Spring Into Spring

What do the fish go though as they are coming out of their winter hibernation?
Fish are cold-blooded creatures, or poikilotherms. They don’t produce their own heat and so they have developed enzyme systems to provide them with bodily functions such as the production of energy, regardless of temperature. Fish survive winter because they are capable of producing enzymes that function in cold water. These enzymes are called isoenzymes. They are produced as needed and they work in colder conditions. As the water gradually warms up, these coldwater isoenzymes are no longer produced, and the regular enzyme systems begin to function to support the fish.

Before the fish get “warm” in summer, there’s a narrow period of time where the water temperatures are too cold for the fish immune system to function optimally, but parasitic and bacterial populations are proliferating at explosive rates. During this period in the spring, the fish are prone to develop illnesses from these parasites or bacteria.

Further compounding these difficulties, fish have usually not been fed all winter – so they have been in a catabolic energy balance (net loss of energy) instead of gain. Feeding Koi in the Springtime is important to rebuild these energy stores and support the immune system.

What should the pond owner expect in terms of their fish when the ice melts?
When the ice begins to melt, look for the fish to become more and more active. When the water temperatures edge into the low fifties, consider resuming a feeding regimen with a wheatgerm based, easily digestible food with low protein residues. Reducing protein in very cold water feeding regimens is good for the fish, and for the biological (bacterial) processes which reduce fish wastes and uneaten food. These processes work inefficiently in very cold water and so nitrogen may accumulate in cold pond systems overfed a high protein diet.

Apply salt fights a variety of parasitisms, which may have gained an advantage against the fish in winter. Three pounds of salt per one hundred US gallons of water is recommended if you think there could be parasites in the pond. Salt may harm plants.


Medicated food is HIGHLY recommended along with salt in the Spring. Medicated food contains safe but effective levels of antimicrobials to fight infection in the fish during the Springtime warm-up, when their immune system has not regained its full strength.

Because hidden parasite problems and bacterial infections are the two most common diseases that people encounter with their pet fish in Spring, I recommend salt and medicated food very highly. Other medications can be selected, but salt and medicated food are easy to use.

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