G: Pond keepers A to Z … Genetics to Gut


Genetics is the science of inheritance. Although the production of artificially occurring ornamental fish started well before the complexities of genetics were fully understood, the pioneers of goldfish breeders in particular grasped some of the fundamental principles and used them to supreme effect.

Wild-type dark carp will predominate from a wild-type spawn as they will have inherited a dominant wild-type gene from either parent. Such a wild-type fish would be dark if either 1 or 2 of the gene pair were dominant.

Koi first ‘appeared’ when mutants expressing abnormal colour variations were found amongst the many thousands of wild-type fry. Consequently these colourful mutants were so low in number because of the low probability of two recessive genes coding for abnormal colouration being found in one individual. Such rare colourful variations are determined by recessive genes. For a recessive gene to be expressed, both parents must pass one onto the offspring.

As a result, the breeding of koi revolves around the expression of recessive genes. For this reason, breeders must carefully select their broodfish so that recessive genes are passed on from both parents, producing a colourful offspring, otherwise dull and unpatterned offspring are likely to be produced.

For this reason, koi are highly inbred where closely related fish are crossed to ensure a desirable feature is stabilised. If broodfish which are not closely related are crossed, then it is more likely that the desirable recessive trait will also be lost and not expressed. Consequently, koi are very weak as they are extremely in-bred where the desirable fish are the most colourful and well patterned but also the least vigorous, being more susceptible to disease and health problems.

As with all organisms, a koi’s genes dictate its colour, pattern, growth rate, shape and size potential. There is a general trend between the vigour and colour/pattern of a koi which is controlled by a fish’s genes: Dull, pale and less appealing koi are more likely to grow faster and to a greater ultimate size than their higher grade cousins. It is as a result of these genetic factors that chagoi produce consistently large koi while jumbo kohaku, sanke or showa are much rarer and worth considerably more value.


Gills are a fish’s equivalent to our lungs. They are the organs of gas exchange, adapted in structure to enable the efficient uptake of oxygen dissolved in the water and the release of carbon dioxide back into the water from the blood.

The delicate feather-like structure of the gills, with a large surface area, enable fish to extract 80% of the oxygen dissolved in the water passing through the gill chamber. A spectacular feat when compared to our absorption of 14% of the oxygen we breathe.

It is doing koi an injustice to simply compare their gills with our lungs as gills are also the major site for osmoregulation and ammonia excretion. Unfortunately however, they are also a very attractive site for disease organisms.

Freshwater fish such as koi and other pond fish are constantly fighting to maintain a suitable working concentration of tissue fluids while the water is constantly flooding in across the gills, diluting the fish’s tissues. Most minerals are also lost from the fish’s tissues to the environment via the gills. It is the gills’ osmoregulatory function that enables koi to redress the mineral balance, actively absorbing valuable minerals from pond water. In such a way, koi are both victims and benefactors of their gills’ intricate structure.

Most ammonia that a fish excretes is lost directly across the gill membrane into the pond water. Fish depend on there being a steep gradient (difference) between the concentrations of ammonia in the water and that in the blood. If there is a build up of ammonia in the pond water, then ammonia cannot diffuse out of the blood, leading to an unhealthy build-up of ammonia in the blood. This causes koi to gasp at the surface.

Gill Disease

The fine, intricate structure of the gills with their thin outer membranes makes them very attractive to disease organisms. A steady stream of high quality, pre-digested nutrients are constantly passing in the blood through the gills, almost irresistible to a number of suitably equipped parasites.

Gill flukes (Dactylogyrus), gill maggots (Ergasilus) and various bacterial infections of the gills can all cause problems to koi. Not surprisingly, typical symptoms exhibited by koi suffering from gill diseases and infestations include gasping or increased breathing rates.

Although there should be no cause for gills ever to be seen, they can be used as a good indicator of a koi’s general level of health. Bright blood-red gills are healthy whereas brown or purple or gills showing white fringes all suggest health problems.


Only a selection of the more widely spread scale or colour types are included in this series and one such well loved variety is Ginrin.

Ginrin is an abbreviation of Kinginrin and is the term given to a fish showing predominantly sparkling scale types. Various qualities of Ginrin are seen throughout the trade, with specimens ranging from the quality of the ‘sparkle’ within the scale to the extent of the distribution and coverage of the fish with Ginrin scales. Ironically, some good examples of non-Ginrin fish can be spoilt by the occurrence of the odd Ginrin scale.


As technology and innovation change the face of a hobby, there are naturally casualties along the way, where the old industry standards become obsolete, almost overnight. This has been true for gravel. Gravel has suffered as a filter medium at the hands of more effective media such as flocor and reticulated open-cell biofoam. Gravel is often used as the standard against which the surface area claims of new filter media are compared. Considering that its colonisable area is only found on its surface, then it is no surprise that lighter, more perforated and versatile media have taken its place. It is still used in many highly effective home-made filter-beds where its qualities as a cheap, dense and readily available medium are used to good effect. Some gravels also have added pH buffering qualities. However, it can have a tendency to block and clog and its weight can make a gravel filter bed an ordeal to clean. Nevertheless, the koi hobby is still indebted to gravel as the founder media of pond filters.


A host of factors control the growth rate of koi. These include environmental factors such as water temperature, stocking density, water quality and nutrition as well as genetic influences that determine a fish’s vigour and its ultimate size.

Achieving good growth is arguably as important as good colour and body shape to many koi keepers, where larger koi are often more memorable than smaller higher grade koi.

Remarkable growth rates can be achieved in koi if stocked in a warm, aerated, lightly stocked pond and fed on a quality high protein diet. Koi are warm water fish, growing best at 27 degrees C and excellent growth rates can be achieved in the UK when farmed in mud ponds covered with polytunnels.

Growth rates are greatest in juvenile fish where the majority of protein digested from a balanced diet is laid down as new body tissue. The growth rates of sexually mature fish slow down as protein is diverted away from growth and to the deposition of eggs and the production of sperm. As growth is seasonal and temperature-dependent, growth rates peak in summer and cease in winter. This stepped growth pattern can be visualised by examining the bands of rings on scales. These can be counted just as tree rings, to assess the age of the fish.


When seen under a microscope on a mucus scrape, Gyrodactylus can be one of the most frightening parasites to observe. Attached to the skin by a vicious pair of hooks, skin flukes can cause fish excruciating irritation.

Gyrodactylus proliferate on stressed or weakened fish, where additional species of parasites are also likely to be found in a sample of mucus, taken from the fish’s skin. Skin flukes only have fish as a host (unlike other koi parasites) and when viewed under a microscope the another pair of hooks can often be observed within the body of this parasite. That extra pair of hooks belongs to its offspring which is developing inside, ready to infect another fish the minute it is released. These livebearing flukes are easily treated using a formalin-based (or better still, organophosphorous) pond treatment.


A koi’s gut (or intestine) when dissected out is typically twice the length of the fish’s body. A long gut is typical of an omnivore, where at times the food may be of mixed quality, requiring significant digestion and absorption to extract sufficient nutrition. Food is broken down physically by pharyngeal teeth (located in the throat) and chemically by a range of digestive enzymes, with the soluble products of digestion being absorbed into the blood. Koi, unusually do not possess a stomach and their gut is very similar in structure along its entire length. As a result, koi cannot store a meal for long in their intestine and digest food more efficiently when fed a little and often.

Kill blanketweed and string algae.