Getting the greens in the catfish

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What are you feeding your suckermouths? Rebecca Bentley knows a thing or two about catfish diets, and she has some advice to share…

Catfishes display a vast diversity in morphology and diets, and dominate the planet, inhabiting all the continents except Antarctica. We have catfishes in Europe (like the Wel’s catfish, Silurus glanis), Asia (like the families of Bagridae, Pangasiidae, and Siluridae), Africa (including the Mochokidae and Clariidae,), North America (Ictaluridae) and the staggeringly popular catfishes of South America (such as the Loricariidae, Trichomycteridae, and Callichthyidae). There are even marine catfishes such as the venomous Plotosus lineatus.

Across the many families, catfish size ranges from miniature fish not much over 1cm to absolute giants of at least 300cm and potentially weighing hundreds of kilograms. With such a diverse morphology it stands to reason that not all catfish have evolved to eat the same foods, and within their ranks you’ll find a range of feeding adaptations including parasitism, carnivory, herbivory, detritivory and omnivory.

Plants, bacteria, fungi, protozoa and bacteria make up an important part of any ecosystem, and freshwater is no exception. A large number of fishes exploit these plentiful resources, and these are referred to as the herbivores. Within herbivory you can divide even further into the algivores (that eat algae), frugivores (fruit), folivores (leaves) and granivores (seeds). Fungivores, species which feed on fungi could be considered herbivores, while those fish that specialise on decaying matter are known as detritivores.

Many feed on structures known as periphyton, a complex material that grows on submerged surfaces and contains a mixture of algae, bacteria, protozoa and microbial life. Fish diets can be a little more complicated, as the occasional invertebrate — creatures such as sponges, bryozoa and rotifers — can make their way into their diet by also inhabiting the periphyton.

International catfish taste

When it comes to catfishes there is one group that is largely known for exploiting a herbivorous diet: the Loricariidae of South America, known colloquially as the plecos.

The majority are either algivores or detritivores although a few are omnivorous or carnivorous. With so many species found in overlapping ranges, each of them specialises, partitioning their dietary niches.

Fish diets are reflected in their morphology. For example, a rasping mouth with numerous fine teeth will be oriented towards feeding from a surface, likely on periphyton. When it comes to the Loricariidae, or plecos, there are misconceptions about which genera eat what. Out of the subfamily Hypostominae, which contains the most common species labelled as plecos, those in the genera Scobinancistrus, Leporacanthicus, Pseudacanthicus, Lithoxus and Exastilithoxus are, as far as we are aware, true carnivores. Hypancistrus, Peckoltia and similar genera are omnivores/detritivores. Panaque, Hypostomus, Panaqolus and Lasiancistrus heteracanthicus tend to be detritivores who feed on microbes that decay wood. Baryancistrus, Ancistrus, and Chaetostoma are herbivorous, and the majority swing towards being algivores.

In Africa a few members of the family Mochokidae eat aufwuchs, such as Euchilichthys guentheri, while Synodontis brichardi and Synodontis schoutedeni will consume algae as part of an omnivorous diet. If you look carefully at these two Synodontis they also have rasping mouthparts suggesting that they also graze on some resource growing on a surface.

Healthy gut, healthy body

The diet of a fish is reflected in the digestive tract — the stomach and the intestines. For example, longer intestines suggest that a fish has evolved to ingest food that takes longer to process, and that’s usually a herbivorous diet. By contrast, a large stomach and short intestine hints at a carnivorous fish.

If we look into their molecular physiology, all organisms generate proteins known as enzymes, and these are used to break down their food. These enzymes have evolved over thousands of years to break down a specific part or parts of a diet, and where an animal has not evolved the enzymes to process specific parts of a diet, that diet will not be processed by the digestive tract. The unprocessed food will either pass straight out or it will result in fluid and gas production, bloat and digestion issues.

Inside the intestines there also resides a variety of bacteria and archaea directly involved in the digestive process. Like enzymes, these bacteria have also evolved over generations to feed on specific aspects of a fish’s diet — some deal with vegetable matter, others with flesh, and so on.

Feeding a herbivorous species on a carnivorous diet (for example, foods high in fish/krill/insect meal content) will likely not provide suitable levels of nutrition as it will not be digested.Fundamentally, malnutrition will likely occur where any herbivorous species cannot break down carnivorous food.

What to look for

Labels on fish foods can lead to confusion. For example, a food labelled as a ‘pleco diet’ must be herbivorous, right? After all aren’t members of the Loricariidae (plecos) herbivores? Well, no, and therein we find a problem. Marketing a veg-based food as suitable for all plecos ignores the presence of omnivorous and carnivorous species — all of which are plecos.

A majority of branded diets are more on the omnivorous side, though if you look at the ingredients, you’ll see that some lean more towards carnivory. Usually when it comes to the ingredients list on any packet of food will list the ingredients in order of abundance. So, if you see ingredients listed something like ‘Cereals, spirulina, lobster eggs’ then that indicates that cereals make up the greatest proportion, followed by spirulina and then lobster eggs. Alas, the main ingredient of many ‘pleco diets’ is usually fish/krill/insect meal with few algae added (if any) and usually a small number of vegetables.

Fish/krill/insect meal (along with derivatives) in small amounts is unlikely to cause issues and is even a good source of Vitamin B12. Using these ingredients is a cheap way of increasing protein levels, though in the case of fish and krill it can also have an environmental impact. However, these proteins might not be digestible in herbivorous fishes, and other aspects of a carnivorous diet are equally likely to have limited digestibility in herbivorous species.

In the wild many of these herbivorous species would likely be eating large amounts of algae. Spirulina and chlorella meals are high in protein and other minerals, and kelp and seaweed (macroalgae) are also available. The ideal diet would contain algae as the first ingredients (spirulina, kelp, seaweed and chlorella) and ideally these should be dominating the top of the list. Fish/krill/insect meal should be lower down. I am a fan of diets containing a large number of ingredients, so providing a wider range of vitamins, minerals and nutrients.

Vegetables would be beneficial but algae in a captive diet would be more beneficial with the range of vitamins and minerals. Due to the specialisations in the guts in these fishes, algae would be easily digested and provide the most nutrition. I also find that these are the best way to get herbivores feeding after transit. Wood, or more precisely cellulose and lignin, can be a big thing in fish diets, and especially so in dedicated Loricariidae diets.

This seems to be the case regardless of the scientific evidence indicating that these fish aren’t being able to digest cellulose at all — instead they feed on bacteria, fungi and other microbes that are breaking down the wood. Therefore, cellulose and lignin are little more than fillers with no nutritional value. Fillers do have their place for a digestive purpose but these are different from other fillers and will likely bulk up the overall filler content.