A fish kill caused by dissolved oxygen (DO) depletion can be a catastrophe for pond managers. Fish kills can occur any time the DO demand is greater than the water can supply — but 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,” said Grant Huggins, a wildlife and fisheries specialist with the Noble Foundation in Ardmore, Okla.

Typically, pond owners stock fingerling catfish and begin a feeding program. Initially, the pond water supplies plenty of oxygen given the small size of the catfish. In seven or eight years, however, 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,” Huggins said.

Plants, especially phytoplankton (the microscopic plants suspended in the water column that give water a green color), produce oxygen 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,” Huggins said. “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 “turn over,” 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 phytoplankton die-off, creating a large oxygen demand to decompose the plants,” Huggins said. “This demand can be higher than the system can support, leading to a fish kill.”

Huggins suggests these tips for preventing oxygen-related fish kills:

  • Do not fertilize ponds where fish are supplementally fed. The feed itself acts as fertilizer when passed through the fish.

  • Do not supplementally feed fish in ponds that have or have a history of visibility less than 12 inches due to phytoplankton bloom.

  • Keep the total pounds of fish per surface acre of water below 1,000.

  • Discontinue feeding when fish “go off feed” and monitor DO or early morning fish activity. Fish going off feed sometimes indicates oxygen stress. Fish that are oxygen stressed “gulp” at the surface of the water.

  • Monitor the pond's water color through the growing season. If the color suddenly changes from green to tan, brown or black, expect an oxygen problem.

  • Monitor oxygen levels or fish activity at dawn during cloudy weather when visibility is below 12 inches due to phytoplankton.

  • Monitor DO or fish activity during fall cool fronts.

  • Remove domestic waterfowl from ponds with a thick phytoplankton bloom unless they are more important than the fish. Waterfowl waste fertilizes ponds.

These steps will reduce the likelihood of a fish kill, but will not eliminate the possibility,” he added.

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,” Huggins said.