Dead Fish in Ponds, Lakes and Canals

Most fish kills are the result of natural processes

When a fish kill occurs in a lake, pond or canal, the first assumption is  that something is terribly wrong with the water body. Suspicions are raised as to whether human activity, such as a chemical spill, may have caused the fish to die. However, most often fish kills are the result of natural processes that cause the oxygen dissolved in the water to drop to levels insufficient for fish survival. A dissolved oxygen, or DO-related fish kill can occur in virtually any aquatic environment, but water bodies located in residential areas are particularly vulnerable. Developed areas create rainfall runoff that may contain high amounts of nutrients from septic tanks and fertilizers. Water bodies with high nutrient levels can produce a dense growth of algae. When sunlight is available, the algae use the nutrients in the water to produce oxygen through the process of photosynthesis. Most of the oxygen available to fish comes from this process. However, nighttime and cloudy or low sunlight days causes the algae to switch from photosynthesis to respiration, which results in the algae consuming the oxygen needed by the fish population. Clean-up of fish kills occurring in private residential ponds and lakes is generally the responsibility of the property owner or homeowners association. Fish kills in District canals should be reported by calling 561-498-5363 or emailing