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ARTICLES OF FISH HEALTH BY:
Dr. Jesse Sanders
Jessie Sanders, DVM, CertAqV
Effects of Pond Temperature on Koi Immune Function
Due to the outdoor nature of koi ponds, over the course of a normal year, your pond temperatures will fluctuate. What effects of these temperature changes have on your fish and their immune function?
Koi Immune Function
Most fish have more advanced immune function than we give them credit for. They are equipped with the most basic innate immune function, like many of their invertebrate cousins, and more advanced humoral immunity, allowing for effective vaccination. The immune function responds to invasions by foreign pathogens, including bacteria, fungi, parasites and viruses.
Innate Immune Function
The innate immune system is the most basic and relies on the process of phagocytosis. White blood cells, specifically heterophils, eosinophils and basophils in most fish species, detect foreign particles, engulf them and digest them. This occurs for any foreign pathogens, including bacteria, fungi and parasites. In fish, this takes place mainly in the cranial kidney and spleen.
Humoral Immune Function
Humoral immune function is responsible for memory response and is utilized in the protective vaccine effect. This immune response relies on B- and T-lymphocyte white blood cells that carry specific receptors after a fish has been exposed to a foreign particle, such as a virus or vaccine. These specialized cells respond quickly if the virus re-appears, providing protection for the fish. This type of immune response is not effective against pathogens on the outside of a fish, including most fungi and parasites.
Barriers to Infection
Fish have built in barriers to prevent invading pathogens from causing disease. In addition to their robust immune response, fish have a unique protective skin mucus. In addition to providing a physical barrier into the fish, a fish’s slime coat contains many phagocytic cells that effectively eliminate pathogens before they have a chance to infect the fish. You may have noted changes in your fish’s appearance and slime coat with various pond temperatures and known infections.
Ectothermic Metabolism
Fish immune function is tied to their body temperature, a result of their ectothermic nature. Unlike humans, who are endotherms, fish cannot regular their internal body temperature. Their metabolism, immune function and reproductive capabilities directly correlate with the water temperature.
When temperatures are warmer, koi have better metabolic and immune function. This is in an ideal range between 65-75°F (18-24°C). Below 60°F (15°C), koi have very little to no immune function. This is why they are unable to heal from injury or ward off infections in colder water temperatures. Thankfully, most bacteria and parasites also do not replicate at these lower temperatures, protecting your fish from a serious infection.
Above 80°F (27°C), something similar happens, where a koi’s immune and metabolic function do not work at their optimal performance. This range will vary genetically and individually, depending on what temperatures fish are routinely exposed to and how rapidly temperatures change. It is not uncommon to have one “canary” in your pond who is always the last to return to normal activity in the warming spring temperatures and always tends to get sick the same time of year.
Seasonal Temperature Variations
The riskiest time to be a koi in an outdoor pond is early spring. As the water temperature starts to warm, all the pathogens in your pond who have been hanging out in suspended animation start to replicate and reproduce. Since many of them are simple, tiny organisms, replication can take place very quickly. However, your fishes’ immune system may lag behind the replicating hordes of parasites, bacteria and other infections. This may allow the pathogens to get a jump on the fish and cause severe disease. Thankfully, with good supportive care, mainly good water quality and high-quality diet, most koi can recover from these infections without any intervention on your part.
But what if your “warming” temperatures start bouncing up and down? Just as a koi begins to ramp up their immune function, a cold couple of days knocks that back down. This pattern may be a bigger challenge for your fish to recover from. Keep a close eye on your pond temperature and how your fish are behaving. Be on the lookout for signs of developing disease, such as flashing behavior, loss of appetite or ulcerations.
Best Practices to Mitigate Threats of Disease
The best way to limit the spread of disease in your pond comes down to three key points:
Maintain your water quality. Water quality is the #1 influence on fish health. Do your regular water changes and keep an eye on your significant parameters, including ammonia, nitrite, nitrate, pH, kH and temperature, with a liquid-based test kit. Be sure to get a new test kit once a year!
Feed a good quality diet. Just because a koi food is expensive does not guarantee it is a good diet. High quality diets have fewer protein sources and promote pre- and probiotics.
Quarantine all new fish additions. Avoid adding fish until the warmer summer months so your fish’s immune functions can be at full capacity. Keep new additions in a separate system for 4-6 weeks to avoid passing on parasites, bacteria and the deadly Koi Herpes Virus (KHV).
Truth Behind UV Sterilization in Koi Ponds
What is UV Sterilization?
UV sterilizers are lights that emit ultraviolet light within the 400 and 10 nanometer wavelength. They fall in between visible light and x-rays. Within this category are three different subtypes:
UV-A (320-400 nm) – With the longest wavelength, these rays are the least harmful. Also known as “black light,” this wavelength can bring out fluorescent colors with a vibrant glow. Many bird and insect species can detect this type of light.
UV-B (290-320 nm) – This subtype is responsible for causing sunburns, and with prolonged exposure, increased risk of skin cancer and cellular damage. Almost all the UV-B rays produced by the sun are absorbed by the ozone in the atmosphere.
UV-C (100-290 nm) – Although the most harmful, this subtype is almost entirely absorbed by the earth’s atmosphere. These wavelengths are commonly used to disinfect food, air and water by destroying nucleic acids within pathogenic cells.
How is UV Sterilization Incorporated into Koi Ponds?
UV sterilizers made for water can come in a variety of wavelengths. An effective sterilizer will fall within the UV-C spectrum, where the wavelengths are effective at destroying cells. Most UV lights for ponds are sold by the bulb wattage. The size you need for your pond will depend on the total volume of water, available nutrients from fish, plants and fertilizers, and your filtration flow rate. Depending on the UV component manufacturer, your wattage may vary significantly. You may be looking at adding multiple bulbs in parallel if your system needs heavy filtration. When sized correctly, you will not need serial UV lights. Contact your local professional if you need help selecting the correct size unit.
Koi pond UV sterilizers are housed in compact units that connect into your filtration system. It is recommended they are plumbed after your main filter (i.e. matting, shower, pressurized). All units are typically self-contained with an influx and efflux, bulb and quartz/glass sheath to protect the bulb. By containing the UV light in a sealed container, it contains the harmful UV-C rays. The specialized quartz sleeve allows the UV light to effectively penetrate the surrounding water. Over time, these sleeves may become coated in algae or other particulates or cracked. If cracked, it is likely you will have to replace the sleeve AND lamp. Replacement parts are available as are units with specialized squeegee inserts to keep your sleeve clean.
Over time, your UV bulb will burn out and need to be replaced. A UV light not at full effectiveness will allow algae to start covering the quartz sleeve. Most UV lights should be replaced once a year or every 13 months. If you start to see more than the usual amount of algae in your pond, check your UV light first. If you burn out more than one bulb a year, have a professional check the UV setup to make sure it is properly maintained and hooked up to your electrical supply correctly.
UV Sterilization Effects on Water
As described previously, the UV-C spectrum works to disrupt the DNA and RNA in cells. This effectively works to obliterate the small particles running through your filtration, mainly algae cells. It will also effectively kill any pathogens, such as bacteria, fungi or parasites, that pass through the filter.
If there are medications in the water, such as antibiotics or antiparasitics, UV light may alter the chemical structure of these drugs and make them ineffective. It is recommended that UV lights be turned off for any water-based medication use.
UV Sterilization Effects on Koi
Since the UV light does not come in contact with the fish, it is very safe for use with any fish, including koi. UV light will not affect any pathogens living on the fish, including common parasites such as Trichodina, White Spot or Costia. It is ineffective against viruses such as Koi Herpes Virus (KHV). It will not change the fish’s behavior in any way.
Many koi owners assume that adding a UV light will “treat” bacteria and parasites on their fish. This is a harmful myth that is not true at all. Some juvenile stages, such as Anchor Worms (Lernea spp.) and free-swimming theront stages of White Spot Disease (Ichthyophthirius multifiliis), will be zapped by the UV light, but not the reproducing adults on the fish themselves. UV light is NOT an effective method of pathogen control or treatment in koi ponds.
Best Use of UV Sterilization
UV sterilization is the best method of control for suspended cellular algae. When combined with manual removal or barley straw, it may be effective at controlling string algae, provided the cells make it to the UV filter.
The key point of UV sterilization is that it only treats the particles exposed to the UV-C rays. This only includes pathogens, algae and zooplankton in suspension. If your filtration is inadequate or your UV sterilizer too weak, you will not have your desired results. Consult with your local pond professional for selecting the best UV model for your pond and correct installation.
