ROB TOONEN, Ph.D.
Feeder fish (guppies and goldfish)
It is much more intuitive to realize that nutritional value is a major concern for feeding our animals. One of the most common mistakes that people make with feeding large predatory fishes (such as groupers, snappers or lion fishes), is to give them feeder goldfish. There are many reasons that people usually give feeders to their marine fishes. First, virtually all of these predatory fishes are still wild-caught, and when first brought into captivity, they do not receive the proper cue to initiate feeding from flake or frozen foods, and the addition of a struggling live fish (such as a freshwater goldfish tossed into a marine tank) is a powerful feeding cue to get these fish to immediately attack the prey. That means that the owner doesn't have to invest any time in training the lionfish to accept other foods. Second, because goldfish are so slow relative to most marine fishes, it is easy for the lionfish to catch. Third, the use of goldfish is a simpler and more cost effective way to feed these guys than to use a marine "feeder" fish. I think that all of these factors come into play with the decision of most pet shops to feed goldfish to their lionfish. Because a good feeding response is one of the criteria that most of us use to decide whether or not a fish is healthy and worthy of purchase, this is an important thing for any pet shop to be able to demonstrate to us as potential buyers. So, adding a feeder goldfish to a tank of lionfish is one of the best sales techniques available to a retailer. You have to admit, there are few feeding responses among marine aquarium fishes quite so dramatic as watching a lionfish hunt and then engulf a goldfish whole! Whether we are willing to admit it or not, deep down, most people get a thrill from watching a predator hunt down and devour live prey. In fact, I suspect that is a large part of the reason that many people decide to purchase a lionfish in the first place.
I am not saying that there is anything wrong with the thrill of seeing a lionfish hunt. In fact, many zoos and public aquariums now realize that providing live prey to active and intelligent predators makes for a much healthier and happier animal. It seems that the thrill of the hunt extends to our pets as well, and without the challenge of hunting live prey (or some similar puzzle-solving challenge or enrichment) many zoos have found that their animals develop behavioral and/or health problems. However, the nutritional value of feeder goldfish for a marine predator is a much more serious concern. So, now let's get into the discussion of why you should not use feeder goldfish for your lionfish (or any other marine predatory fish) - I'll explain it in more detail below, but the short-and-sweet answer is that freshwater fish make a lousy food for marine predators.
A buddy of mine is a fish parasitologist who used to volunteer with a couple of veterinarians at public aquaria to do autopsies on dead fish. He was telling me that the single most common cause of death he's seen among marine fishes at public aquaria is "fatty liver disease." Although not really a disease, fatty liver is a serious condition in which the liver becomes enlarged, often to the point that it interferes with, or even crushes, the other internal organs and is apparently the cause of death. This condition seems most commonly to be the result of poor diet, and the consensus of several well-known fish pathologists is that the single most common cause of fatty liver disease is a diet high in saturated fats, although biotin and/or choline deficiencies, toxemia and "unknown nonspecific causes" are also possible factors. My buddy said that he also sees the same fatty liver disorder in a variety of marine fishes (most commonly groupers and lionfishes) from pet shops and hobbyists who fed these predators on a diet of primarily live goldfish. Although bacterial diseases and parasitic infections claim many more fish than nutritional deficiencies(Francis-Floyd and Klinger 2003), fatty liver disease is probably one of the most common of fatal nutritional problems.
"... the short-and-sweet answer is that freshwater fish make a lousy food for marine predators."
Aside from the fatty liver "disease," providing the wrong proportions of the various fats in the diets of marine fishes have been shown to result in reduced growth, lower percentages of muscle tissue, liver degeneration, higher susceptibility to bacterial and viral infection, and a decrease of hemoglobin in the blood cells among other nutritional problems. All of these things suggest there is a very real, and potentially fatal, consequence to feeding your favorite marine predator primarily (or only) on freshwater feeder fish (such as goldfish or guppies).
Because there is no real data for the nutritional profiles of aquarium fishes, I did a survey of the aquaculture literature to find the nutritional composition of feeder fishes. Of course, I couldn't find the composition of guppies and goldfish, so I did the best I could with other fish species that are regularly examined for nutritional profile by the US Department of Agriculture for human consumption. A quick comparison of farmed freshwater catfish & carp to marine cod & snapper (these seemed to be the most reasonable proxies for feeders and lionfish that I could find the exact nutritional composition in my search) shows some major differences in the nutritional profiles. Unfortunately, there is little interest (or money) to develop similar data for aquarium fishes, so although these are not actually the exact values for guppies and goldfish, the general trends shown in the summary tables below between the freshwater fish and the marine predators should be informative enough:
Table 1: Total amount of various nutrients in a 100g sample of tissue from selected species of potential food fish as compiled by the US government (Dept of Agriculture) for nutritional comparisons of foods that are available to consumers.
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There are some obvious differences between species with freshwater, brackish and marine origins. For example, marine species tend to have less total energy per unit weight, but more protein and substantially less fat. Not surprisingly, brackish species tend to fall between the two extremes, although in most cases other than protein content, the brackish species tend to more closely resemble freshwater species in their nutritional makeup. The most striking and important difference between marine, freshwater and brackish species however, is the much lower fat content of marine foods. A closer look at the lipid profiles of these species groups gives a better picture of how the groups differ and where it is possible to artificially reduce that difference.
Table 2: Amount of saturated fat and a number of essential highly unsaturated fatty acids (HUFA) for each of the species groups listed in Table 1. Values for Saturated fats, LA (Omega-6, linoleic acid - 18:2), ALA (Omega-3, alpha-linolenic acid - 18:3), EPA (Omega-3, eicosapentaenoic acid - 20:5), and DHA (Omega-3, docosahexaenoic acid - 22:6) are again measured in grams from a 100g tissue sample as presented in Table 1, above. These fatty acids are among those typically included in HUFA enrichment products to supplement the diet of marine fishes in captivity. Small, non-zero numbers are denoted by < 0.01.
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The lipid profiles between freshwater and marine species are very different, and the amount of saturated fat in the average freshwater prey fish is roughly 8 times that of the average marine fish. But, if we use catfish and carp as a reasonable proxy for feeder goldfish, then the picture is even worse, with a single feeding of goldfish providing more than 20 times the saturated fat as a feeding of the average marine prey fish. It is hard to imagine that incorporating 20 times more saturated fat into the diet of any animal (you or your fish) is not going to have a substantial effect on long-term health!
Table 3: Averages of the nutrient values presented in Tables 1 & 2. If mixed species averages were available in the U.S. government (Dept of Agriculture) database, I have used them here. Where mixed species averages were not available, I tabulated the records for up to a dozen species (I stopped after 12 if there were more) in each category and calculated the average for each for presentation here.
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In stark contrast to the advice I have heard dispensed in several pet shops to supplement goldfish and/or guppies with Selcon or Zoecon (highly unsaturated fatty acid supplements) to make up for the fact that these animals come from different habitats, in this case such "enrichment" will only make the situation worse, not better! Supplementing the fat profile of a goldfish with HUFA would be roughly equivalent to making your BigMac more nutritious by dipping it into vegetable oil first. Sure, it's better than dipping it into lard, but still not going to change the fact that you're getting more fat in that single meal than you're ideally supposed to eat in an entire day, and it only makes things worse than if you had not dipped the burger at all. . . .
So obviously feeder goldfish are not the best choice of a staple food for your marine pets, but what about guppies or mollies? These are brackish fish that are frequently adapted to saltwater - do these provide better nutrition than feeder goldfish? The simple answer is "I don't know." My gut feeling is that because brackish-water fishes are somewhat intermediate, but are generally closer to the lipid profiles for freshwater than they are for saltwater species, guppies and mollies would likely be inordinately high in saturated fats as well. Probably closer to the roughly six times the amount of saturated fats found in the average brackish fish rather than the roughly twenty times as high likely to be found in goldfish, but still high nonetheless. Perhaps that makes these fish a better choice than goldfish for a live food item to be fed to your lionfish until it can be weaned onto frozen silversides or some other marine staple food. David Cripe of the Monterey Bay Aquarium tells me that he has used saltwater acclimated guppies as food for marine fishes that he has been rearing, but these are usually fed for only a short period of time until the larvae are switched onto other foods. It seems unlikely to me that the occasional feeding of saltwater acclimated guppies or mollies will prove to be a problem for marine predatory fish, but as I said earlier, this is just my gut feeling, and I am really just guessing here because there are no fatty acid profiles available for any of the aquarium species we're discussing here. However, given the data above it seems that ghost shrimp or even freshwater crayfish would be the best choice to feed your lionfish (or whatever) until you can train it to take frozen marine prey fish.
The reason I say that is because the nutritional profile is not nearly so different between freshwater and marine crustaceans, and in fact, in this case, the freshwater animals are simply deficient in the amount of fats provided. This is good news for aquarists because it means that by simply supplementing a diet of crayfish or ghost shrimp with an occasional boost of some HUFA enrichment product (either by gut-loading, soaking or injecting the "feeder" animals), you're likely to provide a perfectly suitable diet for long-term care of a marine predator such as a lionfish or grouper. This is easy an inexpensive to do, and once your fish is eating well on these prey, you can slowly start trying to hand feed it. Start off by hold the live shrimp in your fingers and let the fish come close to them before letting the shrimp go. After a few times, the fish will usually recognize that your fingers in the tank means food, and start coming to investigate them as soon as you put them into the tank. Start to hold onto the shrimp longer and longer until the fish starts to come and take the live food from your fingers before you let it go. Once your lionfish will take live ghost shrimp from your fingers, then you can try some recently dead ones, and then eventually move on to frozen shrimp. It takes some time, effort and patience, but I have yet to find a fish that I could not train in this way, if you invest the effort.
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