Koi digestive system. How it works

The more we know and understand about certain aspects of our life, the more we can appreciate them. I remember the first time I learnt and understood how an engine worked, I was amazed that our family car actually worked knowing its old age and history. Years later I am now able to apply that knowledge further and get more out of my own car by knowing what to look for when things go wrong or what work needs to be carried out to prevent things from deteriorating.

I am able to take my appreciation further by just carrying out a service myself and also by actually knowing and understanding why I am changing the spark plugs etc.

This is also true about many aspects of koi keeping and no more so than how a koi’s diet relates to its digestive system. As with a car’s engine, the more we can understand and appreciate about a koi’s digestive physiology, the more we can respond to it and benefit from understanding why we can feed a different number of diets.

Fortunately, through many years of extensive research, primarily on carp but more recently on koi and other ornamental carp species, the nutritional requirements of koi have been identified and can quite adequately be provided through a number of commercially available diets. What our koi can and cannot digest from the food is largely the interaction of two factors: the quality of the ingredients and the ability a koi has of digesting them.

What is digestion?

For a fish to benefit from a well-chosen food, it must be able to re-distribute the basic nutrients from the food to every cell in the body. Digestion is the series of processes through which a fish breaks down insoluble nutrients such as starches and proteins into their soluble building blocks such as sugars and amino acids. These soluble products of digestion can then be transported in the bloodstream to every needy cell.

What is a koi’s digestive system like?

Digestion in humans starts in the mouth with chewing action to physically break up the food. This is also true for koi where preparation of the food for the gut commences with the grinding action of paired pharyngeal teeth. Unlike humans and some other fish, these teeth are not situated in the mouth but in the throat area. It is not possible to be bitten by a koi, just sucked! The pharyngeal teeth act as a grinding mill ensuring that all food entering the gut is suitably fragmented prior to digestion. In fact it is sometimes possible to spot koi struggling to grind up a stubborn item with their teeth as they ‘chomp’ with exaggerated jaw movements. This is more easily spotted when watching them feed in an aquarium, where they may also be seen to expel unwanted material through their opercula.

Koi are opportunistic omnivores, in that they will feed on either living or dead animal and plant matter and will try anything once! Koi have a remarkable ability to make the most of whatever food happens to be available.

Considering the diverse nature of their food source, it is surprising to find that koi do not have a stomach as this is probably one of the most effective parts of any digestive system. The stomach is an area of extreme acidity (pH2) where a meal rests once ingested and is broken down by acid and enzyme activity. It is a little mystery why carp are so successful yet lack a stomach.

Consequently, digestion in the koi is very simplified. The intestine is a relatively featureless structure and besides having a dilating receptive sac, the intestine looks the same all along its length. Koi have a long intestine and if dissected out it is usually twice the length of its body, a typical feature of a grazing omnivore such as the carp.

This differs significantly from predatory or carnivorous fish such as trout or salmon. These fish possess a far more differentiated intestine, with a stomach to receive its kill. The rest of the intestine is relatively short, being limited in its absorption of a range of digestive products. Such fish are suited to being fed in one or two large feeds a day, where the ‘kill’ is stored temporarily and digested in its stomach.

The koi’s digestive physiology functions more efficiently if it is able to process a more regular supply of food. This is why koi constantly root around, looking for any morsel that they can sample and digest. For this reason it is better to feed koi on a little and often basis. If this is not always practical why not try an auto feeder?

This may lead us to summarise that if koi are so adaptable to a range of naturally occurring foodstuffs, does it really matter what we feed them?

Just because koi are able to ingest and physically break up a food does not necessarily mean that koi are able to digest it efficiently. It is for this reason that a well researched, blended and balanced diet tailored to the digestive requirement s are available. Wholefood treats such as bread and sweetcorn may be ingested but may not be fully digested. For this reason, it is vital that a balance is struck between a highly digestible diet with little fibre content and a high fibre diet with little nutritional value.

This may contradict some high performance low fibre diets where the formulation is geared for growth at all costs. This philosophy stems from the culture of intensively reared food fish where economics of inputs and outputs are the controlling factor in diet formulation and the fish’s life is relatively short. However, koi are kept and reared for the long term by koi keepers, where both growth and health are equally important factors. The health of a koi must be built upon and reinforced through what we feed and the quality of the diet is not only reflected in aspects of health that we can see such as growth and colour but also in the things we cannot see such as maintaining a healthy digestive system. A balanced diet containing significant fibre and roughage will help achieve this.

What koi cannot digest.

Koi, being omnivores can breakdown many different foods, but will have difficulty in absorbing excessive quantities of digested products. These are likely to be excreted adding a burden to the filter.

Protein. Too much protein, especially at low temperatures will not be completely absorbed, leading to waste and water quality problems.

Oil. Foods containing less that 10% oil must be fed as excessive oil in the diet can lead to health and water quality problems.

Fibre. Koi, like us can only gain limited nutritional benefit from fibre in the diet, but it is included in the diet for its indigestible bulking properties. Fibre in the diet allows the gut to grip the food and keeps the food moving through the system. Many diets boast a low fibre, high digestibility formula but is this really a good idea?

Food Colouring. Some koi foods contain both natural and artificial food colourants to enhance their shelf-appeal. Some colourants are indigestible or included in high concentrations. These may colour the pond water giving it a yellow or orange tinge.

Beneficial Ingredients

Many of the naturally occurring foods such as worms, insect larvae and fresh vegetable matter are generally only partly digested by koi. The indigestible elements of the diet act as roughage to keep the intestine active and healthy. Carp will also ingest significant quantities of silt and other indigestible detritus from their natural environment which again will act to bulk out the diet.

As koi are generally kept in relatively hygienic, silt-free ponds and fed highly digestible diets there are several ingredients that can give a koi’s digestive system that natural experience, even when feeding on artificial diets.

Grass meal/alfalfa meal. A beneficial source of fibre.

Ash. A beneficial source of minerals, but will also act as a bulking agent in digestion. Watch out for excessive ash content (>10%) that may be included to replace other more vital ingredients

High digestibility claims – Beware!

Why do we, who are omnivores too, deliberately try to eat so much fibre? A high fibre content aids digestion and gives the intestine some bulk to grip onto. Fibre is of value to the fish even though the fish does not gain any nutrition directly from it. For the same reason, fish must be given a level of roughage to maintain a healthy intestine. Some koi diets claim extremely high digestibility rates which may cause some long-term problems if these are the only foods that koi consume.

Jargon Busters

Pharyngeal Teeth. Grinding teeth located in the throat area of a koi

Opercula. Plural of operculum, which is the bony plate protecting the gill chamber

Enzyme. A chemical produced by the koi’s digestive system to aid the breakdown of insoluble foods into their soluble building blocks.

Kill blanketweed and string algae.