You might be surprised to hear that fish, including koi, can suffer from sunburn.
How could this be?
Water does provide a barrier to UV rays but middle and long UV wavelengths can penetrate water for a few centimetres, especially if the water is very clear. Which it would be in a koi pond! Koi wouldn’t normally choose to spend too much time near the surface of the water but may be forced to if the water is too shallow or they are struggling to get enough oxygen from it.
Koi are coldblooded creatures which means that their metabolisms speed up as the water temperature increases. They then require more oxygen but the oxygen available will be decreasing as the water gets warmer. The struggling fish move to the surface and become vulnerable to the UV rays. Fish may also move closer to the surface of the water if the pond is overcrowded.
Symptoms of sunburn in Koi
Fish may develop skin sores in the areas exposed directly to the sun including the top of the head, dorsal fin and upper back. The affected skin will first turn whitish in colour and then eventually becomes patchy, thickened and creamy. Fins will become frayed and as the sores develop, the skin flakes off leaving a whitish or pink coloured ulcer.
In extreme cases, sunburn can result in serious imbalances in the blood and then kidney damage. Bacterial and fungal infections may develop and these might mask the original lesion, concealing the root cause of the problem. If you spot lesions in hot weather, always consider the possibility of sunburn. These secondary infections can be more serious than the original sunburn. Most fish will survive a case of sunburn if cared for properly but will often be permanently scarred as a result of their ordeal.
Certain chemicals can impact photosensitization in fish which means that the effects of the sun will be increased. If anything is to be added to the pond water, it is worth checking the photosynthesising properties of the substance before using it.
How to protect Koi from sunburn
It is important to ensure that your pond isn’t overcrowded as this will cause an excessive demand for oxygen. If you’re your pond is shallow, deepen it if possible. Avoid contaminating the water with photosensitizing agents and top up the pond with cooler water if it is overheating or evaporating too quickly.
You should check that your pump is working correctly and provide shade over the pond to protect it from direct sunlight. Better still, fit a koi shelter in the pond or construct a cave to provide a refuge for the fish.
If your koi have become lethargic in hot weather or appear generally unwell, they may be suffering from sunburn. In addition to the aforementioned preventative measures, consider the use of a fish stress reducer to help restore your stock to good health.