Language of Koi | Effects of Seasonal Transitions on Fish Health

A natural, healthy, balanced pond is a continuous joy to any hobbyist.

What happens to our fish when the seasons change? Why do they get sick and have such a tough time in these seasonal transitions?

When the seasons change, there’s a lot going on in our ponds and with our fish. Depending on where you live, this transition may be especially harsh. Let’s think about what these fish are going through. Fish are dependent on the temperature of their environment and get stressed when there are large, rapid thermal shifts. When the fish are close to freezing for much of the winter, they are very stressed, as this approaches their lethal temperature. Add a relatively rapid increase with a few warm days, and now the fish are really stressed. The same can be said in the fall when very warm water can chill with some very cold nights.

Most koi keepers stop or reduce feeding for the winter, so when spring arrives, the fish may not have been fed for months. When you add a nutritional deficit on top of the temperature change, it can drastically affect the immune system. In the fall, stress is added following the elimination or reduction of feeding habits.

Things to Look Out For

Aside from the thermal stress, a pond must cope with a variable biofilter function, which slows down in the fall and basically has to start from scratch each spring. With the lack of efficient biofiltration, water quality can deteriorate quickly thanks to the elevated ammonia and nitrite levels as a result of the warming water and fish beginning to feed.

Many pathogens like Costa and Aeromonas are ever-present in our ponds and often take advantage of stressed fish, inducing disease. In summer when all is in balance, there is typically very little issue with the nasties that live with our fish, as stressed fish can fight off infection and generally stay healthy. But in the spring and fall, with all this stress, infections can occur.

microscope slide fluke

This histology of a gill shows a gyrodactylis fluke.

Minimize the Stress

It is often difficult to do anything about temperature itself. Some pond owners like to use heaters to offset temperature effects, but this is often not practical (and can be very expensive). We can add shade around ponds to avoid a rapid temperature increase. If we were only dealing with temperature-related stress, we wouldn’t have to worry as much, as fish can tolerate some stress without getting sick. So if we can then focus on controlling some of the other stressors, we can decrease the odds of health issues popping up.

There are many high-quality diets for koi on the market, and with proper feeding, they may help the fish through a difficult spring or fall. A fish is only as strong as the food it is fed. This is true not only during seasonal transitions, but all year long, also. A fish that is fed a high-quality diet throughout the pond and water-garden season is going to be in better condition for the transition to winter, through the winter and into the spring. So, encourage your customers to feed a quality diet all year.

Many feed producers provide choices intended for feeding during the winter and spring. Targeting the fish’s needs during this time can offset some nutritional stress. There are many immune boosters available, such as Glucan, which can be found in many brands of feed. Start with one of these feeds in the spring and fall, as it may help bolster a struggling immune system.

white seabags with bacterial infection

This white seabass has cytophaga-type bacteria (Tenacibrculum maritimum) on its mouth.

Monitor Water Quality

While filters are transitioning, keep a close eye on the pond’s ammonia and nitrite levels, as well as alkalinity and pH. Remember that with alkalinity below 100 mg/L, biofilters will not operate at peak efficiency. Peak efficiency is essential during transitions.

We can help things along a bit with a nitrifying bacteria-boost product. There are several out there. I have had great success with ATM Colony. When the product is added to a brand-new pond filter, no ammonia or nitrite spikes were observed several days after introducing fish. Of course, for poor water-quality conditions, a good water change always helps. Remember that a functioning biofilter is about so much more than just the nitrification process. There is a whole ecosystem in there that we will probably never understand — no two are alike.

koi with ulcer

This koi has an ulcerative bacterial lesion usually
associated with Aeromonas hydrophily.

Finally, we can try and reduce the number of disease-causing organisms in the pond. In addition to thermal stress, the types and number of disease organisms present in the pond can lead to illness. An outbreak often occurs because one type of organism is in large numbers and can induce disease with weak and stressed fish. If we reduce the total number of disease-causing organisms, we may avoid problems altogether.

If you tend to experience a lot of problems during transition periods, prophylactic treatments may then be a good choice. Try using a single treatment with a broad-spectrum disease treatment like MinnFinn. Multiple applications of other products like malachite green, formalin mixtures and potassium permanganate can also help.

Remember, fish can tolerate some stress without getting sick. If you can control as many stressors in the pond as possible, you can increase the odds of bringing the fish through season transitions with flying colors.

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