L: Pond Keepers A to Z – Lateral Line To Leather Scales

Lateral Line

The lateral line is similar to a dot-to-dot picture, where the dots found along the flanks of a fish link to form a clearly visible line. Each dot is a hole in a scale which links to a tube running beneath the series of holes or ‘dots’ on the fish’s body surface.

Often described as a fish’s sixth sense, the lateral line detects vibrations that are conducted through water alerting the fish to moving objects and the direction from which they come. When a koi is spooked or startled by a knock or a bang around a pond, the fish is alerted to this potential hazard by its lateral line sensing the vibration. As a line runs on either side of the fish’s body, it can determine the direction from which a movement is occurring.

Koi have a single lateral line running along the midline of each flank. Other freshwater fish may have up to 7 lateral lines on either flank while the blind cave fish uses its lateral lines to detect solid surfaces, preventing it from hitting otherwise ‘unseen’ objects.

If ice forms over a pond in winter then it is out of respect for the sensitive lateral line that the ice should not be broken by smashing a hole. It is better to prevent the ice from completely covering the pond by using a low-wattage pond heater. Liner A pondliner is by far the most popular material to use when constructing a pond. Liners allow unlimited variations in design, giving a free hand to any would-be pond designer. And there lies the real benefit of using a liner when building a pond, in that a professional finish can be easily achieved cheaply, quickly and relatively simply by a novice.


As long as a number of basic guidelines are followed in designing the shape, preparing the excavation and edging the pond then a liner should outlive its guarantee, which depending on the grade of liner, may be as long as 25 years.

Bought as a pre-cut packaged piece of liner or off the roll, the best quality liner that can be afforded should be purchased to prevent the unnecessary disruption of a matured and balanced pond and all its inhabitants in years to come.

The two most common liners available are PVC and Butyl although more recently a more durable product EPDM has been available giving a good selection of liners.

Of the three, PVC is expected to have the shortest life, and is quite a thin liner, being about 0.5mm thick. Extra care and expense must be taken to protect this liner during installation by using a thick underlay.

Underlay should also be used with a butyl liner. Although butyl may be twice as expensive as PVC, it is twice the thickness of a PVC liner offering better flexibility and UV stability and the near certainty of a ‘fit it and forget it’ quality (until you want to extend the pond).

Besides the main advantages, liners have few disadvantages and these include:

Folds. Liners are best suited to large ponds with sweeping corners and curves, where folds and creases can be kept to a minimum. Folds are unavoidable in smaller ponds and will collect dirt and detritus and may even snag equipment during pond maintenance.

Easily Punctured. Compared with other pond materials, liners are more likely to be punctured accidentally by 4 legged pets or garden implements. Fortunately this eventuality is catered for as aquatic outlets that sell liners will also sell repair kits.


The term livefood usually refers to a range of ‘treats’ offered to koi and other pondfish as a means of giving variety in a koi’s diet.

Typical livefood treats available in aquatic shops such as daphnia or bloodworm will usually be so small that they may be overlooked by medium to large fish. Real favourites include chopped worms and maggots, often causing a feeding frenzy. There is a wide range of opinions as to whether such ‘treats’ are suitable for koi in terms of disease and their digestibility.

- Disease. A useful rule to follow to minimise the risk of introducing parasites to koi through feeding a livefood treat is by feeding livefood from a terrestrial origin. Earthworms, grubs and maggots are less likely to be carrying intermediate parasitic stages compared with aquatic livefood such as daphnia or bloodworm.

- Digestibility Although livefoods are likely to be less digestible than artificial dry diets, koi are well capable of breaking down and digesting such food items. After all, a range of livefoods form a good proportion of their natural diet.

-Livefoods as first foods.

If it was not for livefoods, then koi farming would not produce such wonderfully shaped and perfectly formed specimens in the numbers that are currently produced.

Mud ponds in which the fry are reared are managed so that they produce abundant quantities of livefood on which the fry can gorge themselves. Ponds are manured and fertilised to produce a good ‘bloom’ of zooplankton such as rotifers and daphnia, producing excellent survival and growth in fry through these very vulnerable stages.


Leeches are bloodsucking ectoparasites that attach themselves to unsuspecting hosts such as koi and pondfish by means of a powerful sucker. Leeches that parasitise koi appear to be clearly segmented, with their slim-line bodies divided into numerous sections. They are easily identified by a disc-like sucking attachment which secretes an anticoagulant to prevent the koi’s blood from clotting, allowing a lengthy and unhindered feed.

Leeches are quite rare in the more formal koi pond set-up as they are generally transferred and introduced on plants. Therefore, traditional well-planted garden ponds are at more risk from infestation as leeches and their eggs enter on plants and soil.

Leeches can be controlled on individual fish through dipping fish in a salt bath or by using organophosphorous pond treatments. However, if eggs persist in the pond, then koi are likely to be reinfested. To be sure that reinfection does not occur, eggs are best eliminated by removing and discarding all pond plants and replacing with safe, new specimens.


The duration of light that koi experience each day (photoperiod) has a profound effect on a koi’s physiology, particularly growth and breeding performance. Koi are sensitive to the duration of light each day and use it in combination with other environmental factors such as temperature as a cue for growth or reproduction.

Light is picked up by a photosensitive organ called the pineal eye which triggers the hypothalamus in the brain to stimulate the pituitary gland to release growth and sex hormones appropriately.

Studies have shown that koi require a minimum photoperiod to stimulate growth, and if they are not exposed to a sufficient photoperiod, then irrespective of water temperature, koi will not undergo true growth.

The photoperiod also determines the maturation rate of eggs within mature female koi. If koi are held within a completely controlled (blacked-out) environment where the photoperiod is determined by artificial light then their eggs can be matured earlier than natural, prolonging the growing season for the subsequent fry.


Every koi pond and filter system should contain a form of limestone (calcium carbonate) as it acts as a source of pH buffering as and when the pond water requires it. Limestone chippings, cockleshell, tufa rock etc can all be reliably used, retained in an inert net bag to prevent it from spilling into other areas of the filter or pond.

The natural tendency for the pH of koi ponds is to drop and turn acidic through biological processes such as biofiltration and respiration. Regular water changes with treated tapwater (which is buffered by water companies anyway) can alleviate undesirable pH swings but the addition of a limestone source will definitely prevent such swings, maintaining a health and stable pH.

Care must be taken to ensure that a bacterial film does not cover the surface of the limestone, preventing it from buffering the water. For this reason, the net bag containing the limestone should be shaken and disturbed regularly.

Leather scales

Referred to as leather scales in the carp world, koi exhibiting leather scales are referred to as Doitsu.

A scale variety in koi is reputed to have been introduced from Europe, most frequently found in Shusui and Metallic varieties. Classic Doitsu scale patterns show enlarged mirror-type scales arranged along either side of the dorsal fin along the back as along the lateral line.

Because the majority of skin on a Doitsu koi is scale-less, pattern on Doitsu fish generally show better definition. In addition, from experience, when injecting broodstock with hormone to induce spawning activity, Doitsu fish are a lot easier to work with as there is no need to locate the needle beneath a scale for injection.

Did You Know?

When breeding Doitsu fish, during a Doitsu cross, 25% of the fertilised eggs will contain a lethal gene which prevents those affected from developing beyond the embryo stage. So do not be alarmed if after a Doitsu x Doitsu spawn you find large quantities of larvae on the bottom of a hatchery tank 2-3 days after hatching as their death is through a quirky genetic trait and is entirely natural (but strange!)

Kill blanketweed and string algae.