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The Feeder Fish Debate: Are They Essential, Cruel, Or Dangerous?


By Neale Monks


Most fish are predators. There are a few fish that feed primarily on algae or soft-leaved plants -- plecs, mollies, and mbuna, for example -- but the vast majority eat other animals. Because aquarists keep relatively small fish, the preferred prey tends to be rather small as well, with small crustaceans and insects topping the list. Live or frozen bloodworms, mosquito larvae, brine shrimps, and Daphnia are enthusiastically eaten by virtually all of the popular community fish.

However, there is no escaping the fact that many fish are piscivores; that is, their natural diet isn't insects and crustaceans, but fish. Caring for these fish raises an ethical dilemma that has divided the hobby for decades: should aquarists feed predatory fish with live 'feeder fish' such as goldfish, guppies, and minnows.


Is live food essential?

Piscivorous fish can be divided into three types according to the degree to which live fish are an important part of their diet. The first group are the opportunistic piscivores. These are species that only eat smaller fish when the opportunity presents itself. Small fish will be eaten, but they are not a major part of their diet in the wild, and in captivity they can be maintained easily without any need for feeder fish. By far the best-known opportunistic piscivores are angelfish; while they have evolved to feed primarily on mosquito larvae and other small invertebrates, in aquaria they will steadily work their way through any livebearer fry or small tetras (such as neons) in the tank with them. Other widely traded, but not always recognised, opportunistic predators include polka-dot catfish, Asian killifish, giant danios, spiny eels, ropefish, archerfish, knight gobies, and mudskippers. With opportunistic predators, it isn't so much providing them with feeder fish that is the issue, but making sure they won't 'accidentally' eat their tankmates that's the problem!

The second group of piscivorous fish are the facultative piscivores. These are fish that feed primarily on smaller fish but also take a wide variety of other prey items as well. Because they are generalists, when kept in aquaria they will accept a wide variety of foods, from dried foods like pellets and flakes through to various frozen and live invertebrates such as shrimps and insects. Oscars are popular examples of facultative piscivores, being happy to take live feeder fish if they are offered, but just as easily maintained on good-quality carnivore pellets as well as frozen lancefish and whitebait, shelled prawns and mussels, earthworms, river shrimps, crickets, and insect larvae. Other widely traded facultative piscivores include bichirs, arowanas, African butterflyfish, piranhas, snakeheads, climbing perch, knifefish, gars, predatory catfish, and sleeper gobies.

The third and final group are the obligate piscivores. These are fish that have evolved to feed more or less exclusively on smaller fish. These include pike cichlids, needlefish, pike livebearers, leaffish, prehistoric monster fish, and freshwater stonefish. Any aquarist buying these fish needs to be aware of the fact that the responsibility to provide them with a healthy diet of clean feeder fish may well be part of the deal. For reasons outlined below, simply relying on cheap goldfish and guppies from your local tropical fish store isn't a good idea.

Broadly speaking then, unless you are keeping obligate piscivores, there is absolutely no reason to use feeder fish at all. On the other hand, if you are planning on keeping an obligate piscivore, then you will need to decide whether to use feeder fish or not.


The case FOR feeder fish

The biggest advantage to using feeder fish is that they are readily accepted by otherwise difficult species. For any aquarist attempting to keep obligate piscivores, weaning their new fish onto an alternative diet is a major challenge. This is especially true with wild-caught species that simply will not recognise things like frozen whitebait as potential food. Given that the fish in your local tropical fish store might not have had a proper meal in weeks, providing it with at least a few feeder fish can be a quick and effective way to return it to perfect health and condition.

A second big advantage to feeder fish is that they are inexpensive and easily obtained. Virtually every aquarium shop carries goldfish and guppies, but only a few have live river shrimp, and if you want earthworms, you'll probably need to find your own. Feeder fish are also very convenient; they can be simply placed into the tank, and the piscivorous fish will pick them off at its leisure. Training an obligate piscivore to take a piece of prawn or whitebait is much more difficult because they do not recognise inanimate objects as prey.

Given that these predatory fish have evolved to eat smaller fish, it is reasonable to assume that feeder fish will provide them with a healthy diet. This isn't necessarily true, because feeder fish can be nutritionally unbalanced and a source of parasitic infections, but in general, there's no question that many fish will thrive when regularly fed appropriate feeder fish.


The case AGAINST feeder fish

The main disadvantage of using live fish as food is the risk of introducing parasites and bacteria into the aquarium. The cheap fish likely to be used as feeders have, almost by definition, been raised intensively, and that means they will have been exposed to all kinds of disease-causing organisms. Whitespot and velvet are extremely easily transmitted between feeder fish kept in cramped quarters, and over the long term, feeder fish can expose predatory fish to skin and gill lice, parasitic intestinal worms, neon tetra disease, and a whole host of opportunistic parasites, bacteria, and viruses.

Goldfish are especially popular as feeder fish, being cheap and relatively large, but they are actually among the worst of all prey species for most predatory fish. They are fatty and rich in a chemical called thiaminase, an enzyme that destroys the essential nutrient thiamine (also known as vitamin B1). Rosy red minnows, a popular alternative to goldfish, also contain a lot of fat and thiaminase. In the wild, because piscivorous fish eat a variety of species, any shortcomings with one prey item will be counterbalanced by the next; so while a gar or channel catfish might catch a minnow one day, its next meal might be small perch, carps, or killifish. But in the aquarium, where only one or two species of prey are used, there is a very real risk of vitamin deficiency. By contrast, aquarists taking the time to train their predators to take a selection of frozen foods supplemented with pellets or flake can guarantee that their fish will receive a varied diet that will keep their fish healthy throughout its lifetime.



The majority of aquarists are against the use of feeder fish, but then again, most aquarists keep community tank species that thrive on flake, bloodworms, and other non-contentious food items. Irrespective of the pros and cons outlined above, the basic argument comes down to whether or not it is cruel. The advocates of feeder fish usage maintain that the prey fish is killed quickly, and that killing a small fish is fundamentally no different from killing an earthworm, river shrimp, or any other small animal. Since most aquarium fish foods are based at least in part on animal protein, usually fish meal, even flake food has, at some point, involved the death of animals somewhere along the line. Is sending a goldfish to its death in the jaws of a snakehead really any different to freezing and suffocating a haul of silversides and whitebait dragged out of the North Sea?

Opponents maintain that very often death doesn't come quickly, particularly where the predatory fish dismembers the prey fish before it is eaten. We have no idea if fish swallowed by a larger fish dies slowly or quickly. Do they die of shock at once, or suffocate slowly? Irrespective of how quickly the fish is killed, simply being dumped into a strange aquarium and then pursued by a predatory fish must be intensely stressful.

The argument that feeding live fish to a predator is simply doing what happens in Nature is largely fallacious. 'The wild' is a big place, and almost every time, prey fish usually manage to escape by swimming away to safety. They are also able to use anti-predation behaviours, such as forming large schools or foraging in streams too shallow for bigger fish to use. Under natural circumstances it might be several days, even weeks, before a predatory fish is able to make a successful kill. Simply dumping a couple of goldfish into a small aquarium doesn't in any way mimic Nature because it skews the odds dramatically in favour of the predator.

A critical concern is the degree to which fish feel pain. It is often assumed that fish cannot feel pain because they lack the pain receptor nerve cells that higher vertebrates possess. In recent years, however, the evidence has begun to swing scientific opinion in the other direction, with many now believing that fish do indeed feel pain. Fish will avoid using injured parts of their body, for example, in just the same way as we won't put our weight on a twisted ankle, suggesting that they are able to 'sense' damaged tissues and react accordingly.


Best practise

If you absolutely have to use live feeder fish, then the important thing is to avoid using the cheap, feeder-quality, goldfish and guppies often recommended because of the risk of introducing parasites. The only safe approach is to breed your own livebearers, such as mollies and guppies, and use their offspring. Indeed, anyone breeding these fishes will likely end up with large numbers of deformed or otherwise undesirable fry anyway, in which case using them as feeders does make some kind of sense. While some people have also used cichlid fry as feeders, the spiny fins on these fish are problematic for those predators not especially adapted to deal with them.

Livebearers are also much better than goldfish from a nutrition point of view: they do not contain thiaminase, and since most are vegetarians, it is easy to fatten them up on vitamin-rich green foods like algae and soft vegetables. It is also important to 'gut load' feeder fish before giving them to piscivores. All predatory animals, even strict carnivores like cats, depend upon the gut contents of their prey for certain nutrients not present in meat. This is why cats eat the guts and liver of mice and birds they catch yet ignore the bits that seem nicer to us, like the wing muscles.


Training obligate piscivores to accept alternative foods

Opportunistic and facultative piscivores are easy to work around because it is in their nature to try all sorts of different prey items. All the aquarist needs to do is provide them with a variety of dried, frozen, and live foods and see which ones they like. Gars and catfish, for example, scavenge as often as they hunt, and quite readily take all kinds of floating foods, from small pieces of white fish through to pellets. Climbing perch will do very well on a diet of bloodworms and crustaceans, either live or frozen; and African butterflyfish happily eat small insects dropped onto the surface of the water, and can eventually be taught to eat flake and pellets. The piscivorous pufferfish readily accept mussels, cockles, lancefish, and whitebait, and though snakeheads are famous for their predaceousness, in aquaria are easily satisfied with similar foods.

Obligate piscivores are a trickier because they do not recognise alternative foods as being edible. Where opportunist and facultative fish belong to the 'suck it and see' school of logic, obligate piscivores need to receive very specific sight and smell stimuli before they will attempt to attack and swallow something. Safe live foods like earthworms, mealworms, and river shrimps are one very effective option, and most obligate piscivores will take these with little effort. Needlefish, Xenentodon cancila, for example, are often cited as one of the species that will only eat live fish, but in captivity these fish will eat pretty much any live food at or close to the surface of the aquarium. Small crickets and other insects work especially well. Once they learn to associate the aquarist with feeding time, then frozen or dried foods of various kinds can be used, including krill, small shrimps, eve lancefish and silversides. Dropping food into a current of water often helps to convince these predators that the prey is 'alive'.

For mid-level and benthic predators, such as leaffish and toadfish, earthworms and river shrimps are usually the best option as far as live food goes. Both are safe and nutritious, and exhibit just enough movement to trigger the feeding response. Dangling small frozen fish on cotton thread can sometimes work well, too (though the prey should be tied to the thread loosely enough that the predator can pull it free!). Often, starving the fish for a few weeks helps to whet its appetites, encouraging it to try whatever foods you put on offer. Large predatory fish can easily manage without a meal for several weeks, but for the anxious aquarist this 'trial of wills' can be rather nerve-wracking! Sometimes though, even these tricks won't work, and the predator has to be kept on a feeder fish diet. With these fish, truly the only safe option is breeding your own feeders. Buying cheap goldfish and guppies just isn't an acceptable alternative.

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