How sensitive are fish to fluctuations in temperature?

This winter, so far, seems to be relatively mild but we have had some very cold weeks and I expect there may be more to come. These changes in temperature I am sure will affect my pond temperature and I wonder how it might affect my fish?

Fish can detect small changes in temperature – some as small as 0.03C. Some fish do not do well with temperature changes. They get stressed and therefore are more susceptible to stress related diseases, while others tolerate change fairly well, as long as it’s gradual.

As fish species have evolved to live at different water temperatures, each has a different tolerance. The maximum tolerable temperature is called the Upper Incipient Lethal Temperature (UILT) and the minimum is called the Lower Incipient Lethal Temperature (LILT). However, some fish are “eurythermal” and can tolerate a wide range of temperatures.

Goldfish, for example, can live in water from 1-40C/34-104F providing they are properly acclimatized. When you first introduce your fish to your pond you don’t want to drop them straight in. Letting them acclimate to the water is always a good idea. Other species, are “stenothermal” and have a narrower temperature band.

Also in many cases the LILT and UILT change as your fish mature, which is why fry and young fish are much more sensitive to changes.

So the gradual changes in temperatures that happen in your pond seasonally shouldn’t adversely affect your pond fish because they are adapted to these changes. However the temperature also has an effect on the oxygen levels in your pond and because fish need more oxygen when they are warm (and warm water holds less oxygen). In the worst case scenario they can succumb to oxygen starvation. Cold water can hold more oxygen but in cold weather your fish will not be as active. However when it is warmer your fish will be active and require more oxygen. Therefore boost oxygen levels by adding an air pump. This will ensure that your fish have sufficient oxygen and by keeping your air pump running continuously you will not be caught unawares if there is a warm spell and consequently oxygen levels drop. Similarly an air pump will mean that your pond water is in constant movement so you can maintain a hole if cold weather causes ice to form. This will allow gas transfer to continue also in the colder weather.

Your pond should remain healthy even in fluctuating temperatures but to keep it that way you need to maintain oxygen levels. Introducing an air pump to your pond will certainly be a beneficial addition.