A catastrophe many pond managers experience each year is a fish kill due to dissolved oxygen (DO) depletion. Fish kills can occur any time the DO demand is greater than the water can supply but pond managers can take steps to avoid such an event in their ponds.
Fish kills commonly occur in ponds where channel catfish are fed supplemental food. Typically, the pond owner stocks fingerling catfish and begins a feeding program. Given the small size of the catfish, initially the pond water supplies plenty of oxygen. However, in seven or eight years, the fingerlings have grown to eight pounds or larger, and unless the owner has been harvesting periodically, the pounds of catfish the pond has to support is significantly greater. A September cold front with several days of cloudy weather rolls in and the pond owner finds most of his prize fish belly-up a sad situation.
Let's examine a pond's daily oxygen cycle to better understand this phenomenon. Oxygen is produced by plants, especially phytoplankton (the microscopic plants suspended in the water column that give water a green color), during photosynthesis. If the DO concentration is below saturation, oxygen is also gained through diffusion from the atmosphere. If the DO is above saturation, oxygen is lost to the air through diffusion. Oxygen is consumed through the respiration of the phytoplankton, other plants, fish, invertebrates and decomposers present in the water. During the growing season, these interactions produce a daily DO cycle that is lowest at dawn, increases to a peak in late afternoon and then decreases until dawn.
Compounding the potential problem is the fact that water holds less oxygen as its temperature increases. Therefore, late summer water holds the least amount of oxygen of the year. Ponds also tend to stratify during summer, producing a deep cool water layer that doesn't mix with the upper warm water layer. The deep layer can become completely depleted of oxygen over the summer and actually create an oxygen debt from non-decomposed fish waste, feed, plants, etc. A cold front that sufficiently cools the upper layer to the temperature of the lower layer can cause the pond to "turnover," mixing the two layers and allowing remaining oxygen to service the oxygen debt of the non-decomposed waste, drastically reducing the DO level of the pond and potentially causing a fish kill.
In some ponds, a phytoplankton die-off causes fish problems. If a phytoplankton "bloom" is relatively heavy, the microscopic plants represent tremendous biomass supported by the pond. Cool fronts, cloudy weather, muddy water, herbicides or nutrient depletion can cause a die-off, creating a large oxygen demand to decompose the plants. This demand can be higher than the system can support, leading to a fish kill.
So, what can be done to prevent oxygen-related fish kills?
These steps will reduce the likelihood of a fish kill, but will not eliminate the possibility. When oxygen depletion is suspected, aeration or transferring fish to another pond are the only solutions. To gain oxygen, air must be pumped into the water or water must be splashed into the finest droplet size possible before reentering the pond. Commercial aerators are available, but pumps rigged to splash water into the air can help in a small pond. Remember, aeration will be required primarily during nighttime hours.