Key Takeaways for Pond Aeration

  • Install aerators and destratifiers to eliminate temperature stratification
  • Responsible pond stewardship should include managing phytoplankton blooms

General Takeaways

  • Most common causes of fish mortality are related to low-dissolved oxygen from dense phytoplankton blooms. Other causes include weather-related events, chemical applications, fish diseases, water chemistry, algal toxins, and watershed pollution.

Highlighted Causes of Fish Kills

  • Phytoplankton Bloom – When fish were fine the previous day and die overnight or in the early morning at or before dawn, the most common cause of fish mortality is low-dissolved oxygen. A phytoplankton bloom generates oxygen with sunlight, yet at night it consumes oxygen. In a bloom environment, sometimes this nighttime consumption of oxygen exceeds the amount of dissolved oxygen remaining from the previous day.
  • Weather-Related Events
    • Calm, Clear Weather – These conditions, combined with a sudden phytoplankton die-off in ponds with dense cyanobacteria (blue-green algae), may cause fish mortality. None, some, or many fish may die. The impact on fish depends on the quantity of phytoplankton that died. When there are dense algal mats on the surface, the high photosynthesis rate elevates pH and dissolved oxygen. This, in turn, can stress the algae and lead to phytoplankton die-off, which causes low-dissolved oxygen. The decomposition of large amounts of algae consumes dissolved oxygen at a time when less is being produced. During this incident, the pond may appear brown with small bubbles on the surface.
    • Summer Weather Front – When the presence of a fish kill is coupled with a cold rain and wind event, the cause is thermal destratification of the water column, resulting in a low-dissolved oxygen condition. During this event, stratified ponds have cold bottom water with little to no dissolved oxygen, ferrous iron, or organic matter. The cold rainwater generates a “pond turnover”* where the anoxic bottom water mixes with oxygenated surface water. This mixing dilutes the overall dissolved oxygen throughout the pond. Dissolved oxygen is further diminished as the ferrous iron and organic matter at the bottom mix with oxygen and begin to oxidize. With this event, the pond may appear brown, and other area stratified ponds will likely be similarly affected.
    • Weeds and Weather – Ponds with dense underwater weeds may be susceptible to fish kills when coupled with a heavy rain event. When the pond’s turbidity increases due to sediment from watershed runoff, the turbid water shades the underwater weeds, reducing photosynthesis and dissolved oxygen production.
    • Chemical Application – The application of herbicides and algaecides reduces the dissolved oxygen as plants die off. If the application is followed by prolonged cloud cover and the reduction of oxygen-generating photosynthesis, the conditions can become serious and lead to fish mortality.
  • Other
    • Disease – Although more common in commercial fish ponds, fish mortality as a result of disease tends to occur over several days and weeks. The author recommends consulting a fish disease specialist when lesions and/or distress is noted on fish population.
    • Water Quality – The author considers it uncommon in sportfish ponds, but water containing high concentrations of ammonia, nitrite, and hydrogen sulfide can be the cause of a fish kill.
    • Algal Toxins – Algal toxins produced from some blue-green and golden algae may cause fish mortality. This condition is difficult to verify and is often assumed when all other potential causes are ruled out.
    • Pollution – Pollution from unknown sources in the pond watershed can be the cause of a fish kill when no other reason is found. It is recommended to investigate potential sources of runoff entering the pond.

* Although the author does not use the term “pond turnover,” it is the commonly used nomenclature to describe this event.