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.
by Bill McLarney
May/June 1983

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

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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

You may well be able to locate free fish for stocking, particularly bluegills and bullheads (rapid reproducers that can quietly overpopulate a body of water if they're not kept in check). Many owners of farm ponds will be glad to relieve their fish population crunch by letting you haul away some of their "stunted" stock. Such fish will resume normal growth patterns when they're again placed under favorable conditions.

Taking your starting supply from public waters is usually illegal, especially if you're after such prized game fish as trout. Still, it can't hurt to check with your local fishery biologist to see if he or she knows of any unwanted surpluses of bullheads or carp. Such specimens can generally be caught with seines, cast nets, or traps baited with liver, fish, or meat offal. (These methods are thoroughly covered in my upcoming book from Cloudburst Press, The Freshwater Aquaculture Book: A Manual for Small Scale Aquaculture in North America.)

Among the fishes discussed here, bullheads, non-hybrid sunfish, and tilapia will readily reproduce in any pond in which they can survive. Channel catfish and carp are a bit more particular about their breeding grounds, but most home growers should be able to get them to reproduce. However, many backyard aquaculturists consider trout to be too tricky and time consuming to breed ... while breeding hybrid sunfish requires specialized skills, and eels are unlikely to reproduce at all in captivity.

On the other hand, none of these fishes will breed in a cage that's kept properly suspended off the bottom. This seeming disadvantage is actually an advantage more often than not, because it lessens the chance of the cage culturist encountering the all-too-common problem of stunted, overpopulated groups of fish. Indeed, such individuals who own farm ponds are in the position to "have their cake and eat it too." The growers can use their ponds as natural hatcheries to produce young with which to stock the cages, and the containers themselves as isolated pens to raise collections of fish to optimal eating size away from the competition present in the pond at large.

And Now the Cage

Once you've gotten access to a body of water and are confident that you'll be able to line up a supply of fish, it's time to build your cage. (You could, of course, buy a fish box, but commercial cages often cost three to five times more than homemade units.) Rectangular cages seem to work best. And while we once raised bullheads in 24-cubic-foot enclosures at New Alchemy — and commercial growers often use cages of as large as 1,000 cubic feet — home growers planning to manage one to six boxes will probably do well with a 3' x 3' x 6' (or 54-cubic-foot) structure. (See our Fish Farming Cage Illustration.)

Although you can make the cage walls out of almost any sort of mesh material, I prefer nylon webbing, such as Du Pont Vexar, which is sold in hardware stores (and by aquaculture supply companies) and often used to keep leaves out of rain gutters. Flexible nylon mesh is easy to work with, slightly buoyant, long lived, and chemically inert. Wire, on the other hand, may be initially less expensive but it's also much heavier than nylon — you'll need extra flotation to compensate for the added weight — and it usually has sharp ends that can injure people and fish. Worse yet, all but the best-coated wires corrode in water. I've heard many sad but true stories about bumper fish crops that fell through cage bottoms at harvest time!

Whatever material you use, though, you'll improve water circulation, reduce your cleaning labors, and save some money if you purchase the largest mesh that's capable of preventing your new baby fish from escaping. (We often used 1/2" mesh at New Alchemy.)

Your cage will need a rigid top frame made of either treated wood or aluminum tubing, and you can frame all of the underwater sides of the cage as well, if you wish. (Non-corroding nylon cable ties sold at electrical supply stores are my personal choice for fastening the mesh to this frame.) Or you can make a "basket" cage by simply "sewing" the sides together with nylon rope or twine. Basket cages are lighter, easier to store, and less expensive to make than full-frame ones, but I've found that they take longer to construct and don't hold their shape as well.

The other three essential cage components are the flotation devices, a top, and a feeding ring. It takes one cubic foot of polystyrene foam (an ideal flotation material) to support a 50-cubic-foot full-frame, nylon web cage. Since the polystyrene may crumble with time, though, you should enclose it in sturdy, water-resistant bags. (It must also be attached in such a way that the top of the cage sits at least four inches above the water level.)

The cage will require a hinged or removable top, because most fish prefer shade to sunlight. (This lid will also help deter theft, predation, or high-leaping escapes.) The feeding ring is simply a band of fine mesh — such as mosquito netting — that prevents floating feeds from slipping out the sides of the cage. You can stitch a length of this material around the inside of the cage at the water line, construct a smaller floating, wood-framed feeding skirt, or cut a feeding "well" hole in the cage's lid and trim that with the netting.

And how much will all this cost? Well, the price will naturally vary depending on where you live, but in 1979 we were able to construct a 48-cubic-foot (4' x 4' x 3) basket cage from Vexar and treated wood and fit it with a lockable plywood top, canvas-wrapped foam flotation, and an attached feeding ring for only $47.

Positioning Your Cages

The ideal site for a cage will be a spot where the water is about six feet deep. Shallower locations will normally have reduced water circulation, while some deeper spots are subject to "turnover" — a sudden reversal of water layers that brings deoxygenated bottom water to the surface. Caged fish that are caught in a turnover may be killed.

Try to choose a site that's exposed to breezes but protected from high waves and motorboat traffic. Consider, too, that if you can set your cage off the end of a dock, you won't have to boat out to feed the fish. Should you need to site the enclosures in open water, do secure them with at least two anchors — one at each end of the cage — fastened with nylon lines. And always put your cages out before you're ready to stock them so you can make sure they float properly.

Stocking the Cages

A delicate, indeed critical, moment in any aquaculture operation is stocking: The transferring of the captured or purchased fish to their permanent home. You should always transport your stock in large, double plastic bags that are filled halfway with water and have plenty of air inside. (Commercial suppliers often pack their shipping sacks with pure oxygen instead of with ordinary air. Don't waste that valuable element by opening such bags prematurely.)

Begin by floating the fish-filled sack in your cage, and checking the water temperatures of the enclosure and of your container. Unless the two temperatures are nearly identical, you'll have to wait for them to equalize. The process (which usually takes from 15 to 45 minutes) can be speeded up by carefully exchanging small amounts of water between the sack and cage, but if you try this, be sure you change the water temperature gradually. Too drastic a jump can either kill the fish outright or lower their resistance so much that many will likely die later on.

When the two temperatures have equalized, submerge the bag, tilt it sideways, and allow a few minutes for the two fluids to mix. Only then should you move the bag to encourage the fish to exit.

In general, stock your cages when the ambient water temperature is slightly below the range of desirable growth temperatures. That would mean introducing trout into 50-55°F water, tilapia in 70-75° ponds, and the other species discussed here in 60-70° water. Consequently, in most cases you'd probably want to stock your cages in the spring.

However, in many areas, larger, more vigorous stock is often available in the fall. If stocked then, these fish can be overwintered and "finished out" during the following spring and summer. Or if you live where hard freezes are not a problem, you can even stock young fish in the fall for overwintering. Remember, though, that fish set out in autumn won't grow appreciably during the cold months. (In warm southern climates, trout can be successfully reared in the winter.)

The size of your purchased fish (if you do buy any) will depend on your budget. Smaller ones are naturally less expensive. However, if you can afford it, stock the largest young fish you can obtain; they'll outgrow their little kinfolk and will have a higher survival rate.

And just how many fish should you stock? Well, to figure that out, you have to calculate backwards from your anticipated harvest, keeping in mind that the carrying capacity of a cage is the weight, not the number, of fish it will support. I've already indicated that you might expect to raise about 200 pounds of fish in a 50-cubic-foot basket. However, since most cage culture depends upon a growing season of six months or less, you probably won't be able to produce fish that average more than 3/4 pound each. Actually, I think 1/2 pound is an excellent pan size for the majority of species. (It's also an efficient size, since fish grow fastest when they're small.) Therefore, if you intend to produce 200 pounds of 1/2-pound fish, you'll want to rear 400 individuals. So, allowing for some mortality, you could stock a 50-cubic-foot cage with 440 bullheads, trout, or whatever.

Feeding Your Fish

Fish in cages have very limited access to natural food. The cage culturist, then, is responsible for providing a diet which is both quantitatively and nutritionally complete.

Most American fish culture is based on commercial processed feeds. These are somewhat expensive, and there are ecological arguments against using them (they're made primarily from fish meal, cultivated grains, and petroleum-synthesized vitamins — products that could certainly be put to more appropriate uses). However, the rations are both effective and convenient, and beginning cage culturists have enough to learn without trying to create their own feeding schemes

The majority of ready-made fish food is tailored to the requirements of either channel catfish or rainbow trout — our two major conventional aquaculture crops — and will thus be only approximately suited to other species. Such feeds often lack certain trace nutrients, as well, since pond-raised fish (for which the foods were created) are expected to gather minor quantities of natural organisms. However, some manufacturers do offer special cage culture feeds that are nutritionally complete. (You may not need this more expensive fish food, though, if you can supply some of the natural supplements I'll discuss below.)

The other characteristic necessary in any feed used in cages is that it float. Sinking rations will be largely lost through the bottom mesh.

Commercial feeds range in size from meal to 3/16-inch pellets. You can pretty well gauge the size you'll need to buy by the mouths of your fish. Since that size will change as your stock grows—and because the feed doesn't store well—you should always buy the smallest convenient quantity at one time. (And keep those rations tightly covered.)

The best time to feed is usually at dawn. Later in the morning or at dusk are second choices. As a rule of thumb, you'll want to give 3% of the total body weight of your fish at each feeding, and feed them six days a week. And how do you determine the total weight of your fish? Simply catch and weigh a sample periodically, then using this information and the number of fish in the cage to calculate the total group's weight.

I strongly recommend that you employ this sampling method during your first season. In subsequent years, you may develop enough of a feel for feeding to skip the sampling and successfully raise fish by simply giving them the amount they'll consume in a half-hour (or one hour, if you're raising carp).

To gain some idea of the kind of feeding schedule you'll have, make a few calculations using the "end point" method. To illustrate how this works, let's suppose you begin with 200 bullhead fingerlings that, all together, weigh 2 1/2 pounds. Now, assume that you want to raise them to an average of 1/2 pound each, thus yielding 100 pounds of fish. Using the 3% weight-a-day feeding rule, you would then give them 0.03 X 2.5 = 0.075 pounds of ration the first day. Furthermore, on the last day before harvest, you'd be feeding 0.03 X 100 = 3 pounds of feed.

If you have some idea of when that harvest date will occur — a good time is just after the date your water temperature falls below that needed for optimum growth — you can then count the number of feeding days between stocking and harvest and calculate an approximate weekly feed allotment that gradually increases from 0.075 to 3 pounds a day. Be sure, however, to allow for the fact that fish put on weight most rapidly at first, and slow down some as they grow.

Natural Feeds

All of the fishes mentioned here, with the exception of the sunfish, can be reared on a 100% processed food diet. (Although most sunfish will eat some commercial feed, they really don't make good use of it.) Of course, all fish will eat available natural foods. The possibilities are legion, but I'll limit myself to describing five of the ways you can provide your caged livestock with "fresh victuals."

Bug lights: The Will-O'-the-Wisp Bug Light Fish Feeder consists of an ultraviolet light and an impeller fan. Night-flying insects attracted by the light are sucked in by the fan and blown into the water. The unit can be mounted directly over a cage and costs pennies a day to operate. And although trout and bluegills particularly relish floating bugs, almost any species of fish will learn to eat them.

The Trail Lake Feeder: The Trail Lake Feeder is a do-it-yourself, low-tech way to provide caged fish with a good supply of insects — in this case, fly maggots. To construct the unit, mount a hardware cloth basket on a pole a few feet above the cage. Wrap some form of rotting meat or fish — road kills are OK — in moistened paper, set it in the basket, and then let nature take its course. As the many maggots that appear mature and seek to pupate, they'll move away from their food and fall into the water. (You'll need to replace the decaying meat weekly.)

Earthworms: If you raise earthworms, you can construct a special feeder by simply punching a few holes through a thin block of polystyrene foam. Spread some worms on that and float it in the cage. As the crawlers burrow their way down through each hole, the fish will eat them.

Scrap and offal: If you have a source of unwanted fish, meat scraps, or offal, you can use these leavings by grinding, chopping, or boiling them. Any food particles that are so small they might pass through the mesh should be placed in a large container and lowered into the cage.

Green plants: Although most of the fishes I've discussed will not eat leafy matter, tilapia are quite fond of plants and Israeli carp will take some. Most soft aquatic plants, comfrey, purslane, carrot tops, and hairy vetch can be used. Serve the vegetation by tying it in bunches and hanging it in the cage.

Although you may not be able to provide much natural feed, keep in mind that even an amount totaling less than 1% of your fish's total diet may improve their growth or flavor — or even enable you to switch from a complete commercial feed to a less costly supplemental one. (Remember to allow for water content when calculating feeding rates, or ratios, with natural feeds. Commercial products are relatively dry, while insects contain approximately 75% water; fish, meat, and worms about 85%; and green plants 90 to 95%.)

Cage Maintenance

 Inspect your cages daily. When necessary, clean the mesh so any accumulated algal growth won't inhibit water flow. (A toilet brush mounted on a long handle works fine for this purpose.) You should also examine the fish to see that their feed is being eaten and that they're in good health. If food is being wasted, cut back on the amount, skip a day's serving, or change their diet.

If your stock remains off its feed after you've taken the above measures, check the fish for disease. It isn't feasible to discuss all of the potential health problems here. (Consult The Freshwater Aquaculture Book, or a diagnostic service if you suspect a malady.) Disease can do damage, but let me put the matter in perspective: In six years of raising bullheads and sunfish at New Alchemy (excluding the mysterious brown bullhead deaths I talked about) we had one disease outbreak, and that resulted in only about 10% mortality.

Harvesting Your "Crop"

Mass harvesting of cage-cultured fish is the simplest process imaginable. Just lift the cage and take out the crop! To catch a few fish for dinner, you can drive some into a corner, using a shallow, square-cornered dip net, and scoop them out.

The Payoff

Although the "bottom line" of your homestead fish culture operation will vary depending on local expenses, I'll try to give you some idea of what to expect by citing two disparate examples.

In 1974, the Kerr Foundation of Poteau, Oklahoma carried out experiments in commercial cage culture of channel catfish. It stocked 9 to 11 large (7"-8") fingerlings per cubic foot in 36-cubic-foot cages and recorded purchases, catfish cage feed, chemicals, and "cage usage charge" costs. (The Kerr people didn't keep track of any fixed or overhead expenses.) According to their estimates, the 1.2- to 1.3-pound fish they produced cost 45¢ to 46¢ a pound over a seven-month growing season.

At New Alchemy we made a homestead-scale production study in 1979, raising yellow bullheads in 48-cubic-foot cages that were stocked four (3"-4") fish to the cubic foot. (We obtained them for free from a wild population.) We used ordinary commercial trout feed — supplemented with small amounts of insects and earthworms — and harvested our bullheads at 0.35 pounds after three months of growth. After figuring in the cost of the cage (amortized over five years), we estimated that our fish cost us 66¢ a pound to produce.

Furthermore, that price doesn't include the satisfaction we gained from growing our own fish or the fact that the consensus at New Alchemy was that those bullheads were the best fish we ever ate!

EDITOR'S NOTE: For a detailed treatment of this entire topic, including plans for cage construction, you can read Bill McLarney and Jeffrey Parkin's The New Alchemy Back Yard Fish Farm Book: Growing Fish in Floating Cages.

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