Chapter 10: Fish Kills in Recreational Ponds
R. Graves Lovell, Joseph E. Morris, and Richard D. Clayton
Sport fish ponds have sudden fish die-offs (natural or human-induced) more frequently than flowing waters and large impoundments because they are often managed to sustain high-density fish populations. Accurate determination of the cause of the fish kill will determine what, if any, action is necessary to save fish that might still be alive. Before assessing a fish kill, it is important to have a good understanding of the causes as well as symptoms that might be detected before, during, or after a kill. The purpose of this chapter is to provide information biologists can use to understand the nature of fish kills and determine what activities might prevent a similar die-off from reoccurring. The steps for responding to a fish kill in a pond are then described later in this chapter.
Low dissolved oxygen is the most common cause of large fish kills in ponds managed for sport fishing, particularly during the summer months. As water temperature increases, rates of respiration and decomposition also increase while water oxygen saturation levels decrease. Oxygen concentrations in a pond fluctuate during a 24-h period; the highest concentration usually occurs during mid to late afternoon, and the lowest concentration is observed just before dawn. Early morning hours are therefore the most critical. The primary source of oxygen in most ponds is from photosynthesis by phytoplankton and submersed vascular plants in the presence of sunlight. Wind action at the pond surface can also be a substantial source of oxygen.
Oxygen is consumed through respiration by fish, aquatic plants, insects and other aquatic organisms, and decomposition of organic matter. Oxygen consumption is determined by the density of aquatic organisms as well as the amount of dead organic matter within the pond. Aquatic plants, both living and dead, typically account for the highest oxygen consumption in most sport fish ponds. In “healthy” ponds, dissolved oxygen levels fluctuate over a 24-h period but usually remain at healthy levels (>5 mg/L) for sport fish. Occasionally, dissolved oxygen can drop quickly and cause a sudden fish die-off. Alternatively, the dissolved oxygen concentration may drop less aggressively to sub-lethal levels, which can induce fish stress. If this pattern continues for several days, fish may die from stress associated with prolonged exposure to low oxygen concentrations (<3 mg/L). Low dissolved oxygen levels may also lead to stress-related diseases.