Farming Fish in Cages

You might think farming fish demands more resources than an ordinary person can pull together. Not so. An experienced aquaculturist explains how.

| May/June 1983

farming fish - two men in boat retrieving fish

The harvest stage of farming fish is pretty simple. Just lift the cage out of the water, or use a dip net if you just want one or two.

Photo by Hildy Maingay

Aquaculture. For many people, this word conjures up images of technologically complex artificial fish-rearing pools (or at the very least, large ponds devoted exclusively to raising fish). Yet one of the most convenient forms of intensive home-scale fish production is probably best called cage culture. Put simply, it’s the practice of farming fish for food in floating cages! This uncomplicated technique is most commonly used in North America by commercial fish farmers, but as years of successful experimentation at the New Alchemy Institute farm in Hatchville, Massachusetts have shown, it could be employed by any individual who has access to an appropriate natural or artificial pond or lake.

Furthermore, the number of fish that can be produced in a single floating cage is astonishing. The current records for rainbow trout, channel catfish, and common carp — three oft-reared species — are all approximately 15.6 pounds per cubic foot. Therefore, a cage that's only three feet on a side (totaling 27 cubic feet) could conceivably be used to raise over 400 pounds of fish! And in more practical terms, even a first-time grower should be able to achieve harvests of nearly four pounds per cubic foot, producing 100 pounds of fish in a 27-cubic-foot cage.

How Cage Culture Works

To understand just how fantastic that rate of production is, imagine a 3' x 3' x 3' hole in your back yard filled with water. Then visualize 100 pounds of fish swimming around in it. That's the "beginner's yield" we're talking about!

Most people are aware that actually trying to produce fish in a small hole would create serious problems long before that 100-pound harvest could be achieved, primarily because the creatures' wastes and any uneaten feed would soon drastically pollute the water. If such contamination didn't kill the fish outright, it would certainly retard their growth.

Of course, you could improve water quality and stock growth by using some combination of costly circulation, aeration, and filtration technologies. Cage culture, however, solves the problem of pollution in a small space because there is a constant exchange of water between the screen-walled cage and the larger body of water in which it floats. After all, even landlocked, supposedly stagnant ponds have some natural circulation as a result of wind and convection currents. But the major source of water movement in a cage is often a "natural pump": The swimming and breathing of the fish themselves! Since the stock's containers are, in addition, normally floated in the upper layer of water that's at least 6 feet deep — where there likely are relatively few other fish and little organic decomposition — the incoming fluid is usually clean and rich in oxygen.

Why Use Cage Culture?

The outstanding advantage of cage culture is that it permits "fish gardening" in bodies of water that aren't suitable for more intensive aquaculture (in which large nets are used to harvest the crop). Such locations include the following:

Public (or large private) waters: Harvesting of fish from public waters is usually restricted to sporting, single-line methods, while privately controlled ponds that cover more than an acre can present formidable difficulties to aquaculturists hoping to harvest with nets. Yet such environments are often ideal for cage culture.

Multipurpose ponds: Maybe your pond was built to attract wildlife, to offer recreational fishing and swimming, or just to provide scenic beauty. Converting that entire body of water to intensive fish culture may be difficult, and doing so would certainly compromise or even render impossible some of its other uses. However, you could easily produce several hundred pounds of cage-cultured protein without lessening your ability to enjoy the pond in these other ways. (Cages may even attract wild fish, and thereby enhance sport fishing!)

Ponds with more than one owner: Suppose your land fronts on a private body of water that's shared by a number of households. Since each family probably has its own ideas about how best to enjoy the pond or lake, you might have a good bit of difficulty getting all of the owners to agree on a joint aquaculture management plan. But you could keep a few cages floating off your own shoreline without interfering with your neighbors' chosen activities.

Very deep ponds: Deep bodies of water such as quarry pits usually are not productive as conventional aquaculture sites. But the same sites are often good for cage culture.

Brushy ponds: Some ponds are so full of brush, logs, or other obstacles that net-harvesting is all but impossible. But as long as there are patches of open water, such locations can be used for farming.

(Incidentally, the fish containers can also be used in the sea and, on occasion, in rivers and streams. But in this article I'll deal solely with standing bodies of fresh water.)

Kinds of Fish

Although most fishes probably can be raised in cages, experience to date suggests that if you're more interested in production than experimentation, you should select from the following seven varieties:

Channel catfish (Ictalurus punctatus): This is the fish most often raised commercially in the United States. Consequently, abundant technical information and good stock are readily available. Channel cats make for excellent eating, too, but they are somewhat costly to feed and relatively susceptible to disease.

Bullheads: In my opinion, bullheads are among the tastiest of all the North American freshwater fishes. What's more, based on our experiences at the New Alchemy Institute they seem to be some of the most suitable for home cage culture. In order to be successful, you'll need to know exactly which species you have. At New Alchemy, we had great success with yellow bullheads (Ictalurus natalis), but we had miserable luck with our sole attempt at raising brown bullheads (Ictalurus nebulosus). Some 90% of the caged browns died within a few weeks of stocking, even though the same species did fine if allowed to swim freely in the pond! (We never tried rearing any other species.)

The three most widely distributed bullheads — yellow, brown, and black — appear quite similar; despite their names, you can't distinguish them by color alone. Your nearest state or provincial fishery biologist (or a good field book) should be able to help you learn the distinguishing characteristics, however.

Trout: The ever-popular rainbow trout (Salmo gairdneri) is the "number two" species in North American commercial fish culture. Like channel cats, trout are easy to purchase, expensive to feed, and somewhat prone to disease. In addition, they require cold water (below 70°F) in order to thrive. As a general rule, these trout do well only in waters that are too cold for rearing most other kinds of fish, and will not even survive in locations suitable for "warm water" fish. (In parts of the south, though, commercial growers take advantage of this trait by raising catfish in cages in the summer and then using the same enclosures to produce a winter crop of trout!)

Sunfish: The bluegill (Lepomis machrochirus) is known to be a good cage culture fish, and other sunfishes may also turn out to be suitable. Indeed, various hybrids that grow much faster than the wild species are already beginning to be available commercially. But they're still quite costly.

Carp: The much-despised common carp (Cyprinus carpio) is, in fact, a tasty food fish and one of the easiest and least expensive to feed. In my experience, the commercially available "Israeli" or "mirror" variety does best, although unselected wild stock can be raised.

American eels: It may surprise you that baby eels (Anguilla rostrata) are captured in U.S. estuaries and exported—fetching a handsome price in the process—to Taiwan and Japan. .. where they're "grown out", often in cages, and used for food. There's no reason we couldn't raise this gourmet food ourselves.

Tilapia: If you can maintain a pond water temperature of at least 75°F for four months or more, you might want to consider raising tilapia. (I particularly recommend the blue variety, Tilapia aurea.) These fish are even more adaptable feeders than are carp, and will make good use of the opaque "pea soup" that occurs in some highly fertile waters. Take caution, though: In many states, it's illegal to import the potentially fast-spreading fish. Furthermore, growers in very warm climates should not use tilapia or other exotic fish if there is the slightest chance that the creatures might escape into natural or public waters.

Obtaining Your Fish

There are three ways to obtain your fish stock: buy them, catch them, or breed them. You should be able to locate dealers who sell channel catfish, rainbow trout, hybrid sunfish, Israeli carp, and even some of the tilapias. You may also be able to find folks who deal in some of the other species I mentioned, but don't count on it.

To find out where to buy fish stock, try Aquaculture North America  or the U.S. Department of the Interior, Fish and Wildlife Service, Division of Fish and Aquatic Conservation

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