By Bob Goemans
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Q&A - Fish Nutrition

Authored by: Rob Toonen


I have a Black Volitan Lionfish that only eats live goldfish. I heard that it is bad to feed him only goldfish, but he shows no interest at all when I have tried to feed him krill, silversides and a couple of other frozen foods. Is it really a big deal to feed him goldfish, and if it is so bad, why do all the pet shops feed live goldfish to lionfish? Is there a better way to feed them?




Well, first off I’d have to say that with most marine aquarium fishes, and your Volitan Lionfish (Pterois volitans) certainly falls into this category, there is no way for you to sex the fish. So, unless you happen to have some specific reason for suspecting your fish is male, “he” ought to be an “it.”

Of course that doesn’t tell you anything about what to feed your fish, so to answer your question, yes it is a big deal to only feed goldfish to a predatory marine fish like a Volitan Lionfish. As for why the pet shops use this food for their animals, I suspect it is a combination of a couple of things. First, many predatory fishes, such as lionfish, when first brought into captivity 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 pet shop doesn’t have to invest any time in training the lionfish to accept other foods, and that keeps the overhead down. 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 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 the pet shop to be able to demonstrate. You have to admit, there are few feeding responses among marine aquarium fishes quite so dramatic as watching a lionfish hunt and swallow a goldfish whole, and whether we can 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.

Ok, that aside, let’s get into the discussion of why you should not feed goldfish to your lionfish – 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 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. Aside from the fatty liver "disease," fatty acid malnutrition in 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 lionfish only 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 table below between the freshwater fish and the marine predators should be informative enough:

Table 1: An average of the total amount of each nutrient in a 100g sample of tissue as compiled by the US government (Dept of Agriculture) for nutritional comparisons of foods that are available to consumers. Mixed species averages were used where available, and individual results from several species were averaged to generate a mixed species mean for the remaining categories.

EnergyProtein - Total Lipid - Vit. B - Vit. C

Freshwater fish 548 kj 16.69 g 6.60 g 3.15 mg 1.10 mg

Brackish fish 589 kj 19.16 g 6.51 g 5.87 mg 1.30 mg

Marine fish 381 kj 19.21 g 0.99 g 2.00 mg 2.25 mg

Freshwater Crustacean 301 kj 14.85 g 0.97 g 2.60 mg 0.50 mg

Marine Crustacean 457 kj 20.46 g 1.62 g 3.90 mg 2.00 mg

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: Average 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.

Saturated FatLA ALA EPA DHA

Freshwater fish 1.43 g 0.70 g 0.19 g 0.16 g 0.16 g

Brackish fish 1.22 g 0.28 g 0.14 g 0.32 g 0.53 g

Marine fish 0.18 g 0.01 g 0.01 g 0.06 g 0.19 g

Freshwater Crustacean 0.16 g 0.08 g 0.03 g 0.12 g 0.03 g

Marine Crustacean 0.29 g 0.02 g 0.01 g 0.26 g 0.16 g

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. 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 fat into the diet of any animal (you or your fish) is not going to have a substantial effect on long-term health!

Unlike the advice I have heard dispensed in several pet shops to supplement goldfish and/or guppies with a HUFA (highly unsaturated fatty acid) enrichment product 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, but like I said, 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 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. 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 lionfish that I could not train in this way, if you invest the effort.

The best advice I can offer is probably to avoid feeding freshwater feeders to marine fish and vice versa because of the different nutritional profiles of the prey items. I think that the key to maintaining healthy fish and/or inverts long-term in aquaria has more to do with a properly varied diet than any specific nutritional component or additive. Given the limited data on the nutritional profiles of freshwater and marine prey items that I could find, there seems to be good evidence that feeding a lot of freshwater fish to a marine predator will result in a diet unnaturally high in fats.

Together with the experience from public aquarium fish autopsies, it would appear prudent to avoid feeding primarily freshwater fishes to marine predators in general. The primary recommendation from these sort of discussions (again and again) seems to be that aquarists need to simulate the natural diet and vary it as much as possible. Given the (admittedly) limited data available, I think it just a reasonable precaution to avoid using such feeders as a sole or primary food item for extended periods of time. I doubt that the occasional guppy or molly is likely to cause any long-term problems (sorta like the occasional trip to a drive-thru burger place), but if it is used as a staple of the diet or the primary food, your fish are probably destined for the same fate as you if you were to eat exclusively at one of the burger chains for the rest of your (substantially shortened) life. . . .


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