. These wounds are unsightly and reduce the marketability of the fish, also if they become infected with bacteria the fish may die.
Algae or bryozoa, a grey jelly-like mass, may sometimes accumulate on the cage. It should be raked off with a stick or broom periodically to insure adequate water circulation through the cage.
Cage floats may have to be adjusted or replaced during the growing season. Also regularly inspect the cage door to make sure it is tightly closed. A strong wind can blow the lid open and allow fish to escape.
An in depth discussion of diseases and cures is complex and beyond the scope of this fact sheet. The best advice is to contact a fish disease specialist at the first sign of sick fish. The first indication of sick fish is a sharp feeding reduction for no apparent reason. Common physical signs of disease include skin lesions, fuzzy patches on skin, small white, salt like spots, red mouth or fins and dead fish.
It is common to lose an occasional fish, especially after stocking or handling.
Contact Langston University Aquaculture Extension (405) 466-6127 if a disease problem is suspected.
Dissolved oxygen level is the most important water quality parameter to understand when raising fish in cages. Oxygen abundance is expressed interchangeably as parts per million (ppm) or milligrams per liter (mg/L) and can be measured electronically or by chemical analysis. During summer months, oxygen reaches saturation levels at about 7 mg/L. The colder the water the more oxygen it can hold before reaching saturation. Winter oxygen levels may reach 8-12 mg/L. Oxygen levels below about 5 mg/L can reduce feeding and growth of fish. Dissolved oxygen levels below 3 mg/l can stress fish while levels below 2 mg/L can result in mortality of caged fish. Oxygen measurements should be recorded weekly or even more often when conditions suggest a possible problem. Observable signs of a potential oxygen problem included:
When dissolved oxygen levels drop below 3 mg/L stop feeding the fish. If dissolved oxygen level drops below 2 mg/L, aeration will be needed to save fish in cages. Do not resume feeding fish until oxygen levels recover.
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