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Are Most Fish Omnivorous?

To the saltwater fish aquarist, Atlantic Blue Tangs (Acanturus coeruleus) are considered to be primarily herbivorous. Although they are indeed browsers, with lips and dentition designed for snipping off the tips and branches of algae, through feed trial studies we know that they require much more than algae to be maintained in captivity. A study was performed by Dr. Ruth Francis-Floyd and Chris Tilghman, from the University of Florida, involving captive nutritional management of herbivorous reef fish using Atlantic Blue Tang (Acanthurus coeruleus) as test subjects, divided into three groups. The first Group was fed washed seaweed (ulva spp.). The second group was fed commercially prepared food designed for herbivores and the third group was fed another commercial diet that was an all purpose diet (marine protein was in the formula). At the end of the study, the first and the second group suffered a high mortality rate, (approximately 80%), with the surviving fish showing clinical signs of malnourishment. Some had become emaciated to paper-thin condition. The third group had only an approximately 30% mortality but the remaining fish had 400% weight gain! While the information from this study was made available during a lecture on November 29, 2001, at the Marine Ornamentals International Conference, held in Lake Buena Vista, Florida, outside of that conference room the results were never made public.

While ichthyologists are busy describing and classifying new species of fish each year, there is much that is still not known about their biology and behavior in the wild.

While most fish do in fact have specialized feeding methods in the wild, and do ingest a certain amount of matter more than others, in the wild almost all fish are opportunistic feeders. Even the more specialized feeders, such as Atlantic Blue Tangs, ingest a certain amount of nutrients from other sources.

In Africa, the fish found in Lake Malawi and Lake Tanganyika are some of the most specialized feeders found on the planet, yet they too are all opportunistic when it comes to feeding. Many people get too caught up in the amount of greens some species eat, vs. where their protein source in the wild truly comes from. The reality is that even though algae dominates the stomach contents of the majority of certain species of African cichlids, and many of these species have indeed been classified as herbivores, the actual foods that make them grow are insect nymphs and larvae, crustaceans, snails, mites, micro-organisms, and zooplankton, not vegetable matter. This is something that many hobbyists fail to understand.

Some hobbyists may consider feeding Surgeonfish a diet of algae to be more natural than a pellet or flake food, but that couldn't be further from the truth, and the results from the study performed at the University of Florida bear this out.

In the wild, herbivores have to eat constantly for at least 12 hours per day in order to ingest sufficient nutrients. During this period they discharge waste constantly. Under artificial environments, as in an aquarium, they no longer have an unlimited amount of food to graze upon, and even if you could feed them every hour on the hour, it would add massive amounts of pollution to your aquarium. Keep in mind that an aquarium is not even a tiny fraction of a drop in comparison to the volume of water found in an ocean or large lake. In order for aquarium-raised fish that are classified as herbivores to thrive, they must take in sufficient nutrients from their diet, and what takes place in the wild can simply not be duplicated in an aquarium setting.

Carnivores may in fact eat fish in the wild, but those fish will usually be gut-loaded with various smaller life forms, such as zooplankton and phytoplankton. Their prey is part of the natural food chain, and these feeders in the wild provide much more balanced nutrition than frozen silversides found at your local grocery store. Just as captive-raised specimens, even carnivores in the wild will consume a certain amount of vegetable matter to acquire various nutrients that their diet may be lacking in. The reality is that carnivores do not just eat meat, any more than herbivores just eat algae.

Over the years many hobbyists have been led to believe that fish classified as herbivores must eat a diet that's mainly made up from vegetable content in order to truly thrive in captivity, but this is simply not the case. In the past I have failed miserably when using this approach with herbivores, and I have learned from my mistakes. While I could have gained a great deal financially by creating yet another fish food formula marketed specifically for herbivores, using mostly vegetable content and green dye, it would have been unethical to do so. I simply could never manufacture and market a product that will not truly benefit the fish.