Pond fish Olfaction and Digestion

‘What can smell, but doesn’t have a nose? – Fish (ie kippers)’. OK, probably one of the world’s worst jokes, but I couldn’t resist, and you’ll soon discover it’s relevance to the article. Koi (and the carp family) are renowned for their acute sense of smell. It plays a vital role in their detection and interaction with their environment, not only in relation to feeding, but also at times of spawning.

Initially, the association between a koi’s sense of smell and the action of its intestine may not be apparent yet as koi keepers we can certainly interact with both through our actions and by understanding how they work, particularly the intestine, we can have an impact on their performance.

Smell. The sense of smell works in conjunction with taste, and is used by koi above all other senses (including sight) to detect its food. Carp are supremely well adapted for life in highly turbid, silty water that sunlight struggles to penetrate, giving them a dark and dingy existence. In these conditions, sight has little use (as those who have dived in similar waters will appreciate).

Koi use their acute sense of smell as a long-distance taste, sampling and responding to compounds that are dissolved and carried on the water. If a substance dissolves in the water, it has the potential to be detected by their nose. Such substances include amino acids, sugars and other organic and inorganic compounds. Similarly, if a substance is insoluble in water, it cannot be detected by the koi’s nose. This has been recognised by anglers for generations where pungent baits have been used to great affect, covering vast areas of water in a bid to attract and hook their fish.

The biological term describing the sense of smell is olfaction, and the olfactory organs in koi are located within the nostrils. However, unlike our nostrils that form an entrance to the throat and lungs, experiencing a tidal flow of air, koi’s nostrils (called nares) serve a single dedicated role of detecting smell and do not channel water to other parts of the body. Essentially, they can be thought of as a U-tube, situated at the front of the koi’s head, just forward of each eye (see Figure 1). To each U-tube, there are 2 nares, one inhalent and one exhalent. Within each nostril, situated at the base of each tube is the delicate and finely structured olfactory epithelium which detects chemicals dissolved in the water that are then interpreted as an odour.

Water moves through these delicate structures due to a combination of the koi’s movement through water, the diffusion of compounds through the water and the gentle action created by small hair-like structures situated within the olfactory organ.

Olfaction and the koi keeper

Even though the highly filtered, clear, recirculating water of a koi pond is far removed from the less glamorous environment of a clay pond, olfaction still performs an important role for koi. I remember one instance when working on a koi farm that on close inspection of a 2 year old koi having been harvested into clear indoor ponds, it was apparent that it was completely blind, formed without eyes with completely smooth skin where the eyes should have been. And yet, this koi’s development had kept pace with the rest of its year class, demonstrating how essential olfaction is to koi for locating their food. I doubt similar development would have been seen in a koi born without nares.

Even though koi will use their sight a great deal more in a koi pond for orientation and food location, smell is still the key sense for koi. Koi appear to show far more ‘interest’ and ‘enjoyment’ when offered a paste food compared with a pelleted diet of the same formula. The enhanced vitality and action when feeding a paste food is due to it producing a greater smell in the pond water. When a paste food is prepared, the nutrients and odours will readily dissolve from the finely milled ingredients, being released and dispersed that much quicker when it is fed to the pond. The flood of appetising compounds dissolved in the water causes a feeding frenzy as the koi appear to over-react to the smell.

The koi’s ability to detect the presence of ripe females at spawning time also results in similar frenzied activity. Females communicate their readiness for mating by releasing pheromones into the pond water that are acted upon by males once picked up by the olfactory system. A similar response in male koi can be achieved by adding only a small volume of water from a female-only system into a male-only system. As the water contains female pheromones, males respond by rubbing against each other convinced that one of them must be a female!

The intestine.

The intestine is the place where food is digested (broken down into soluble building blocks) and absorbed into the blood stream. By being aware of the atypical characteristics of a koi’s digestive system, we can enhance their ability to utilise the food we offer.

Compared to a predatory fish that will kill, eat and rest as its kill digests in its receptive stomach, koi, being a stomachless fish are geared to feed in a completely different way. Rather than feeding on discrete meals throughout the day, koi digest and assimilate food that is ingested on a little and often basis.

Naturally, carp routinely root and forage, aquatic pig-style, in soft and silty sediment, ingesting whatever their snout and nose locates. Consequently, there is a regular flow of food items (many of which are difficult to digest) as well as a steady stream of silt and sediment that is ingested inadvertently.

Because koi do not possess a stomach, they are not well adapted to receiving their food in large amounts (even though these aquatic pigs will still happily help themselves!) Feeding in such a style means that the food is more likely to travel relatively quickly through the gut, spending less time in contact with digestive enzymes resulting in incomplete digestion. This phenomenon could be compared to the way water passes through a purifier. Raw tap water has a better chance of being treated effectively if it passes through the unit slowly and at a regular pace, rather than at speed and at great volume. In a similar way, koi digestion is more effective if food is allowed to be processed in small but regular amounts. Not only does this have implications for koi growth, but also water quality. The better the opportunity koi have at breaking down and absorbing nutrients from their diet, the less solids and soluble waste is likely to be excreted by koi into the pond.

Yet feeding koi as regularly as they would eat naturally is just as difficult as say brushing your teeth 3 times a day as recommended or drinking water regularly throughout the day to maintain a steady state of hydration. What happens in reality is a compromise between what is ideal and what is practical.

A useful way of ensuring koi eat in an efficient way is to install an autofeeder. They can be set to dispense a day’s worth of food at regular intervals throughout the day. Yet it is no surprise that few of us own autofeeders as feeding time is probably one of the most enjoyable and interactive times and is something we prefer to do ourselves rather than rely on a machine. Hence our koi may get fed at 2 occasions in the day. Something that is practical rather than ideal.

In summary, the function of the nose and intestine are closely linked, with the food that is located finding its way into the gut. Our activities in and around the pond can both stimulate them and enable them to work efficiently, even though at times, this is not always the most efficient alternative.