Are Koi Wusses? Part IV-Oxygen

Oxygen comprises 21% of the Earth’s atmosphere by volume and is vital to the existence of all higher animals. Some animals have developed, over time, an amazing efficiency in the utilization of Oxygen. Fish make up a large part of this group, with some being even more efficient than others. Members of the Carp family are some of these fish.

Fish respiration is very efficient. They possess the ability to remove up to 80% of Oxygen in water. Compare this to an efficiency rate of 27% for humans and it is quite impressive, but it is also necessary given the fact that, in fresh water, the normal level of oxygen, by volume, is 6-8 mg/L which is a tiny fraction of 1%.

Added to this amazing ability is the fact that members of the Carp family have the means to survive, for a period of time, at very low (hypoxic) oxygen levels. Goldfish (Carassius auratus auratus) can actually survive for extended periods with NO Oxygen (Apoxia).

Oxygen in the aquatic environment is usually measured in mg/L and in percent of saturation. Saturation is the normal maximum level of Oxygen that water can hold at a given temperature and atmospheric pressure. Water with no oxygen is termed  anoxic, 1% to 30% saturation is termed hypoxic, 31% to 100% is termed normoxic and any level over 100% is termed hyperoxic.

A “healthy” aquatic system should rarely experience less than 80% saturation. This means, for example, that a Pond that is located at sea level with a water temperature of 20C/68F should have an oxygen level of at least 7.3 mg/L. This same Pond at 26C/79F should have a minimum level of 6.6 mg/L. This lower level is because water loses its ability to retain Oxygen as temperatures increase, so saturation levels decrease.

One study states that Carp (Koi) can survive for 5hrs at 15C/59F under anoxic conditions. This adaptability is in inverse relation to temperature. As temperature rises, the time period of toleration decreases.

What about hyperoxia or super-saturation? Experiments have shown that “hyperoxia did not cause any visible changes in fish behavior, but at the biochemical level a serious stress was indicated. In fact, tissue-specific changes……did not return to normal even after 36 hrs of normoxic recovery”(Lushchak et. al. 2005).

So what does all of this mean?

As stated previously, Carp (Koi) are extremely adaptive. They can tolerate extremes in Oxygen levels for limited periods of time with no adverse effects given sufficient recovery time. Oxygen levels between 31% and 100% saturation are acceptable. The lower limits of this range are, however, not recommended. This is due to the fact that in any 24 hour period Pond Oxygen levels will fluctuate as temperatures change. A 40% saturation at noon will most probably mean an anoxic condition existed pre-dawn. 70% saturation is suitable for Carp (Koi), but 80% saturation is best for the entire eco-system.

How important is pH? Next!