Fish require good quality water to survive, grow, and reproduce. Good quality water is free of pollutants such as toxic materials, excessive organic matter, and silt. Water should also have high oxygen content. Oxygen deficiency is a common water quality problem encountered in ponds. Most fish species require at least 5 parts per million (ppm) of dissolved oxygen for good health and vigorous growth. They can tolerate 1 or 2 ppm for short periods, but they will become stressed, will cease feeding, and may become susceptible to diseases.
The amount of oxygen contained in the pond depends upon the water temperature and the depth. During late winter and early spring, the water in a pond will have the same temperature from top to bottom. In late spring, increased atmospheric temperatures begin warming the pond from the surface down. Water in shallow ponds located in open areas exposed to the wind may continue to mix through ice-free periods, but by the summer the surface water in deeper, less windswept ponds is considerably warmer (and much lighter) than the bottom water, so a thermal stratification occurs. This is a fairly stable condition with the warm upper layer (epilimnion) floating on the cool bottom layer (hypolimnion) separated by the transition zone (thermocline). As the wind blows, only the upper layer is mixed and oxygenated. The lower layer does not receive additional oxygen and in fact slowly loses its oxygen by the decay of organic matter on the bottom. By mid-summer oxygen is consumed in the lower layer so fish are confined to the upper layer and thermocline. This is why ponds built deeper than 15 feet waste space during the summer. On the other hand, a pond that is too shallow (less than 10 feet deep) may encounter summer-kill problems. This topic is discussed further in the “ Fish Kills” section.
In late summer and fall, the surface water cools until its density is similar to the bottom water. Strong winds are then able to mix the water from top to bottom. This carries oxygenated water to the bottom and fish are again able to inhabit the entire pond.
After ice forms in the winter, water on the bottom of the pond is slightly warmer than water just under the ice. Fish usually prefer to locate near the bottom for this reason. Since ice prevents a mixing action from occurring, organic wastes again settle to the bottom, much as occurs during the summer. Decomposition of organic wastes uses oxygen, and excessive decomposition can drive fish off the bottom, up the water column, in search of oxygen. Severe cases of decomposition in combination with lack of oxygen production by plants results in an oxygen deficiency throughout the pond and eventually winterkill. Particulars on this subject are discussed in the “ Fish Kills” section.