Pond ecosystems are delicate and complex systems that support a wide range of aquatic life.
Among the many natural processes that occur in ponds, turnover is one of the most significant and
potentially harmful events. Pond turnover refers to the sudden mixing of water layers within a
pond, typically triggered by abrupt weather changes such as cold fronts or heavy rainfall. While
turnover is a natural phenomenon, it can have deadly consequences for fish populations if not
properly managed, often resulting in mass fish kills. When dissolved oxygen levels fall below 3
mg/L, fish begin to experience stress, and if levels drop below 2 mg/L, large-scale fish kills may
occur. This is particularly dangerous for sensitive species.
In most cases, farm ponds in the south become stratified in the summer. Stratification happens when the water column divides into an oxygen-rich upper layer and an oxygen-deficient lower layer, separated by a sharp boundary known as the thermocline. Typically, the longer stratification persists in a pond, the more the thermocline rises, expanding the volume of low-oxygen water at the bottom.
Signs of Pond Turnover and Impending Fish Kills:
- Sudden color changes (water may become darker or murkier)
- Rotten egg smell (from hydrogen sulfide in bottom layers)
- Fish gasping at the surface or staying near inflows or aerators
- Dead fish appearing within 12–24 hours after a cold front or storm
How to Prevent or Reduce Fish Kills from Turnover
- Use surface or bottom aerators to circulate water and prevent stratification
- Reduce organic load by minimizing runoff, avoiding overfeeding fish, and removing excess vegetation
- Monitor oxygen, especially during seasonal transitions or after weather events
- Deep water access to ensure fish have areas with adequate oxygen
Emergency Response to Turnover
- Aerate immediately if you suspect turnover, run aerators 24/7
- Flush the pond if possible and add fresh oxygenated water
- Remove dead fish to prevent further oxygen depletion from decaying